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The role of cathodic current in plasma electrolytic oxidation of aluminium ldquoScanning wavesrdquo of the current density on complex-shape substrates
Aleksey B Rogovabc Aleksey Yerokhina Allan Matthewsa
a) School of Materials the University of Manchester Manchester M13 9PL United Kingdomb) Nikolaev Institute of Inorganic Chemistry Novosibirsk Russia 630090
c) Scientific and Technical Centre ldquoPokrytie-Ardquo Novosibirsk Russia 630015
Abstract This paper is focused on the factors influencing the surface distribution of the
current density during plasma electrolytic oxidation of 2024 Al alloy under alternating
polarisation It was found that employment of combined current mode including relatively long
(100-2000 ms) pulse trains improved coating uniformity even when electrolyser provided severe
non-uniform primary electric field Experimental investigation employing a sectioned sample
showed that the non-uniform distribution of the primary electric field could be compensated by
the changes in the coating properties induced by previous cathodic polarisation Temporary
changes in the secondary distribution of current density across the sample surface (attributed to
the coating properties) caused dynamic redistribution of the anodic current density during
following AC pulse train resulting in so-called ldquoscanning waverdquo effect ie migration of the
maximum current density along sample surface Factorial experimental design finite element
modelling and analysis of the transient current-voltage curves were applied Mechanistic
explanation underlying considered phenomenon has been suggested In addition the discussion
on the increase in PEO efficiency under soft sparking conditions were provided as
ldquoelectrocatalysisrdquo of anodic oxidation by previous cathodic treatment
Keywords Plasma electrolytic oxidation coating uniformity aluminium alloy scanning wave
effect
- corresponding author
1 Introduction
Plasma electrolytic oxidation (PEO) is an electrochemical surface treatment under high anodic
polarisation conditions (up to 1000V) [12] A specific feature of the PEO is the presence of
localised current channels caused by dielectric breakdown of the forming oxide layers The
breakdown appears as localised microdischarges where energy is dissipated triggering plasma-
assisted chemical reactions and heat effects that influence the charge and mass transfer through
the metal-oxide-electrolyte system and may cause phase transformations sintering calcination
and dehydration of the formed oxide layers As a result new composition and microstructure
may develop in the coatings leading to new properties For instance hard and dense sintered
oxide layers enriched with α-Al2O3 can be obtained on the surface of aluminium alloys at near to
ambient bulk substrate temperatures Due to ecologically friendly nature of the process as well as
high wear- and corrosion resistance of the coatings they become suitable for a wide range of
applications
Despite some positive results a broad industrial application of PEO is limited by a number of
technological issues caused by complexity of the process and a lack of basic information about
the coating formation mechanisms One of the most promising current mode in PEO is so-called
ldquosoft sparkingrdquo regime wherein the development of large destructive discharges at the later
stages of the process is suppressed providing a deeper substrate oxidation leading to more
uniform coatings with higher hardness and better adhesion (see overview in [34]) PEO under
alternating polarisation conditions including soft sparking mode is described by a number of
process parameters in terms of initial (electrolyte composition substrate materials) and boundary
(polarisation mode) conditions As a result it is difficult to find an appropriate set of the process
parameters satisfying requirements of the final coating quality One of the most important
generic properties of coatings is thickness uniformity on the substrates with complex shape
geometry such as blind holes cavities sharp edges and large protrusions which represents the
main challenges in PEO technology
It is clear that any non-uniformity of the PEO coating is caused by non-uniform distribution
of the current density which in turn depends on both the primary electric field distribution in the
electrochemical cell and the local properties of the resulting coating Therefore there are two
main approaches to achieve the uniform PEO coating The first one consists in deliberate
arrangement of the uniform primary electric field distribution by using conformal auxiliary
counter electrodes which shape fits the substrate geometry thereby creating a uniform primary
field distribution [56] Wei et al [7] mentioned that the smaller interelectrode distance the better
corrosion resistance of the coating moreover they have found that current pulses earlier reached
maximum amplitude on the nearer parts of substrate then on the farther ones Importance of the
interelectrode distance in PEO under galvanostatic conditions was reported as well in [8][9]
The extension of this technique is so called ldquoscanning micro arc oxidationrdquo wherein a small-area
counter electrode is moved through the surface of larger substrate allowing uniform coatings to
be produced on the large and complex substrates [1011] Numerical calculation of the primary
electric field distribution through the surface of complex shape substrates with deep internal dead
holes under PEO conditions has been performed in [12]
The second approach utilises specific coating properties to influence the secondary
distribution of current density at the sample surface providing a self-aligning coating growth
behaviour Those approaches are based on the local difference in transient current-voltage curves
(CVC) of the particular coating regions It is known that typical anodic (characterising substrate
oxidation) CVC for PEO of aluminium possess severe non-linear character that may include both
the sharp increase in current once threshold voltage is achieved and the regions with negative
differential resistance [313ndash16] The CVC at any given local point within PEO coating depends
on the set of available electrochemical reactions under given particular conditions Those
reactions are defined by properties of the coating (composition microstructure) as well as by the
electrolyte composition It was suggested that transition to the soft sparking PEO which occurs
at certain excess of cathodic current density over anodic one is accompanied by associated
processes under anodic and cathodic polarisations which in turns significantly change current
response on applied voltage (CVC) Only a few papers concerning coating uniformity in view of
electrolyte composition or polarisation conditions can be found Yerokhin et al [17] noted that
coating uniformity can be improved with increase in alkali and decrease in silicate
concentrations in electrolyte solution Terleeva et al [18] has found that application of combined
current mode including alternating current and additional cathodic current pulse trains (ACC)
allows the coating uniformity on the internal surface of a conical shape substrate to be improved
A few works were devoted to the influence of electrolyte ageing on the appearance of non-
uniform features in the coating regions far away from the substrate edges [1920]
Disadvantages of the primary field control lay in the necessity of manufacturing a counter
electrode which shape may be even more complex than the targeted substrate and any changes
in substrate shape would require the counter electrode to be redesigned and remanufactured At
the same time a control over the secondary field distribution by means of adjusting polarisation
conditions and tailoring electrolyte composition to the specific alloy provides considerable
advantages especially in situations where substrates can vary to some degree Such variation can
be caused for example by a bespoke shape of medical implants or in other applications (eg
marine or aerospace) by the fact that batches could be formed by parts of similar but not exactly
the same geometry due to the small-scale nature of manufacturing (eg by 3D printing)
In this paper experimental investigation is performed in regards of the PEO coating
uniformity and secondary current density distribution under intentionally non-uniform
distribution of the primary electric field and how this distribution is affected by timings and
current densities of the pulse trains in combined current modes Some general considerations
regarding the mechanisms underlying PEO treatments and efficiency of the coating formation
are also provided
2 Experimental
All substrates were made from A2024T3 aluminium alloy (AlCu4Mg) Particular sample
geometry is described in following subsections An aqueous electrolyte solution comprising 15
gdm3 of technical water glass (specific gravity 141 gcm3 SiO2Na2O = 302) and 2 g dm3 of
potassium hydroxide was contained in a cubic stainless steel tank (300 dm3) served as a counter
electrode During the PEO process electrolyte solution was pumped throughout the internal part
of the screen along the coated surface in order to prevent electrolyte overheating above 40oC
The power supply provided a combined current waveform composed by pulse trains of
symmetrical alternating current (AC) characterised by equal positive and negative average
currents J+ = J- = JAC with addition of either cathodic (C) or anodic (A) half-wave current pulses
with frequency 50 Hz (period = 20 ms) for all pulse trains (see Fig1) An obstruction to the
primary electric field was formed by a special insulating screen made from PTFE which
surrounded the sample as shown in Fig2 In order to exclude bottleneck effect but provide
suitable obstruction the cross-sectional area of the open end of the screen was equal to the total
surface area of the sample exposed to the treatment
Fig 1 Schematic representation of the complex alternating current modes with additional
cathodic (AC-C) and anodic (AC-A) pulse trains
21 Factorial experiments Application of the combined current mode (AC-C or AC-A) is
complicated by a number of characteristic parameters that hindering optimisation or general
relations to be found Moreover some parameters are interconnected for instance cathodic
charge may be increased either by increasing in τC or Jc Therefore the factorial experimental
design may help us find out the most valuable effect of individual parameters as well as effects
of their combinations
The samples were fabricated in the form of tubes (Oslash14x12x30mm) which cylindrical sides
subjected to PEO treatments Variable parameters in experiments were as follows current
densities in AC and C pulse trains (JAC and JC) and their durations (τAC and τC respectively)
However some particular effects in factorial experiments were represented by combination of
two variable parameters since we already know the importance of those combinations in PEO
Detailed explanation of factorial experiments and real experimental conditions are given below
and listed in Table 1
Fig2 Drawings of the insulating screen and sample assembly used for a) factorial experiment b) for studying the current density distribution It is sectioned for demonstration the internal construction
As the central point (designated as 0) of the factorial design the following set of
experimental parameters was chosen JAC0 = 90mAcm2 Jc0 = 18 JAC = 1125 mAcm2 τAC =
1000 ms τC = 280 ms t = 251 hours because this was one of the main current modes for a
semi-industrial application that provided hard and well adhered coatings on the outer surfaces of
many industrial components made of aluminium alloys with low (lt12) content of silicone
However such current mode could not often provide satisfactory quality of the coatings for
substrates with complex geometry especially on the inner and concave surfaces The main
effects (Xn) and their levels (plusmn1) in factorial design were chosen as follows (Xn = Xn0 plusmn ΔXn)
1) current density within AC pulse train X1 JAC = JAC0 (1 plusmn 13)
2) ratio of current densities within C and AC pulse trains X2 JCJAC = 18(1 plusmn 13)
3) ratio between durations of AC and C pulse trains X3 τACτC = 357 plusmn 114
a) b)
PTFE
Al
1234
Al ringsPTFE
PTFEcap
PTFEcap
wire
Spacers
inner end
outer end
4) duration of C pulse train X4 τC = 280 plusmn 140 ms
Total process duration for every experimental condition were chosen to achieve equivalent
total anodic charge in the PEO processes This allows us to estimate the process efficiency by
comparison of the average coating thicknesses
The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication
could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which
appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)
and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The
sequence of treatments among total 24 experiments was defined by randomization Runs at each
experimental condition were repeated three times As responses the following characteristics
were taken
1) The average increase in diameter Y1
2) The mean average coating thickness Y2
3) Dispersion of the diameter increase through the sample length Y3
4) Dispersion of the coating thickness increase through the sample length Y4
5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the
worst) to 1 (the best) scale with 01 step at equal intervals Y5
The first two responses (Y1 Y2) represented general process efficiency (since anodic charge
was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The
coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand
which provided reproducible measurements in respect to the sample axis with an error 4microm
Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5
along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy
of plusmn4microm The schemes of measurements are depicted on Fig 3
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
1 Introduction
Plasma electrolytic oxidation (PEO) is an electrochemical surface treatment under high anodic
polarisation conditions (up to 1000V) [12] A specific feature of the PEO is the presence of
localised current channels caused by dielectric breakdown of the forming oxide layers The
breakdown appears as localised microdischarges where energy is dissipated triggering plasma-
assisted chemical reactions and heat effects that influence the charge and mass transfer through
the metal-oxide-electrolyte system and may cause phase transformations sintering calcination
and dehydration of the formed oxide layers As a result new composition and microstructure
may develop in the coatings leading to new properties For instance hard and dense sintered
oxide layers enriched with α-Al2O3 can be obtained on the surface of aluminium alloys at near to
ambient bulk substrate temperatures Due to ecologically friendly nature of the process as well as
high wear- and corrosion resistance of the coatings they become suitable for a wide range of
applications
Despite some positive results a broad industrial application of PEO is limited by a number of
technological issues caused by complexity of the process and a lack of basic information about
the coating formation mechanisms One of the most promising current mode in PEO is so-called
ldquosoft sparkingrdquo regime wherein the development of large destructive discharges at the later
stages of the process is suppressed providing a deeper substrate oxidation leading to more
uniform coatings with higher hardness and better adhesion (see overview in [34]) PEO under
alternating polarisation conditions including soft sparking mode is described by a number of
process parameters in terms of initial (electrolyte composition substrate materials) and boundary
(polarisation mode) conditions As a result it is difficult to find an appropriate set of the process
parameters satisfying requirements of the final coating quality One of the most important
generic properties of coatings is thickness uniformity on the substrates with complex shape
geometry such as blind holes cavities sharp edges and large protrusions which represents the
main challenges in PEO technology
It is clear that any non-uniformity of the PEO coating is caused by non-uniform distribution
of the current density which in turn depends on both the primary electric field distribution in the
electrochemical cell and the local properties of the resulting coating Therefore there are two
main approaches to achieve the uniform PEO coating The first one consists in deliberate
arrangement of the uniform primary electric field distribution by using conformal auxiliary
counter electrodes which shape fits the substrate geometry thereby creating a uniform primary
field distribution [56] Wei et al [7] mentioned that the smaller interelectrode distance the better
corrosion resistance of the coating moreover they have found that current pulses earlier reached
maximum amplitude on the nearer parts of substrate then on the farther ones Importance of the
interelectrode distance in PEO under galvanostatic conditions was reported as well in [8][9]
The extension of this technique is so called ldquoscanning micro arc oxidationrdquo wherein a small-area
counter electrode is moved through the surface of larger substrate allowing uniform coatings to
be produced on the large and complex substrates [1011] Numerical calculation of the primary
electric field distribution through the surface of complex shape substrates with deep internal dead
holes under PEO conditions has been performed in [12]
The second approach utilises specific coating properties to influence the secondary
distribution of current density at the sample surface providing a self-aligning coating growth
behaviour Those approaches are based on the local difference in transient current-voltage curves
(CVC) of the particular coating regions It is known that typical anodic (characterising substrate
oxidation) CVC for PEO of aluminium possess severe non-linear character that may include both
the sharp increase in current once threshold voltage is achieved and the regions with negative
differential resistance [313ndash16] The CVC at any given local point within PEO coating depends
on the set of available electrochemical reactions under given particular conditions Those
reactions are defined by properties of the coating (composition microstructure) as well as by the
electrolyte composition It was suggested that transition to the soft sparking PEO which occurs
at certain excess of cathodic current density over anodic one is accompanied by associated
processes under anodic and cathodic polarisations which in turns significantly change current
response on applied voltage (CVC) Only a few papers concerning coating uniformity in view of
electrolyte composition or polarisation conditions can be found Yerokhin et al [17] noted that
coating uniformity can be improved with increase in alkali and decrease in silicate
concentrations in electrolyte solution Terleeva et al [18] has found that application of combined
current mode including alternating current and additional cathodic current pulse trains (ACC)
allows the coating uniformity on the internal surface of a conical shape substrate to be improved
A few works were devoted to the influence of electrolyte ageing on the appearance of non-
uniform features in the coating regions far away from the substrate edges [1920]
Disadvantages of the primary field control lay in the necessity of manufacturing a counter
electrode which shape may be even more complex than the targeted substrate and any changes
in substrate shape would require the counter electrode to be redesigned and remanufactured At
the same time a control over the secondary field distribution by means of adjusting polarisation
conditions and tailoring electrolyte composition to the specific alloy provides considerable
advantages especially in situations where substrates can vary to some degree Such variation can
be caused for example by a bespoke shape of medical implants or in other applications (eg
marine or aerospace) by the fact that batches could be formed by parts of similar but not exactly
the same geometry due to the small-scale nature of manufacturing (eg by 3D printing)
In this paper experimental investigation is performed in regards of the PEO coating
uniformity and secondary current density distribution under intentionally non-uniform
distribution of the primary electric field and how this distribution is affected by timings and
current densities of the pulse trains in combined current modes Some general considerations
regarding the mechanisms underlying PEO treatments and efficiency of the coating formation
are also provided
2 Experimental
All substrates were made from A2024T3 aluminium alloy (AlCu4Mg) Particular sample
geometry is described in following subsections An aqueous electrolyte solution comprising 15
gdm3 of technical water glass (specific gravity 141 gcm3 SiO2Na2O = 302) and 2 g dm3 of
potassium hydroxide was contained in a cubic stainless steel tank (300 dm3) served as a counter
electrode During the PEO process electrolyte solution was pumped throughout the internal part
of the screen along the coated surface in order to prevent electrolyte overheating above 40oC
The power supply provided a combined current waveform composed by pulse trains of
symmetrical alternating current (AC) characterised by equal positive and negative average
currents J+ = J- = JAC with addition of either cathodic (C) or anodic (A) half-wave current pulses
with frequency 50 Hz (period = 20 ms) for all pulse trains (see Fig1) An obstruction to the
primary electric field was formed by a special insulating screen made from PTFE which
surrounded the sample as shown in Fig2 In order to exclude bottleneck effect but provide
suitable obstruction the cross-sectional area of the open end of the screen was equal to the total
surface area of the sample exposed to the treatment
Fig 1 Schematic representation of the complex alternating current modes with additional
cathodic (AC-C) and anodic (AC-A) pulse trains
21 Factorial experiments Application of the combined current mode (AC-C or AC-A) is
complicated by a number of characteristic parameters that hindering optimisation or general
relations to be found Moreover some parameters are interconnected for instance cathodic
charge may be increased either by increasing in τC or Jc Therefore the factorial experimental
design may help us find out the most valuable effect of individual parameters as well as effects
of their combinations
The samples were fabricated in the form of tubes (Oslash14x12x30mm) which cylindrical sides
subjected to PEO treatments Variable parameters in experiments were as follows current
densities in AC and C pulse trains (JAC and JC) and their durations (τAC and τC respectively)
However some particular effects in factorial experiments were represented by combination of
two variable parameters since we already know the importance of those combinations in PEO
Detailed explanation of factorial experiments and real experimental conditions are given below
and listed in Table 1
Fig2 Drawings of the insulating screen and sample assembly used for a) factorial experiment b) for studying the current density distribution It is sectioned for demonstration the internal construction
As the central point (designated as 0) of the factorial design the following set of
experimental parameters was chosen JAC0 = 90mAcm2 Jc0 = 18 JAC = 1125 mAcm2 τAC =
1000 ms τC = 280 ms t = 251 hours because this was one of the main current modes for a
semi-industrial application that provided hard and well adhered coatings on the outer surfaces of
many industrial components made of aluminium alloys with low (lt12) content of silicone
However such current mode could not often provide satisfactory quality of the coatings for
substrates with complex geometry especially on the inner and concave surfaces The main
effects (Xn) and their levels (plusmn1) in factorial design were chosen as follows (Xn = Xn0 plusmn ΔXn)
1) current density within AC pulse train X1 JAC = JAC0 (1 plusmn 13)
2) ratio of current densities within C and AC pulse trains X2 JCJAC = 18(1 plusmn 13)
3) ratio between durations of AC and C pulse trains X3 τACτC = 357 plusmn 114
a) b)
PTFE
Al
1234
Al ringsPTFE
PTFEcap
PTFEcap
wire
Spacers
inner end
outer end
4) duration of C pulse train X4 τC = 280 plusmn 140 ms
Total process duration for every experimental condition were chosen to achieve equivalent
total anodic charge in the PEO processes This allows us to estimate the process efficiency by
comparison of the average coating thicknesses
The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication
could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which
appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)
and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The
sequence of treatments among total 24 experiments was defined by randomization Runs at each
experimental condition were repeated three times As responses the following characteristics
were taken
1) The average increase in diameter Y1
2) The mean average coating thickness Y2
3) Dispersion of the diameter increase through the sample length Y3
4) Dispersion of the coating thickness increase through the sample length Y4
5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the
worst) to 1 (the best) scale with 01 step at equal intervals Y5
The first two responses (Y1 Y2) represented general process efficiency (since anodic charge
was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The
coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand
which provided reproducible measurements in respect to the sample axis with an error 4microm
Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5
along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy
of plusmn4microm The schemes of measurements are depicted on Fig 3
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
interelectrode distance in PEO under galvanostatic conditions was reported as well in [8][9]
The extension of this technique is so called ldquoscanning micro arc oxidationrdquo wherein a small-area
counter electrode is moved through the surface of larger substrate allowing uniform coatings to
be produced on the large and complex substrates [1011] Numerical calculation of the primary
electric field distribution through the surface of complex shape substrates with deep internal dead
holes under PEO conditions has been performed in [12]
The second approach utilises specific coating properties to influence the secondary
distribution of current density at the sample surface providing a self-aligning coating growth
behaviour Those approaches are based on the local difference in transient current-voltage curves
(CVC) of the particular coating regions It is known that typical anodic (characterising substrate
oxidation) CVC for PEO of aluminium possess severe non-linear character that may include both
the sharp increase in current once threshold voltage is achieved and the regions with negative
differential resistance [313ndash16] The CVC at any given local point within PEO coating depends
on the set of available electrochemical reactions under given particular conditions Those
reactions are defined by properties of the coating (composition microstructure) as well as by the
electrolyte composition It was suggested that transition to the soft sparking PEO which occurs
at certain excess of cathodic current density over anodic one is accompanied by associated
processes under anodic and cathodic polarisations which in turns significantly change current
response on applied voltage (CVC) Only a few papers concerning coating uniformity in view of
electrolyte composition or polarisation conditions can be found Yerokhin et al [17] noted that
coating uniformity can be improved with increase in alkali and decrease in silicate
concentrations in electrolyte solution Terleeva et al [18] has found that application of combined
current mode including alternating current and additional cathodic current pulse trains (ACC)
allows the coating uniformity on the internal surface of a conical shape substrate to be improved
A few works were devoted to the influence of electrolyte ageing on the appearance of non-
uniform features in the coating regions far away from the substrate edges [1920]
Disadvantages of the primary field control lay in the necessity of manufacturing a counter
electrode which shape may be even more complex than the targeted substrate and any changes
in substrate shape would require the counter electrode to be redesigned and remanufactured At
the same time a control over the secondary field distribution by means of adjusting polarisation
conditions and tailoring electrolyte composition to the specific alloy provides considerable
advantages especially in situations where substrates can vary to some degree Such variation can
be caused for example by a bespoke shape of medical implants or in other applications (eg
marine or aerospace) by the fact that batches could be formed by parts of similar but not exactly
the same geometry due to the small-scale nature of manufacturing (eg by 3D printing)
In this paper experimental investigation is performed in regards of the PEO coating
uniformity and secondary current density distribution under intentionally non-uniform
distribution of the primary electric field and how this distribution is affected by timings and
current densities of the pulse trains in combined current modes Some general considerations
regarding the mechanisms underlying PEO treatments and efficiency of the coating formation
are also provided
2 Experimental
All substrates were made from A2024T3 aluminium alloy (AlCu4Mg) Particular sample
geometry is described in following subsections An aqueous electrolyte solution comprising 15
gdm3 of technical water glass (specific gravity 141 gcm3 SiO2Na2O = 302) and 2 g dm3 of
potassium hydroxide was contained in a cubic stainless steel tank (300 dm3) served as a counter
electrode During the PEO process electrolyte solution was pumped throughout the internal part
of the screen along the coated surface in order to prevent electrolyte overheating above 40oC
The power supply provided a combined current waveform composed by pulse trains of
symmetrical alternating current (AC) characterised by equal positive and negative average
currents J+ = J- = JAC with addition of either cathodic (C) or anodic (A) half-wave current pulses
with frequency 50 Hz (period = 20 ms) for all pulse trains (see Fig1) An obstruction to the
primary electric field was formed by a special insulating screen made from PTFE which
surrounded the sample as shown in Fig2 In order to exclude bottleneck effect but provide
suitable obstruction the cross-sectional area of the open end of the screen was equal to the total
surface area of the sample exposed to the treatment
Fig 1 Schematic representation of the complex alternating current modes with additional
cathodic (AC-C) and anodic (AC-A) pulse trains
21 Factorial experiments Application of the combined current mode (AC-C or AC-A) is
complicated by a number of characteristic parameters that hindering optimisation or general
relations to be found Moreover some parameters are interconnected for instance cathodic
charge may be increased either by increasing in τC or Jc Therefore the factorial experimental
design may help us find out the most valuable effect of individual parameters as well as effects
of their combinations
The samples were fabricated in the form of tubes (Oslash14x12x30mm) which cylindrical sides
subjected to PEO treatments Variable parameters in experiments were as follows current
densities in AC and C pulse trains (JAC and JC) and their durations (τAC and τC respectively)
However some particular effects in factorial experiments were represented by combination of
two variable parameters since we already know the importance of those combinations in PEO
Detailed explanation of factorial experiments and real experimental conditions are given below
and listed in Table 1
Fig2 Drawings of the insulating screen and sample assembly used for a) factorial experiment b) for studying the current density distribution It is sectioned for demonstration the internal construction
As the central point (designated as 0) of the factorial design the following set of
experimental parameters was chosen JAC0 = 90mAcm2 Jc0 = 18 JAC = 1125 mAcm2 τAC =
1000 ms τC = 280 ms t = 251 hours because this was one of the main current modes for a
semi-industrial application that provided hard and well adhered coatings on the outer surfaces of
many industrial components made of aluminium alloys with low (lt12) content of silicone
However such current mode could not often provide satisfactory quality of the coatings for
substrates with complex geometry especially on the inner and concave surfaces The main
effects (Xn) and their levels (plusmn1) in factorial design were chosen as follows (Xn = Xn0 plusmn ΔXn)
1) current density within AC pulse train X1 JAC = JAC0 (1 plusmn 13)
2) ratio of current densities within C and AC pulse trains X2 JCJAC = 18(1 plusmn 13)
3) ratio between durations of AC and C pulse trains X3 τACτC = 357 plusmn 114
a) b)
PTFE
Al
1234
Al ringsPTFE
PTFEcap
PTFEcap
wire
Spacers
inner end
outer end
4) duration of C pulse train X4 τC = 280 plusmn 140 ms
Total process duration for every experimental condition were chosen to achieve equivalent
total anodic charge in the PEO processes This allows us to estimate the process efficiency by
comparison of the average coating thicknesses
The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication
could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which
appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)
and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The
sequence of treatments among total 24 experiments was defined by randomization Runs at each
experimental condition were repeated three times As responses the following characteristics
were taken
1) The average increase in diameter Y1
2) The mean average coating thickness Y2
3) Dispersion of the diameter increase through the sample length Y3
4) Dispersion of the coating thickness increase through the sample length Y4
5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the
worst) to 1 (the best) scale with 01 step at equal intervals Y5
The first two responses (Y1 Y2) represented general process efficiency (since anodic charge
was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The
coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand
which provided reproducible measurements in respect to the sample axis with an error 4microm
Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5
along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy
of plusmn4microm The schemes of measurements are depicted on Fig 3
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
In this paper experimental investigation is performed in regards of the PEO coating
uniformity and secondary current density distribution under intentionally non-uniform
distribution of the primary electric field and how this distribution is affected by timings and
current densities of the pulse trains in combined current modes Some general considerations
regarding the mechanisms underlying PEO treatments and efficiency of the coating formation
are also provided
2 Experimental
All substrates were made from A2024T3 aluminium alloy (AlCu4Mg) Particular sample
geometry is described in following subsections An aqueous electrolyte solution comprising 15
gdm3 of technical water glass (specific gravity 141 gcm3 SiO2Na2O = 302) and 2 g dm3 of
potassium hydroxide was contained in a cubic stainless steel tank (300 dm3) served as a counter
electrode During the PEO process electrolyte solution was pumped throughout the internal part
of the screen along the coated surface in order to prevent electrolyte overheating above 40oC
The power supply provided a combined current waveform composed by pulse trains of
symmetrical alternating current (AC) characterised by equal positive and negative average
currents J+ = J- = JAC with addition of either cathodic (C) or anodic (A) half-wave current pulses
with frequency 50 Hz (period = 20 ms) for all pulse trains (see Fig1) An obstruction to the
primary electric field was formed by a special insulating screen made from PTFE which
surrounded the sample as shown in Fig2 In order to exclude bottleneck effect but provide
suitable obstruction the cross-sectional area of the open end of the screen was equal to the total
surface area of the sample exposed to the treatment
Fig 1 Schematic representation of the complex alternating current modes with additional
cathodic (AC-C) and anodic (AC-A) pulse trains
21 Factorial experiments Application of the combined current mode (AC-C or AC-A) is
complicated by a number of characteristic parameters that hindering optimisation or general
relations to be found Moreover some parameters are interconnected for instance cathodic
charge may be increased either by increasing in τC or Jc Therefore the factorial experimental
design may help us find out the most valuable effect of individual parameters as well as effects
of their combinations
The samples were fabricated in the form of tubes (Oslash14x12x30mm) which cylindrical sides
subjected to PEO treatments Variable parameters in experiments were as follows current
densities in AC and C pulse trains (JAC and JC) and their durations (τAC and τC respectively)
However some particular effects in factorial experiments were represented by combination of
two variable parameters since we already know the importance of those combinations in PEO
Detailed explanation of factorial experiments and real experimental conditions are given below
and listed in Table 1
Fig2 Drawings of the insulating screen and sample assembly used for a) factorial experiment b) for studying the current density distribution It is sectioned for demonstration the internal construction
As the central point (designated as 0) of the factorial design the following set of
experimental parameters was chosen JAC0 = 90mAcm2 Jc0 = 18 JAC = 1125 mAcm2 τAC =
1000 ms τC = 280 ms t = 251 hours because this was one of the main current modes for a
semi-industrial application that provided hard and well adhered coatings on the outer surfaces of
many industrial components made of aluminium alloys with low (lt12) content of silicone
However such current mode could not often provide satisfactory quality of the coatings for
substrates with complex geometry especially on the inner and concave surfaces The main
effects (Xn) and their levels (plusmn1) in factorial design were chosen as follows (Xn = Xn0 plusmn ΔXn)
1) current density within AC pulse train X1 JAC = JAC0 (1 plusmn 13)
2) ratio of current densities within C and AC pulse trains X2 JCJAC = 18(1 plusmn 13)
3) ratio between durations of AC and C pulse trains X3 τACτC = 357 plusmn 114
a) b)
PTFE
Al
1234
Al ringsPTFE
PTFEcap
PTFEcap
wire
Spacers
inner end
outer end
4) duration of C pulse train X4 τC = 280 plusmn 140 ms
Total process duration for every experimental condition were chosen to achieve equivalent
total anodic charge in the PEO processes This allows us to estimate the process efficiency by
comparison of the average coating thicknesses
The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication
could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which
appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)
and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The
sequence of treatments among total 24 experiments was defined by randomization Runs at each
experimental condition were repeated three times As responses the following characteristics
were taken
1) The average increase in diameter Y1
2) The mean average coating thickness Y2
3) Dispersion of the diameter increase through the sample length Y3
4) Dispersion of the coating thickness increase through the sample length Y4
5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the
worst) to 1 (the best) scale with 01 step at equal intervals Y5
The first two responses (Y1 Y2) represented general process efficiency (since anodic charge
was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The
coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand
which provided reproducible measurements in respect to the sample axis with an error 4microm
Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5
along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy
of plusmn4microm The schemes of measurements are depicted on Fig 3
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
charge may be increased either by increasing in τC or Jc Therefore the factorial experimental
design may help us find out the most valuable effect of individual parameters as well as effects
of their combinations
The samples were fabricated in the form of tubes (Oslash14x12x30mm) which cylindrical sides
subjected to PEO treatments Variable parameters in experiments were as follows current
densities in AC and C pulse trains (JAC and JC) and their durations (τAC and τC respectively)
However some particular effects in factorial experiments were represented by combination of
two variable parameters since we already know the importance of those combinations in PEO
Detailed explanation of factorial experiments and real experimental conditions are given below
and listed in Table 1
Fig2 Drawings of the insulating screen and sample assembly used for a) factorial experiment b) for studying the current density distribution It is sectioned for demonstration the internal construction
As the central point (designated as 0) of the factorial design the following set of
experimental parameters was chosen JAC0 = 90mAcm2 Jc0 = 18 JAC = 1125 mAcm2 τAC =
1000 ms τC = 280 ms t = 251 hours because this was one of the main current modes for a
semi-industrial application that provided hard and well adhered coatings on the outer surfaces of
many industrial components made of aluminium alloys with low (lt12) content of silicone
However such current mode could not often provide satisfactory quality of the coatings for
substrates with complex geometry especially on the inner and concave surfaces The main
effects (Xn) and their levels (plusmn1) in factorial design were chosen as follows (Xn = Xn0 plusmn ΔXn)
1) current density within AC pulse train X1 JAC = JAC0 (1 plusmn 13)
2) ratio of current densities within C and AC pulse trains X2 JCJAC = 18(1 plusmn 13)
3) ratio between durations of AC and C pulse trains X3 τACτC = 357 plusmn 114
a) b)
PTFE
Al
1234
Al ringsPTFE
PTFEcap
PTFEcap
wire
Spacers
inner end
outer end
4) duration of C pulse train X4 τC = 280 plusmn 140 ms
Total process duration for every experimental condition were chosen to achieve equivalent
total anodic charge in the PEO processes This allows us to estimate the process efficiency by
comparison of the average coating thicknesses
The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication
could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which
appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)
and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The
sequence of treatments among total 24 experiments was defined by randomization Runs at each
experimental condition were repeated three times As responses the following characteristics
were taken
1) The average increase in diameter Y1
2) The mean average coating thickness Y2
3) Dispersion of the diameter increase through the sample length Y3
4) Dispersion of the coating thickness increase through the sample length Y4
5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the
worst) to 1 (the best) scale with 01 step at equal intervals Y5
The first two responses (Y1 Y2) represented general process efficiency (since anodic charge
was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The
coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand
which provided reproducible measurements in respect to the sample axis with an error 4microm
Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5
along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy
of plusmn4microm The schemes of measurements are depicted on Fig 3
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
4) duration of C pulse train X4 τC = 280 plusmn 140 ms
Total process duration for every experimental condition were chosen to achieve equivalent
total anodic charge in the PEO processes This allows us to estimate the process efficiency by
comparison of the average coating thicknesses
The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication
could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which
appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)
and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The
sequence of treatments among total 24 experiments was defined by randomization Runs at each
experimental condition were repeated three times As responses the following characteristics
were taken
1) The average increase in diameter Y1
2) The mean average coating thickness Y2
3) Dispersion of the diameter increase through the sample length Y3
4) Dispersion of the coating thickness increase through the sample length Y4
5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the
worst) to 1 (the best) scale with 01 step at equal intervals Y5
The first two responses (Y1 Y2) represented general process efficiency (since anodic charge
was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The
coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand
which provided reproducible measurements in respect to the sample axis with an error 4microm
Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5
along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy
of plusmn4microm The schemes of measurements are depicted on Fig 3
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of
the sample
Usually current mode is characterised by a value ofR=J C J A where JC and J A are the
average negative and positive current densities In this experimental design because of small
variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)
100 may be more convenient
Taking into account that the AC pulse train contains both positive and negative polarisation
an estimation of ΔR values for different experimental conditions was carried out in accordance
with expression
∆ R ( )=( J C
J A
minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC
J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)
Table 1 Factorial design parameters and corresponding experimental conditions for fractional
replication 2-1
X0JAC JCJAC τACτC τC JAC
mAcm2JC
mAcm2τCms
τACms t h ΔR X1 X2 X3 X4
1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
22 Finite element method (FEM) FEM calculations were performed using ldquoComsol
Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample
and screening holder Evaluation of the current density distribution along the sample length has
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a
layer with uniform thickness such situation is typical for PEO right after the voltage had reached
breakdown level The thick coating was modelled as a layer with non-uniform thickness which
is three times thicker at the outer end compared to the inner The difference between anodic and
cathodic polarisations was modelled by different conductivity the average values of which were
taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous
medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter
electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04
m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding
to the current density of about 100 mAcm2 which is similar to those maintained in the
experiments The results of calculations are presented as 2D map of current line in electrolyser
as well as normal component of the local current density in respect to the metal-coating
interface
23 Effects of R on coating structure In this case we were interested only in the effect of
ΔR-value on the coating microstructure with no regards to the non-uniform current density
distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied
between -462 and 135 on the coating microstructure was studied using combined AC-A and
AC-C current modes with constant timings and variable current densities within AC A and C
pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000
with EDS facility
Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz
JAC
mAcm2
JC mAc
m2
JA mAc
m2 τAC ms τC ms τA ms ΔR 123456
117117117927461
37120000
0005898122
280280280280280280
120120120000
000
120120120
1354500
-213-364-462
24 Redistribution of current densities Substrates were fabricated in the form of rings
(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and
individual electrical connections (See Fig 2b) This provided the cell layout similar to that
applied in the factorial experiment (Fig2a) but allows differentiating substrate currents
depending on the distance from the open end All connections were made in accordance with the
wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding
to run 5 in the factorial experiment which was found to provide the best results
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig4 Wiring diagram for experiments with sectioned samples
3 Results
31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High
quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are
typically of uniform light-grey colour whereas appearance of brownish regions indicates
deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or
insufficient coating thickness The brownish colour might possibly be attributed to the formation
of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of
substrate containing about 4 of Cu
From Fig5 it is clear that spatial distribution of defects associated with the brownish regions
is strongly influenced by polarisation conditions The samples can be divided to three groups
with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect
densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample
Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table
1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one
Moreover comparison of the pairs of samples produced at the same R value but with
different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
coating quality and distribution of defects depend on the parameters of the combined current
mode rather than solely on the value of R
Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at
the outer end of the sample) and the increment in the sample diameter (averaged within three
repetitions) along the sample axis
Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively
Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are
presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4
Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)
Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the
sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual
estimation Error level for Y1-Y4 was 4 μm
ΔD μm h μm σ(ΔD) μm σ(h) μm
Visual estimation
Y1 Y2 Y3 Y4 Y5
0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages
(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence
interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|
asymp ΔXimin)
Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5
X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102
X2 (JCJAC) 3 -13 -4 -2 0102
X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061
X1X2 1 -24 -5 -4 -0102
X1X3 1 -18 5 4 -0020
X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100
From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and
increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the
AC current density (X1) decreases The inverse dependence of the coating growth rate on the
current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current
density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values
of duration of the cathodic pulse train improve the process efficiency as well However the
effect of current density during C-pulse train is ambiguous namely its increase causes a
decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt
ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and
the coating becomes enriched with electrolyte components (eg silica)
The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or
Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)
from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and
increase in JCJAC however the significance in the latter effect is lower as its value is close to
the corresponding error level (marked as in Table 4) Other individual effects are
insignificant (marked as in Table 4) The variation in coating thickness is not influenced by
any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it
is clear that different conditions produce different coating thicknesses This however can be
accounted for by interactions of individual factors
It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2
X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance
of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness
coefficients bi2 shows considerable values for every effect and their interactions Therefore there
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3
The other important feature of interacting effects is that the above independence of σ(h) or Y4 on
any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects
It is obvious that both the final local coating thickness and changes in diameter indicate local
process efficiency at given local polarisation conditions averaged within the treatment duration
Therefore the variation in the process parameters in complex combined polarisation conditions
allows local current densities to be redistributed on the surfaces with concave geometry
32 Finite element modelling (FEM) of the current density distribution
Before experiments illustrated above it was expected that coating non-uniformity in
particular distribution of the defects would have similar trend for every set of conditions
However the difference in the defects distribution was qualitatively in some cases they were
concentrated at the outer end in other cases they were concentrated at the inner end or they were
uniformly distributed It was clear that such behaviour could not be explained only by non-
uniform distribution of the primary electric field therefore considered phenomenon appeared to
be more complex
Following calculations were performed to clarify the influence of the coating properties on the
current density distribution It is known that metal-oxide-electrolyte system under PEO
conditions possess severe non-linear properties We will use simplified approach taking into
account only valve effect (difference in effective conductivity under positive and negative
polarisation) and thickening of the coating at the outer end which were found in above
experiments Accordingly we used two values for coating conductivity and two types of coating
geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on
Fig7 Moreover distributions of the normal component of current density for thin and thick
coatings under anodic and cathodic polarisation are shown in Fig8
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under
anodic (ac) and cathodic (bd) polarisation
Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick
coatings under anodic and cathodic polarisations
It can be seen that thicker coatings tend to provide more uniform current density distribution
however a relative difference is still quite considerable A specific feature of the current density
distribution consists in the fact that not only a decrease in current along the sample axis can be
seen but also a variation in local values of R = JcJa As a result formation of the coating
fragments located at the inner end of the sample occurs not only at a lower growth rate but also
under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is
about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of
about 5mm (characteristic point) after that the main part of the internal coating is formed under
conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known
that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO
coating (R lt 1) in structure composition and properties
33 Influence of R-factor in combined current mode
In order to clarify how the coating microstructure is influenced by the R-value in combined
current modes (AC-C and AC-A) a series of treatments has been carried out under experimental
conditions provided in Table 2 Values of ΔR were varied from -462 to 135
The question about application of optical spectroscopy instead of electron microscopy sounds
quite often For copper containing aluminium alloys this is reasonable because of lack in
contrast in SEM images which can be easily seen in optical microscopy as dark and white
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We
assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX
analysis (Table 5) revealed no considerable difference in the coating sub regions This could
happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers
however we had no successful evidence of the assumption due to low copper concentration
which also unable to provide enough contrast in BSE images Therefore optical microscopy
appears to be the best way to see internal microstructural features of the PEO coatings on Cu-
containing alloys
Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)
Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12
57896104
32733143
785579
034017
034043
037028
048085
Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that
the coating produced at ΔR = -462 has non-uniform morphology where white and dark
regions are mixed together PEO processes with ΔR = -364 and higher produced layered
coatings composed from well distinguishable white interfacial layer and dark dense inner layer
(loose outer layer was partly removed) Moreover the total coating thickness increases from 70
to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating
thickness Since the charge passed in the anodic direction was the same for all current modes
listed in Table 2 the increase in the coating thickness indicated improvement of the process
efficiency As a result non-uniform distributions of both local current densities and R-values
cause considerable variation in the coating microstructure (and therefore properties)
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024
alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-
current gauge) bottom percentages indicate fractions of the white interfacial layer
34 Redistribution of the current density
From the results of the factorial experiment discussed in Section 31 it could be seen that the
coating uniformity may vary depending on the current mode Simplified calculations of the
current density distribution (Sec 32) could not help explaining the difference in the coating
uniformity Therefore it became necessary to study in a direct experiment what happens with the
current density distribution during the PEO process under combined polarisation conditions
Experimental investigations of redistribution of current densities caused by the non-uniform
distribution of the primary electric field have been carried out using substrate composed from
rings which were located at specific distances from the open end of the insulating screen (see
Fig1b) One experiment included four individual specimens insulated from each other by PTFE
spacers providing independent electrical connections of the rings (1-4) to the individual current
shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2
and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current
signal from section 3 was left unrecorded however general considerations could still be
provided
Fig11 shows behaviour of the measured electrical parameters during AC pulse train
following the C pulse train (not shown) It can be seen that a well-known gradual increase in
anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in
current through ring 1 and by increase in current through ring 2 besides a slight increase in the
current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
indicates redistribution of the electrical current within the sample surface caused by relaxation of
the coating after the polarisation conditions were switched from C- to AC-pulse train Such
redistribution can also be considered as a wave of the maximum current density as if it
ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set
of electrical parameters
Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55
min (Fig12) captured within steady part (the last period) of the AC pulse train shows that
maximum of hysteresis between upward and downward anodic branches (one of the
characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner
ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft
sparking conditions in addition to the current density redistribution This is not obvious because
very different CVC for anodic and cathodic polarisations generally speaking may provide
waves with different characteristic length magnitude and propagation rate In particular such
local conditions may be illustrated by Fig12a where maximum of cathodic current density
corresponds to the second ring whereas maximum of the anodic current density corresponds to
the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs
becomes more similar to each other illustrating uniform formation of the PEO coating
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2
and 4) during AC pulse train following the C pulse train (not shown) in a combined current
mode at 45 min of the process
Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode
(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows
show time sweep
4 Discussion
41 Redistribution of anodic current density
Improvements in the coating uniformity observed in the above experiment are driven by
current density redistribution under certain polarisation conditions which may be explained
based on the qualitative analysis discussed below This analysis is performed on the basis of
recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should
therefore be considered in close connection with that work The main idea of that concept
consists in the existence of a dielectrically volatile region at the metal-oxide interface where the
main voltage drops It was also assumed that the effective resistance of the active zone could
change due to incorporation of protons under cathodic polarisation providing increase in the
local electric field at the metal-oxide interface hence increasing the rate of electrochemical
reactions
Coating formation under PEO conditions includes electrochemical oxidation of Al substrate
and precipitation of electrolyte components which are followed by plasma assisted reactions It
is obvious that local charge and mass transfers are the more intense the higher the current density
within a particular local region of the specimen Initial coating thickness distribution is caused by
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry
of conductive media and electrodes) influenced by the secondary current density distribution
defined by local properties of the forming layer
Let us consider local changes within the active zone which may occur during the switch
between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to
enable commencement of soft sparking the thickness of the active zone over the sample surface
is relatively uniform and the main difference in the coating thickness should be attributed to the
other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight
after anodic polarisation (without incorporated hydrogen species) is taken as initial point
(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated
hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-
uniform with higher hydrogen concentration at the outer end of the sample The reason for such
distribution is clear considering current density distribution along the sample inside the
insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local
extraction rates of hydrogen species under subsequent anodic polarisation with the higher
current density indicating the higher extraction rate As a result the coating at the outer end of
the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)
After total extraction the coating goes to the initial state (Fig13a) It should be noted here that
exact shape of the line between depleted and enriched regions depends on the mechanism of
hydrogen species extraction however further discussion is not affected by this difference
Fig 13 Local changes within the active zone under cathodic and anodic polarisation
Taking into account that hydrogen enriched regions in the active zone possess significantly
higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be
expected to be accompanied by gradual evolution in distribution of effective resistance along the
sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13
respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)
and changes in resistance (Fig14b) the local current density distributions along the sample can
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum
current density gradually shifts from the outer end towards the inner end of the sample
From the results of factorial experiments (Sec 31) it can be seen that the coating quality
depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal
parameters of polarisation conditions Fig15 provides an example of the evolution of voltage
waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C
and C to AC is accompanied with voltage relaxation during considerable period of time For this
condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after
which the system achieves a steady state
Fig 14 Schematic qualitative representation of the primary electric field effective resistance
and current densities during AC-pulse train
The life-time of hydrogen species in the coating without external polarisation (within pause
between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical
reactions that accompany switching between positive and negative polarisation are responsible
for associated processes under cathodic and anodic polarisation finally resulting in the soft
sparking PEO mode can be assumed to have characteristic times of about few tens of
milliseconds In other words in order to achieve noticeable interaction between the effects of
anodic and cathodic polarisations it is necessary to use appropriate time scale
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig14 Overview of the voltage behaviour during AC-C current mode
There are two possible options in the analysis of this situation On one hand too short
duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-
electrolyte system hence a decrease in the process efficiency would be expected On the other
hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed
switching effects providing an averaging effect on the overall coating formation process Of
course the latter case is inappropriate for processes that include considerable changes in the
coating properties eg accumulation and total extraction of the hydrogen species as discussed
above in this Section
Besides it is worth noting that scanning phenomena considered above should be clearly
distinguished from known feature of some PEO processes where discharges are not occupying
whole sample surface but form some sort of aggregated group acting only within the surface
part Usually the collective behaviour of discharges is accompanied by gradual migration within
sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the
discharges promoting predictable ignition of the following discharges rather than with
switching between positive and negative polarity
42 Influence of cathodic current on the PEO process efficiency
Finally some simplified considerations can be provided regarding the mechanisms underlying
the increase in efficiency of PEO treatments that are carried out at increased R values in general
and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that
almost all energy injected into the system via microdischarge events is irreversibly transformed
into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the
dissipated energy is difficult to recover the efficiency of the coating formation process appears
to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due
to the chemical reaction between aluminium and oxygen is strongly favourable
thermodynamically
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)
Often this leads to a temptation to assess the PEO of metals based solely on the formation
energy of the corresponding oxides [27] However such oversimplified approach is not always
credible due to more complex route of transformations In particular such estimations neglect
the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface
of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs
with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer
which restricts transfer of reacting species Hence additional energy is required to overcome this
barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk
oxide and the metal-oxide interface
Fig16 shows a simplified energy diagram for typical reactions of alumina formation under
PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is
considered Depending on electronic properties of the electrode surface two main
electrochemical reactions can take place under anodic bias If electrode provides noticeable
electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be
oxidation of aluminium metal which in turn is limited by reacting species transport either to or
from metal-oxide interface We are interested in the latter case ie in the coating formation
process From this point of view it is clear why most of electrical energy is transformed into
heat which is dissipated in the electrolyte under the conditions of PEO The system consumes
electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche
breakdown migration etc However there are no ways to transform this excess energy back to
electrical form The only way is to absorb this energy via thermalisation of excited species with
solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk
electrolyte temperature
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash
ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following
dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea
ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface
The activation energy (Ea) of the entire process is defined by the step with the highest energy
For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the
oxide-electrolyte interface After that high-field migration of reacting species may occur
yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-
field migration depends mainly on the local strength of electric field (providing hopping
mechanism) which in turn depends on the ratio between the applied potential difference and
the distance at which it is applied At the same time as previously suggested [4] the role of
cathodic current consists in particular in increasing the effective electric field at the metal-oxide
interface due to narrowing the non-conductive region in the active zone as well as in eliminating
the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of
it As a result the total activation energy appears to be reduced due to exclusion of the EDL
barrier and facilitation of migration under increased electric field at the metal-oxide interface
(ldquosoft PEOrdquo route in Fig16)
In general chemistry the phenomenon of energy reduction for an intermediate state (with the
same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection
of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides
cathodic current the chemical composition of the coating can potentially affect the activation
energy for PEO
5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex
shape subjected to treatments in combined current modes (AC-C and AC-A) has been
considered The following conclusions can be drawn
1 It was found that the application of combined current modes (AC-C) allowed the coating
uniformity to be improved even if the primary current density distribution is substantially
non-uniform The key role in this phenomenon belongs to associated processes under
cathodic and anodic polarisations The former decreases the local effective resistance of
the coating thereby affecting the secondary distribution of the current density under
subsequent anodic polarisation Local switching between low and high resistance of the
coating occurs in such way that the maximum current density is gradually shifted along
the surface during the AC pulse train thereby facilitating formation of more uniform
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
coatings Since the shift of the maximum current density takes some time temporal
parameters of polarisation become critical Experiments have shown that even at equal
average R-value the coatings formed at different durations of pulse trains possess
substantially different quality
2 From the finite element modelling it was found that the complex surface geometry might
influence not only the local current density but also the ratio between cathodic and anodic
currents (R) Therefore at any particular set of processing parameters local PEO
processes may differ depending on the location of the cite not only in the coating
formation rate but also in its structure and quality as indicated by the local R value
3 Increase in PEO process efficiency with application of excessive cathodic current
particularly in the soft sparking mode can be explained by electro-catalytic action of the
cathodic current which consists both in the elimination of the oxide-electrolyte EDL due
to the local acidification and in the injection of protons into the coating active zone
thereby reducing potential barrier at the metal-oxide interface These effects decrease the
activation energy of alumina formation process which causes lowering the anodic
threshold voltage of the PEO process and reduction of total power consumption
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
References
[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430
[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7
[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115
[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284
[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X
[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046
[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103
[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030
[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222
[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821
[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x
[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019
[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306
[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630
[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330
[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492
discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026
[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X
[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64
[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199
[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001
[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes
[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125
[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652
[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020
[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110
[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124
[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492