· Web viewThe films formed on relatively pure aluminium are amorphous and contain linear pores that extend from the film surface to a thin barrier layer next to the substrate [2-4]

Effects of oxygen evolution on the voltage and film morphology during galvanostatic

anodizing of AA 2024-T3 aluminium alloy in sulphuric acid at -2 and 24 °C.

J.M. Torrescano-Alvarez, M. Curioni, P. Skeldon*

Corrosion and Protection Group, School of Materials, The University of Manchester, Oxford

Rd., Manchester M13 9PL, U.K

* corresponding author

E-mail: [email protected]

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Abstract

The effects of oxygen evolution on the voltage-time response and film morphology during

galvanostatic anodizing of AA 2024-T3 alloy at 50 mA cm-2 in sulphuric acid have been

investigated at -2 and 24 °C. The study employed interrupted anodizing experiments and real-

time gravimetric measurements of the oxygen generated. The results showed that similar

amounts of oxygen were evolved at the two temperatures, but with significantly different film

morphologies and voltage responses. At -2 °C, a relatively large voltage increment

accompanied the formation of linear cells in a relatively compact arrangement. The increment

was mainly due to increase in the barrier layer thickness. In contrast, at 24 °C, the voltage

increase was comparatively negligible and a sponge-like film morphology was generated that

contained significant inter-cell porosity. It is proposed that the anodizing voltage and film

morphology are dependent on the transport paths for oxygen gas escaping the film, in

particular the relative proportions of gas escaping from the film via intra-cell and inter-cell

porosity.

Key words: aluminium, AA 2024 alloy, anodizing, film morphology, oxygen.

1. Introduction

Anodizing in sulphuric acid is widely used for the protection of aluminium alloys against

corrosion and wear [1]. The process produces porous alumina films with thicknesses that

range from a few microns to a few tens of microns. Acid electrolytes are most commonly

employed, such as phosphoric or sulphuric acid. The films formed on relatively pure

aluminium are amorphous and contain linear pores that extend from the film surface to a thin

barrier layer next to the substrate [2-4]. In order to obtain films of high hardness, a low

electrolyte temperature may be used to minimize chemical dissolution and the resultant

softening of the film [5, 6]. For corrosion protection, the films may be formed at a higher

temperature, with the porosity subsequently sealed by one of a range of sealing treatments [7-

16].

In the case of aluminium alloys, the film growth may be accompanied by generation of

oxygen gas that is evolved from the film [17]. The oxygen originates at intermetallic particles

[17] or at the matrix, where alloying element species are incorporated into the anodic alumina

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[18]. The generation of oxygen is particularly prevalent during anodizing copper-containing

alloys [17-19]. Furthermore, the films on such alloys frequently exhibit a sponge-like, rather

than linear, porosity [20].

Oxygen generation during anodizing has been mainly studied for barrier films formed on

solid solution Al-Cu alloys. The gas is generated in the film above the alloy matrix following

the incorporation of Cu2+ ions into the anodic film [21]. Anodic alumina is normally non-

conducting for electrons. However, the incorporation of Cu2+ ions into anodic alumina during

anodizing an Al-Cu alloy modifies the electron energy levels of the oxide enabling oxidation

of O2- ions, which are generated originally from dissociation of water molecules at the

film/electrolyte interface under the influence of the electric field, to take place locally within

oxide according to the reaction:

2O2- → O2 + 4e (1)

Nano-bubbles of high pressure oxygen gas form inside the alumina near the alloy/film

interface. Owing to the plasticity of the oxide associated with the migration of Al3+ and O2-

ions across the barrier layer, the bubbles are able to grow in size and may eventually rupture

the film allowing the gas to escape.

The oxidation of copper is preceded by its enrichment in the substrate, which occurs either

during anodizing or during pre-treatments, such as electropolishing, acid pickling or alkaline

etching [22]. The less negative Gibbs free energy per equivalent for oxide formation on

copper compared with that on aluminium [23] results in preferential oxidation of aluminium,

until copper is sufficiently enriched. Copper and aluminium atoms are then oxidized in

proportion to the bulk concentrations in the alloy, hence maintaining a relatively constant

enrichment with further treatment time. Enrichments of up to ≈ 6 x 1015 Cu atoms cm-2 in a ≈

2 nm thick layer (≈ 40 at. % Cu) occur in alloys containing ≈ 1 at. % Cu [23, 24]. The Cu2+

ions migrate through anodic alumina about three times faster than Al3+ ions and may be

ejected from the film surface to the electrolyte [25]. Investigations by transmission electron

microscopy indicated that θˈˈ and θˈ phases were present in the enriched layer [21, 26].

More limited studies of porous films have suggested that a similar copper enrichment process

occurs to that observed in barrier films [27]. However, the enriched copper appears to behave

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differently from that in a barrier film as a result of the scalloped shape of the alloy/film

interface. It has been proposed that as the film grows, the enriched copper is transported

towards the cell boundaries, and is eventually oxidized at the peaks and ridges of the

alloy/film interface [28]. Consequently, oxygen bubbles form preferentially at these

locations. Embryo secondary pores have been observed to grow from the bubbles by plastic

deformation of the alumina by the high pressure gas. The secondary pores eventually connect

to the major pores of the film to release the gas. A study of oxygen generation during

formation of porous films on AA 2024-T3 alloy in sulphuric acid at constant current density

revealed that the rate of oxygen generation was proportional to the current density but

relatively independent of the anodizing time, and hence film thickness [29]. The latter

indicates that oxygen evolution in the porous region of the film, where a relatively low

electric field is present, is negligible compared with evolution from the high-field barrier

region.

The present study has been carried out in order to investigate the effects of oxygen evolution

on the anodizing voltage and to further investigate the relationship between oxygen evolution

and film morphology during galvanostatic anodizing of AA 2024-T3 alloy at 50 mA cm-2 in

H2SO4. The films were formed either at 24 or -2 °C. The film morphologies were examined

by scanning electron microscopy (SEM). Interrupted anodizing and real-time measurements

of oxygen, using a gravimetric method described previously [29-31], were employed to assist

the understanding of the relationship between the gas generation, anodizing voltage and film

morphologies.

2. Experimental

2.1 Specimen preparation

Sheets of AA 2024-T3 alloy (0.06 Si, 0.07 Fe, 4.19 Cu, 0.42 Mn, 1.36 Mg, 0.002 Cr, 0.03 Zn,

0.01 Ti, bal. Al (wt. %)), with a thickness of 1 mm, were cut to produce specimens of size 2.0

x 3.5 cm. The specimens were etched for 60 s in a 10 wt. % NaOH (Fisher Scientific, 98.3 %)

solution at 60 °C, then desmutted for 30 s in a 30 vol. % HNO3 (Fisher Scientific, 70 vol. %)

solution at room temperature. The pre-treatment results in an enrichment of copper in the

matrix [22]. Previous analysis by Rutherford backscattering spectroscopy revealed an

enrichment of 5.3 x 1015 Cu atoms cm-2 following alkaline etching of AA 2024-T3 alloy [20].

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99.94 % aluminium (10 Mg, 20 Fe, 50 Cu, 480 Si (ppm)) was used as a reference material to

compare with the alloy. Specimens of size 1.5 x 5.0 cm were cut from 0.3 mm thick sheets,

then electropolished for 180 s at 20 V in an 80 vol. % C2H5OH (Fisher Scientific, 99.99 %

pure)/20 vol. % HClO4 (Sigma Aldrich, 60 vol. %) solution at 5 °C.

Following the pre-treatments, the aluminium and AA 2024-T3 specimens were rinsed in

deionized water and dried in a cool air stream. The specimens were then coated with 45

stopping-off lacquer (MacDermid) leaving a working area of 1 cm2 on one side.

2.2 Anodizing conditions and oxygen measurements

Specimens were anodized at a constant current density of 50 mA cm-2 in 10 vol. % H2SO4

(Fisher Scientific, 96 vol. %) at either -2 ± 1 or 24 ± 1 °C, using a Metronix 6911 constant

current supply. The electrolyte, of volume 600 cm3, was contained in a double-walled glass

cell through which 50 vol. % C2H6O2 (Fisher Scientific, 99 % pure) could be flowed from a

recirculating cooler (Julabo FL601) to maintain the desired temperature. The cathode was a

cylindrical aluminium sheet of area 200 cm2 that surrounded the cell wall. A magnetic stirrer

was used to stir the electrolyte. The anodizing voltage was recorded with in-house-developed

software. After anodizing, the specimen was immediately rinsed in de-ionized water and

dried in a stream of cool air. Each experiment was repeated twice to check the

reproducibility.

A gravimetric method was used to quantify the oxygen generation during anodizing, as

described in a previous work [29-31]. Briefly, the specimen was placed inside an inverted

cylindrical container that collected the gas. The cylinder was connected to a balance that

measured the weight change due to the buoyancy force from the gas. In addition, in the

proximity of the specimen, a small platinum wire, with the surface exposed to the anodizing

electrolyte, was also attached to the cylindrical container. Either the specimen of the platinum

wire could be connected to the power supply by an insulated copper wire. Thus, two

independent working electrodes were available under the collecting cylinder, the aluminium

specimen and the platinum electrode. After anodizing each specimen at any particular

current, the same anodic current was applied later to the platinum electrode. The current

consumed by generation of oxygen gas was calculated from the rate of change of the

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buoyancy force using Faraday’s law [29]. The oxygen current values obtained from the

platinum electrode were used as reference to assure the accuracy of each experiment. The

electrolyte was not stirred, since turbulence affected the measurement of the weight change.

2.3 Specimen examination

Film cross-sections were prepared using a Leica EMUC6 ultramicrotome, with final

sectioning by a diamond knife (Micro Star). Specimens were then examined by SEM using a

Zeiss Ultra 55 microscope, operated at 1.5 kV.

3. Results and discussion

3.1 Interrupted anodizing with oxygen measurements

Figure 1(a) shows the voltage-time curves for anodizing of the alloy for 600 s at 50 mA cm-2

in 10 vol.% H2SO4 at -2 and 24 °C. Figure 1(b) shows details of the first 15 s, when the

voltage rose rapidly to a plateau at ≈ 9 and 6 V at -2 and 24 °C, respectively. The plateau is

associated with oxygen evolution and oxidation of exposed intermetallic particles [32]. The

voltage then rose at respective rates of ≈ 8 and 7 V s-1 to a peak and then fell to a relatively

steady value due to thickening of the barrier layer on the alloy matrix, the development of

embryo pores in the film and the subsequent establishment of the major pores. During the

growth of the major pores at 24 °C, the voltage increased slightly from 22 V at 15 s to 23 V

at 600 s. In contrast, a much greater increase from 33 to 55 V occurred at -2 °C. Figure 1 also

shows the voltage-time curve obtained during an oxygen evolution measurement at -2 °C,

when the electrolyte was not stirred. The voltage rose more slowly in the absence of stirring,

although the temperature of the electrolyte, measured by a thermometer placed close to the

specimen [29] remained between -2 and -1 °C. The lower voltage in the absence of stirring is

probably related to an increase in the temperature of the barrier layer region of the film [33]

and a slower rate of dispersal of reaction products from the film surface. Voltage-time curves

obtained over longer anodizing times using unstirred electrolyte are presented later,

accompanying the results of oxygen measurements. The curves show that the voltage

eventually rises to values similar to those shown in Figure 1 for the stirred electrolyte. Thus,

the effect of an absence of stirring was to delay the voltage rise. Since the barrier layer

thickness and pore and cell diameters are proportional to the voltage, the film morphologies

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at a given anodizing time will differ between specimens anodized in the absence and presence

of stirring. However, this is not expected to affect significantly the rate generation of oxygen,

which is dependent on the rate of incorporation of copper species into the barrier layer and

the consequent electrochemical oxidation on O2- ions within the barrier region, and hence

dependent on the current density [29].

Ultramicrotomed cross-sections of films formed for 600 s at 24 and -2 °C are shown in

Figure 2. The films were respectively 12 ± 2 and 18 ± 2 μm thick (Figures 2(a) and 2(c)). A

sponge-like porosity is disclosed in the film formed at 24 °C, with a relatively large amount

of inter-cell porosity evident (Figure 2(b)). In contrast, linear cells, relatively compactly

organized, are present in the film formed at -2 °C (Figure 2(d)). Fracture of the film occurred

mainly at cell boundaries at both temperatures.

Figures 3(a) and 3(b) present measurements of oxygen currents at 24 and -2 °C using two

anodizing periods of 300 s, separated by 150 s with the specimen remaining in the electrolyte.

The interruption was made to allow time for trapped oxygen to escape from the film. After

anodizing the alloy, the current was applied to the platinum. The similarity of the oxygen

currents on platinum (48.2 and 51.3 mA at 24 and -2 °C respectively) and the applied current

(50 mA) validated the accuracy of the procedure for the alloy. During the two periods of

anodizing of the alloy, the oxygen current varied between 10 to 15 mA at both temperatures,

corresponding to 20 to 30 % of the applied current.

The voltages during anodizing of the alloy and during application of the current to the

platinum are shown in Figures 3(c) and 3(d). The regions of the interruptions of anodizing of

the alloy are shown in more detail in Figures 3(e) and 3(f). The voltage at the establishment

of the major pores in the first period of anodizing at -2 °C was 30.2 V. The voltage increased

with time, with the final voltage after the second period of anodizing reaching 40.5 V. Hence,

the total voltage increase during the formation of the major pores at -2 °C was ≈ 10.3 V. In

contrast, the voltage increase at 24 °C was only 0.5 V. A voltage transient occurred at the

start of re-anodizing in which the voltage reached a peak and gradually decayed to a

minimum. The transients were completed in 10 and 20 s at -2 and 24 °C, respectively.

The voltages at the start of major pore growth, Vmp, the end of the first period of anodizing,

Vf, the transient peak at the start of the second period of anodizing, Vpk, and the subsequent

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minimum are listed in Table 1. The peak heights (Vpk-Vmin) were 1.0 and 1.1 V at -2 and 24

°C, respectively (Table 1). The voltage difference between Vf and Vmin for both cases was 0.0

V. Table 1 also lists the voltage increment between Vmp and Vmin. This voltage is considered

to be the minimum voltage increase associated with an increasing thickness of the barrier

layer, as discussed later.

Figures 4(a) and 4(b) show the oxygen currents during anodizing periods of 600, 180 and 300

s separated by intervals of 180 and 300 s. The first period was equal to the total anodizing

time in the measurements of Figure 3. At -2 °C, the oxygen current increased from ≈ 6 to 11

mA between the first and final periods. The anodizing voltage also increased, with final

values of 36.1 and 48.5 V at the end of the respective periods (Figure 4(c)). Vmp in the first

period of anodizing at -2 °C was 28.1 V. Hence, the total voltage increases during growth of

the major pores were 8.0 and 20.4 V, respectively. In contrast, at 24 °C, the oxygen current (≈

9.5 mA) was negligibly affected by the interruptions and the anodizing voltage during

formation of the major pores increased by only 1.0 V. Voltage transients at the start of

periods two and three occurred at both temperatures (Figures 4(c-f)). The transients lasted for

9 s at -2 °C and 20 s at 24 °C. The peak heights at -2 °C were 2.1 and 4.9 V (periods 2 and 3,

respectively). The respective voltage drops, Vf – Vmin, were 2.4 and 5.4 V. The peak heights

at 24 °C were 1.1 and 1.0 V in periods 2 and 3, respectively. The respective voltage drops

were 0.5 and 0.2 V

3.2 Interrupted anodizing and specimen drying

The effect of trapped oxygen on the voltage was further investigated by anodizing the alloy at

-2 °C in two periods of 600 and 300 s separated by an interval of either 10 or 600 s. The

variation in the interruption time enables an evaluation of the effects associated to the oxygen

gas trapped in the film on the electrical response during the second anodizing step. For short

interruption times, the oxygen is still largely present in the film when anodizing is restarted,

for long interruption times, the vast majority of the oxygen originally trapped in the film has

escaped by the time anodizing is restarted. The specimens were left in the electrolyte during

the periods of the interruptions. No oxygen measurements were made during anodizing these

specimens. The voltage-time curves are shown in Figures 5(a) and 5(b) for interruptions of 10

and 600 s, respectively. The details of the voltages in the vicinity of the interruptions are

shown in Figure 5(c) and 5(d). The final voltages, Vf, in the first stages of anodizing were ≈

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55.1 and 57.4 V (Figures 5(c) and 5(d), respectively, Table 1). Voltage transients occurred at

the start of the second period of anodizing following the two periods of interruption. The

transients lasted for ≈ 6 s. The heights of the transient were ≈ 3.0 and 8.5 V after interruptions

of 10 and 600 s, respectively (Figures 5(c) and 5(d), respectively). The transient peak

voltages, Vpk, were ≈ 51.6 and 49.9 V following interruptions of 10 and 600 s respectively.

These were lower than Vf by about 4.0 and 7.5 V, respectively. The voltages subsequently

reached minimum values of ≈ 48.6 and 41.4 V, for interruptions of 10 and 600 s,

respectively, which were about 6.5 and 16.0 V lower than Vf. Following the minima, the

voltages gradually increased. The voltage increments between the start of major pore growth

and the transient minima, Vmin-Vmp, were 17.6 and 10.4 V, after interruptions of 10 and 600 s,

respectively

Similar results were obtained if the specimen was rinsed with deionized water and dried in

cool air after the first period and then re-anodized (Figure 6, Table 1). The similarity suggests

that by leaving the specimen in the electrolyte for 600 s, most of the oxygen had escaped

from the film. The lower voltage drop after an interruption of 10 s was possibly due to some

gas remaining in the film. In contrast, no voltage transient (Figures 6(a) and 6(b)) was

observed for re-anodizing of aluminium and only small voltage drops of ≤ 0.1 and 0.3 V were

recorded between the two anodizing periods after interruptions of 10 and 600 s, respectively.

If the voltage drops were due to chemical dissolution that thinned the barrier layer, a

thickness reduction of ≈ 0.2 nm is expected, assuming a formation ratio for the barrier layer

of 0.8 nm V-1 [34]. Hence, a chemical dissolution rate of the barrier layer of ≤ 0.02 nm min-1,

and thus loss of ≤ 0.2 nm in 600 s. A similarly low chemical dissolution is expected for

barrier layer of the alloy and hence, chemical dissolution is not regarded as a significant

factor in the voltage drops measured for the alloy.

3.3 Effects of diffusion and film thickness on voltage drop

A further factor to consider that could affect the voltage drop is the possibility of changes in

the electrolyte composition within pores that affect the electrolyte conductivity. These might

occur due to changes in pH from generation of H+ ions at the pore bases as a consequence of

film growth, ejection of Al3+ ions from the barrier layer to the electrolyte and depletion of

SO42- ions at the pore bases due to their incorporation into the film, with the extent of the

changes depending on the film thickness due to the lengthening path for ionic diffusion in the

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pores. However, the films on aluminium were ≈ 1.2 times thicker than those on the alloy after

the first period of anodizing (Figure 7) due to the absence of significant oxygen evolution

during anodizing the aluminium. The greater thickness should have enhanced any effects of

diffusion for the aluminium relative to the alloy. The absence of a significant voltage drop for

aluminium therefore indicates that any changes in the pore electrolyte make negligible

contributions to the voltage drop for the alloy. Furthermore, gas evolution would be expected

to constantly refresh the electrolyte within films on the alloy. However, the time required for

oxygen bubbles to exit the film should depend upon the film thickness, due to the longer path

between the sites of oxygen generation close to the substrate and the film surface. This could

explain why the voltage drop after an interruption was comparatively small following a first

period of anodizing of 300 s compared with a first period of 600 s (Table 1).

3.4 Relationships between oxygen generation, anodizing voltage and film morphology

The present transients are different to those commonly encountered at the start of anodizing

or on stepped changes in the current or potential [35-39]. The latter transients have been

attributed to factors such as charging of the oxide capacitance, dielectric relaxation of the

oxide and formation of mobile ions. They typically end after passing a charge of ≈ 200 μC

cm-2 [37]. This amount of charge would have been passed in 4 ms under the present

anodizing conditions, too short a time for the transient to be resolved with the apparatus used

in this work. Notably, the time is negligible compared with the duration of the present

transients, which were in the range 6 to 20 s. Significantly, transients were not observed with

aluminium, for which oxygen evolution is negligible [40, 41], suggesting an association of

the transients on re-anodizing of the alloy with influences of oxygen in the film.

It is proposed that the blocking of pores by oxygen bubbles during anodizing and the

consequent non-uniform current distribution caused cells to grow at different rates leading to

an irregular alloy/film interface, with a range of barrier layer thicknesses. The growth rates of

individual cells will depend on the ohmic resistance of the pore electrolyte caused by the

presence of bubbles that impede ionic transport. A bubble may even block a pore until the gas

is released at the film surface, causing a temporary cessation of growth of the cell.

Fluctuations in the numbers of bubbles in the individual pores caused by random variations in

the times required for bubble nucleation, growth and escape from the film lead to

continuously fluctuating ionic current densities in the barrier layer at the pore bases. When

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anodizing ceases following a current interruption, escape of oxygen bubbles causes the

current to re-distribute, preferentially to regions of thinner barrier layer. The transformation

of the cell morphologies as the cells grow in response to the new current distribution is

proposed to be the cause of the voltage transients. The voltage drops in Table 1 therefore

indicate the effect of oxygen bubbles on the current re-distribution and the ohmic resistance

within pores.

The voltage drops due to the escape of oxygen following an interruption are relatively small

compared with the overall increases in the voltage during anodizing at -2 °C. The larger

voltage increments are attributable to the gradual increase in the thickness of the barrier layer

at ≈ 0.8 nm V-1 [34] as anodizing proceeds. Figure 8 shows scanning electron micrographs of

the barrier layer region in a film formed for 600 s at -2 °C to a final voltage of 55 V, and a

film anodized for a further 600 s to a final voltage of 74 V. The barrier layer thicknesses are

about 44 and 55 nm, respectively. The increase in thickness is proposed to be due to pore

blockage by oxygen that impedes oxidation of the alloy. The-re-distribution of current leads

to a local increase in the current density and consequent increases in the voltage and barrier

layer thickness. Thickening may be also promoted by locally increased Joule heating.

The increase in the barrier layer thickness is accompanied by an increase in the cell size,

which is accommodated by the terminated growth of a percentage of the cells. The results of

Table 1 suggest an increase of the voltage due to the increasing cell size of around 10 V

following anodizing for 600 s. The anodizing voltage, V, at the start of the growth of the

major pores is about 30 V, suggesting a barrier layer thickness of ≈ 24 nm and cell width of

about 75 nm. The pore population density decreases in proportion to V-2, hence the cell

population density should decrease by a factor of about 1.8 following the increase in the cell

width, from roughly 2 x 1010 to 1 x 1010 cm-2. After anodizing for 600 s the film is ≈ 19 μm

thick. During each increment of the film by the thickness of the barrier layer ≈ 0.1 % of the

pores must be terminated, assuming a constant rate of termination, in order to terminate a

total of 1 x 1010 pores cm-2. However, the voltage increases more slowly in the earlier stages

of major pore growth, suggesting that the rate of termination is lower than in later stages. An

increase in the termination rate with time might arise due to greater pore blockage and hence

current density fluctuations as more oxygen bubbles are present in the film at a given time

due to the increasing length and reducing number of the remaining pores.

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The re-structuring of cell geometries following escape of oxygen from a film is probably

affected by the pore formation mechanism. Considering that the oxygen measurements

indicate that ≈ 25 % of the applied current is consumed by oxygen evolution, about 75 % is

used in alloy oxidation. Hence, from the charge passed in alloy oxidation for the specimen

shown in Figure 2(c), the ratio of the film thickness to the thickness of oxidized alloy for

anodizing at -2 °C is calculated to be about 2.4. In comparison, the ratio for anodizing at 24

°C is significantly lower at 1.5 (Figure 2(a)). The high ratio is attributable to the higher

transport number of O2- ions in the barrier layer and reduced ejection of Al3+ ions from the

barrier layer at the pore bases. This explanation is consistent with the findings of previous

investigations that showed a significant increase in the anion transport number with

electrolyte temperature in anodic films formed in sulphuric acid [41]. The stress created by

the greater volume of oxide formed at the aluminium/film interface at -2 °C will also provide

a greater driving force for pore generation by oxide flow within the barrier layer towards the

cell walls [42]. The difference in stress may contribute to the differing pore morphologies.

The flow within a cell would be constrained by adjacent cells if different current densities,

rates of oxide flow and cell geometries exist between them. Thus, the peak voltage in a

transient may be associated with initiation of oxide flow in cells across the alloy/film

interface.

Interrupted anodizing at 24 °C led to negligible voltage drops (Figure 4(d)), although oxygen

evolution was broadly similar to that at -2 °C. This suggests that oxygen could escape the

sponge-like film without causing significant current re-distribution due to blocking of pores

by oxygen bubbles. Previous work has identified bubble nucleation at cell boundaries in films

formed on aluminium in phosphoric acid [28]. The gas escaped through secondary pores

produced by plastic deformation of the alumina, which connected to the major pores. The

cells of the present films are much smaller than those formed in phosphoric acid, and the cell

boundary length per unit area of the aluminium/film interface will be greater. The larger

influence of the oxygen generation on the voltage-time response in the present films during

anodizing at -2 °C compared with 24 °C, is possibly related to the difference in paths

available for escape of oxygen from the films. At -2 °C, oxygen may escape mainly via the

major pores, in the process causing fluctuating pore blockage and current re-distribution. In

contrast, at 24 °C, oxygen may be evolved through the extensive inter-cell porosity,

additional to the porosity within cells.

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4. Conclusions

1. Oxygen evolution during anodizing of AA 2024-T3 alloy at 50 mA cm-2 in sulphuric acid

at -2 °C results in a dynamic equilibrium with the current density in individual pores

fluctuating about the macroscopic value due to oxygen bubbles impeding ionic transport in

the pore electrolyte. With time and film thickening, escape of oxygen from the film becomes

more difficult and, on average, more oxygen bubbles are present in the film at any given

time. Thus, localized increases in the current density above the macroscopic average produce

increases in the barrier layer thickness and the anodizing voltage as anodizing proceeds.

2. A relatively compact, linear cell arrangement is produced in the films formed at -2 °C. In

contrast, at 24 °C, a sponge-like morphology results that incorporates an extensive network of

inter-cell porosity. It is suggested that the inter-cell network facilitates the escape of oxygen

from the film leading to relatively minor effects on the voltage and barrier layer thickness at

24 °C. In contrast, oxygen bubbles cause greater major pore blockage in the more compact

cell structure formed at -2 °C leading to increases in the anodizing voltage, cell size and

barrier layer thickness.

3. Despite the differences in voltage-time behaviours and pore morphologies at the two

temperatures, the rates of oxygen evolution are relatively similar at the two temperatures.

7. Acknowledgements

The authors thank the Engineering and Physical Sciences Research Council (LATEST 2

Programme Grant: EPSRC/H020047/1) for support of this work and the European

Community for financial assistance within the Integrating Activity “Support of Public and

Industrial Research Using Ion Beam Technology (SPIRIT)”, under EC contract no. 227012.

J.M. Torrescano-Alvarez acknowledges receipt of a scholarship from Consejo Nacional de

Ciencia y Tecnología (CONACYT) and a fellowship from the Roberto Rocca Education

Program to undertake her Ph.D. studies.

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15

16

Figure captions

Figure 1. Voltage-time response of AA 2024-T3 alloy anodized at 50 mA cm-2 in stirred 10

vol. % H2SO4 at -2 and 24 °C for 600 s.

Figure 2. Scanning electron micrographs (secondary electrons) of anodic films on AA 2024-

T3 alloy anodized at 50 mA cm-2 in stirred 10 vol. % H2SO4 at (a,b) 24 and (c,d) -2 °C for

600 s.

Figure 3. (a,b) Oxygen currents during anodizing of AA 2024-T3 alloy at 50 mA cm-2 in 10

vol. % H2SO4 at (a) -2 and (b) 24 °C. (c,d) Respective voltage-time responses. The specimen

working area was 1 cm2. Anodizing was carried out in two periods of 300 s, separated by an

interval of 150 s. (e,f) Details of the voltage on (c,d) at the end of the first period and start of

the second period. The electrolyte was not stirred during anodizing.



was anodized in three periods of 600, 180 and 300 s, separated by intervals of 180 and 300 s,

respectively. (e,f) Details of the voltage in (c,d) at the end of the first period and start

subsequent periods. The electrolyte was not stirred during anodizing.

Figure 5. (a,b) Voltage-time responses of AA 2024-T3 alloy and aluminium anodized at 50

mA cm-2 in stirred 10 vol. % H2SO4 at -2 °C in two periods of 600 and 300 s, separated by

intervals of (a) 10 and (b) 600 s. (c,d) Details of the voltage at the end first period and start of

the second period.

Figure 6. (a) Voltage-time response of AA 2024-T3 alloy anodized at 50 mA cm-2 in stirred

10 vol. % H2SO4 at -2 °C for 600 s, rinsed with deionized water and dried in cool air (black

line), and then re-anodized for 300 s (red line). (b) Details of the voltage at the end first

period and start of the second period.

Figure 7. Scanning electron micrographs (backscatter electrons) of anodic films on (a)

aluminium and (b) AA 2024-T3 alloy specimens anodized at 50 mA cm-2 in stirred 10 vol. %

H2SO4 at -2 °C for 600 s.

17

Figure 8. Scanning electron micrographs (secondary electrons) of the barrier layer on AA

2024-T3 alloy anodized at 50 mA cm-2 in stirred 10 vol. % H2SO4 at -2 °C for (a) 600 s, then

rinsed with de-ionized water and dried and (b) re-anodized for another 600 s.

18

Table 1. Voltages before and after interruption of anodizing of AA 2024-T3 alloy for

different times at 50 mA cm-2 in 10 vol. % H2SO4 at -2 or 24 °C.

Figure numbe

r

Anodizing time

Interruption time Vmp Vpk Vmin Vf Vpk-Vmin Vf-Vmin Vmin-Vmj

(s) (V)10 vol. % H2SO4 at -2 °C

3 300 150 30.2 33.5 32.5 32.5 1.0 0.0 2.44 600 180 30.2 35.8 33.7 36.1 2.1 2.4 5.34 780 300 30.2 40.1 35.2 40.6 4.9 5.4 6.85 600 10 31.0 51.6 48.6 55.1 3.0 6.5 17.95 600 600 31.0 49.9 41.4 57.4 8.5 16.0 10.66 600 NA 31.0 50.1 44.3 54.1 5.8 9.8 14.0

10 vol. % H2SO4 at 24 °C3 300 150 23.5 24.7 23.6 23.6 1.1 0.0 0.14 600 180 23.1 24.9 23.8 24.3 1.1 0.5 0.74 780 300 23.1 24.9 23.9 24.1 1.0 0.2 0.8

19

0 200 400 6000

20

40

60

(b)

Vol

tage

/V

Time/s

-2 °C -2 °C (O2 measurement - no stirring) 24 °C

(a)

0 5 10 150

30

60

Figure 1. Voltage-time response of AA 2024-T3 alloy anodized at 50 mA cm-2 in stirred 10

vol. % H2SO4 at -2 and 24 °C for 600 s.

20

4 µm

(a) (b)

200 nm

(c)

4 µm 200 nm

(d)

SE SE

SE SE

Figure 2. Scanning electron micrographs (secondary electrons) of anodic films on AA 2024-

T3 alloy anodized at 50 mA cm-2 in stirred 10 vol. % H2SO4 at (a,b) 24 and (c,d) -2 °C for

600 s.

21

0 1000 2000 3000-20

0

20

40

60(b) 24 °C

Cur

rent

/mA

Time/s

0 1000 2000 3000-20

0

20

40

60(a) -2 °C

Cur

rent

/mA

Time/s

0 1000 2000 3000-20

0

20

40

60

Time/s

(c)

Vol

tage

/V

Time/s

(c) -2 ºC

0 1000 2000 3000-20

0

20

40

6024 ºC(d)

Vol

tage

/V

Time/s

AA 2024-T3 PtAA 2024-T3 Pt

AA 2024-T3 PtAA 2024-T3 Pt

200 300 400 50028

30

32

34

36

Vol

tage

/V

-2 ºC(c)

Time/s

(e)°

200 300 400 50020

22

24

26

2824 ºC

-1 ºC

(f)

Vol

tage

/V

Time/s

Vmp

Vf

Vpk

Vmin



working area was 1 cm2. Anodizing was carried out in two periods of 300 s, separated by an

interval of 150 s. (e,f) Details of the voltage in (c,d) at the end of the first period and start of

the second period. The H2SO4 electrolyte was not stirred during the anodizing process.

22

Vmin

0 500 1000 1500 2000

0

20

40

60(a)

Cur

rent

/mA

Time/s

-2 °C

0 500 1000 1500 2000

0

20

40

60-2 °C

Vol

tage

/V

Time/s

(c)

0 500 1000 1500 2000

0

20

40

60(b)

Cur

rent

/mA

Time/s

24 °C

0 500 1000 1500 20000

20

40

60(d)

Vol

tage

/V

Time/s

24 °C

550 600 800 900 1250 130030

35

40

45-2 °C

Vol

tage

/V

Time/s

(e)

550 600 800 900 1250 130015

20

25

30(f)

Vol

tage

/V

Time/s

24 °C



was anodized in three periods of 600, 180 and 300 s, separated by intervals of 180 and 300 s,

respectively. (e,f) Details of the voltage in (c,d) at the end of the first period and start

subsequent periods. The electrolyte was not stirred during the anodizing process.

23

0 300 600 9000

30

60

90V

olta

ge/V

Time/s

AA 2024-T3 Al

(a)

0 500 1000 15000

30

60

90

Vol

tage

/V

Time/s

AA 2024-T3 Al

(b)

550 600 6500

30

60

90

Vol

tage

/V

Time/s

AA 2024-T3 Al

(c)

550 575 600 1200 1225 12500

30

60

90

Vol

tage

/V

Time/s

AA 2024-T3 Al

(d)

Figure 5. (a,b) Voltage-time responses of AA 2024-T3 alloy and aluminium anodized at 50

mA cm-2 in stirred10 vol. % H2SO4 at -2 °C in two periods of 600 and 300 s, separated by

intervals of (a) 10 and (b) 600 s. (c,d) Details of the voltage at the end first period and start of

the second period.

24

0 300 600 900

0

30

60

90

Vol

tage

/V

Time/s

Anodizing Re-anodizing

(a)

550 600 650

0

30

60

90

Vol

tage

/V

Time/s

Anodizing Re-anodizing

(b)

Figure 6. (a) Voltage-time response of AA 2024-T3 alloy anodized at 50 mA cm-2 in stirred

10 vol. % H2SO4 at -2 °C for 600 s, rinsed with deionized water and dried in cool air (black

line), and then re-anodized for 300 s (red line). (b) Details of the voltage at the end first

period and start of the second period.

25

4 µm

(a)

4 µm

(b)

Anodic film

Aluminium

Anodic film

AA 2024-T3

Figure 7. Scanning electron micrographs (backscatter electrons) of anodic films on (a)

aluminium and (b) AA 2024-T3 alloy specimens anodized at 50 mA cm-2 in stirred 10 vol. %

H2SO4 at -2 °C for 600 s.

26

(a)

100 nm

100 nm

(b)

Figure 8. Scanning electron micrographs (secondary electrons) of the barrier layer on AA

2024-T3 alloy anodized at 50 mA cm-2 in stirred 10 vol. % H2SO4 at -2 °C for (a) 600 s, then

rinsed with de-ionized water and dried and (b) re-anodized for another 600 s.

27

Documents

· Web viewThe films formed on relatively pure aluminium are amorphous and contain linear pores that extend from the film surface to a thin barrier layer next to the substrate [2-4]