Polarisation curve interpretation

Corrosion Science 47 (2005) 2125–2156

www.elsevier.com/locate/corsci

A guide to polarisation curve interpretation:deconstruction of experimental curves

typical of the Fe/H2O/H+/O2 corrosion system

Harvey J. Flitt, D. Paul Schweinsberg *

School of Physical and Chemical Sciences, Queensland University of Technology,

G.P.O. Box 2434, Brisbane, Queensland 4001, Australia

Received 14 May 2003; accepted 26 October 2004

Available online 8 February 2005

Abstract

Experimental DC polarisation curves are the sum of the cathodic and anodic components

and can be difficult to interpret. Schematic representations of �typical� curves (together withtheir anodic and cathodic components) are available in the literature for comparison purposes.

A better approach to curve analysis is to generate mathematically the experimental curve

which is then deconstructed into its components. Unfortunately the appropriate computer

programmes are not readily available. We have considered it useful to revisit the collected

curve concept replacing schematic representations with experimental curves. Using an up-

dated programme we have accurately analysed curves representative of the Fe/H2O/H+/O2

corrosion system.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Iron/low carbon steel corrosion; Computerised polarisation curve analysis; Curve deconstruc-

tion/deconvolution

0010-938X/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2004.10.002

* Corresponding author. Tel.: +61 73 864 2111; fax: +61 73 864 1804.

E-mail address: [email protected] (D.P. Schweinsberg).

mailto:[email protected]

2126 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156

1. Introduction

The generation of polarisation curves continues to be important in aqueous cor-

rosion research. The time-consuming potentiostatic method has been largely re-

placed by the potentiodynamic approach where the potential (E) of the corrodingmetal is automatically varied with time. The current (I) needed to maintain the metal

(working electrode (WE)) at each applied potential (Ew) is ascertained and the poten-

tial/current data is plotted to give the experimental polarisation curve. In corrosion

studies it is common practice for the curve to be displayed with the independent

variable (in this case the potential) rather than the dependent variable as ordinate.

Further, the logarithm10 of the current density (log i) is plotted in the positive x-

direction, notwithstanding the convention that anodic current is positive and catho-

dic current is negative.The magnitude of Ew can be regarded as a measure of the oxidising power of the

corrodent [1], with the log i axis reflecting the rate of each reaction in the corrosion

process. Depending on the corrosion system under study it follows that from the

shape of the experimental curve it may be possible to obtain information on the

kinetics of the corrosion reactions, protectiveness of a passive film, ability of a com-

pound to act as a corrosion inhibitor, relative corrosivity of process streams and cor-

rosion rate (icorr) of the metal.

Unfortunately, extracting any of the above from the experimental curve may bequite difficult. This is because at each applied potential the recorded current is the

sum of the anodic and cathodic components of the corrosion reaction and the exper-

imental curve (e.g., for the simple case of pure Fe in O2-free dilute H2SO4) will be the

sum of two true polarisation curves, one describing oxidation of Fe to Fe2+ and the

other reduction of H+ ion. This means that for potentials not greatly removed from

that of the freely corroding WE (corrosion potential (Ecorr)) the shape of the anodic

and of the cathodic portions of the experimental curve will differ from that exhibited

by each true curve. However, for potentials further from Ecorr the effect of the catho-dic reaction on the anodic reaction and vice-versa is progressively lessened, and the

shape of the experimental curve eventually becomes an accurate representation of

the kinetics of the anodic and cathodic corrosion reactions. Of course, if an alloy

is involved or if the corrodent contains more than one oxidant (commonly H+ ion

and dissolved O2) the net experimental curve will more complex, and correspon-

dingly harder to interpret in terms of its components.

An example where failure to correctly analyse the experimental curve can lead to

error is when the curve is employed to evaluate corrosion rate. The Tafel extrapola-tion method is well known but it is often forgotten that the metal is required to be

uniformly corroding and at the corrosion potential either the anodic or the cathodic

reaction needs to be under complete activation control. Further, for accurate estima-

tion of icorr the identified linear portion of the experimental curve should extend over

about one decade on the log i axis. Unfortunately, in practice these requirements are

not always met: the relevant cathodic reaction may be experiencing both activation

and concentration polarisation at Ecorr and extrapolation of what is perceived as a

�shortened� Tafel portion is completely erroneous. Another example pertaining to

H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2127

corrosion rate evaluation is when corrosion-monitoring probes based on the polar-

isation resistance method are used. The reaction kinetics of the corrosion process

must be established before installation as these devices again assume that at Ecorrthe anodic and cathodic corrosion reactions are under activation control. A final

example where the experimental curve can be difficult to interpret is when the metalspontaneously passivates/pits in the corrodent prior to polarisation. The anodic por-

tion of the experimental curve may now exhibit �straight line behaviour� but, becauselocalised corrosion is involved, extrapolation of this portion of the curve does not

lead to a �corrosion rate�. Also, in this case the cathodic portion of the experimentalcurve may exhibit either a confusing �cathodic loop or dip� (negative peak).In practice it is difficult, except for the simplest corrosion systems, to visualize an

experimental curve in terms of its anodic and cathodic components. Schematic rep-

resentations of experimental curves with their schematic �true� anodic and cathodiccurves have been published [1,2]. Thus Liening [1] discusses nine possible experimen-

tal curves for the reaction

MþHþ !Mþ þ 1=2H2These examples may be useful in that it may be possible to associate features of an

experimental curve with one depicted in the collection. However, the best approach

for the interpretation of a polarisation curve is one based on electrochemical theory.

Here the appropriate thermodynamic and kinetic parameters are inserted into the

relevant mathematical functions to synthesise the approximate true cathodic and

true anodic curves for the corrosion system. These curves are then combined to give

the approximate synthesised experimental curve, which is then overlaid on the experi-mental one. Values of the input parameters are now varied, and by trial and error

the shape of the synthesised experimental curve is altered until a good match is ob-

tained with the experimental one. (Note: literature and experimental values may be

used as a guide to the magnitude of the various parameters.) Finally, the matched

curve is deconstructed (deconvoluted) to show its true anodic and cathodic compo-

nents. Various computer-based programmes have been devised to effect the calcula-

tions and the results for a number of corrosion systems are described in the literature

[3–20]. We have also used this approach in SYMADEC, a programme for the syn-thesis, matching and deconvolution of curves for the M/H2O/H

+/O2 system. Earlier

versions of the programme have been successfully used to study the corrosion kine-

tics of carbon steel and low-alloy steels in different aqueous environments [21–27].

Unfortunately computer programmes for curve interpretation are not readily

available. We have therefore considered it useful to revisit the collected curve con-

cept, but instead of employing schematic representations have selected for compar-

ison purposes actual experimental curves (in this case for the corrosion of iron and

carbon steels). Each curve has been synthesised, matched and then deconstructed toreveal the nature of its components. Knowledge of the experimental conditions is

important in curve interpretation and this information is provided in detail. The role

of the Pourbaix diagram for the pure iron/pure water system at 25 �C in curve anal-ysis is also emphasised. The experimental curves were obtained either from

experiments carried out in our laboratories, or from examples published in the


literature. Printed curves were scanned and then digitised using a programme written

for this purpose. Filtering and sampling were applied to the digitised data to mini-

mise the current/voltage set and optimise graphical representation. The curves cho-

sen range in complexity, starting with the simple case indicative of one anodic and

one cathodic reaction and undergoing activation polarisation only at Ecorr to corro-sion systems involving both activation and concentration polarisation and more than

one oxidant. The effect of non-passive surface films is also covered together with the

more usual case of an active/passive transition followed by pitting.

2. Mathematical basis of SYMADEC

The most common cathodic reactions driving the aqueous dissolution of a metalare

2HþðaqÞ þ 2e� ! H2ðgÞ ðequivalent 2H2Oþ 2e� ! H2ðgÞ þ 2OH�ðaqÞÞ

and

O2ðgÞþ2H2Oþ4e�!4OH�ðaqÞ ðequivalentO2ðgÞþ4HþðaqÞþ4e�!2H2OÞ

The relationship between the rate of each of the above reactions, expressed as catho-

dic current density, ic and high values of the activation overpotential, gact,c (>approx.�0.03 V) at the metal/solution interface is

ic ¼ i0 expð�anF gact;c=RT Þ ð1Þ

where a = transfer coefficient; n = number of electrons involved in the reaction;F = Faraday�s constant; gact,c = Ew � Ereversible; R = 8.314 J K�1 mol�1; T = abs.

temp. Rearranging gives the Tafel equation:

gact;c ¼ bc logðic=i0Þ ð2Þ

where bc = Tafel slope = �2.303RT/anF.At higher reaction rates concentration polarisation is present (this is most often

seen for the oxygen reduction reaction) and the relationship between the cathodic

current density and the cathodic concentration overpotential, gconc,c, is

ic ¼ iLf1� expðnF gconc;c=RT Þg ð3Þ

where iL = limiting current density.

Rearranging

gconc;c ¼ ð2:303RT=nF Þ logf1� ðic=iLÞg ð4Þ

Charge transfer and concentration overpotentials are additive, and for a single

cathodic process Eqs. (2) and (4) can be added to give

gtotal;c ¼ �ð2:303RT =anF Þ logðic=i0Þ þ ð2:303RT =nF Þ logð1� ðic=iLÞÞ ð5Þ


It follows [18,28] that the approximate value of the total cathodic current density is

given by

itotal;c ¼ ½i0 expð�anF g=RT Þ=½1þ fi0 expð�anF g=RT Þg=iL ð6Þor

itotal;c ¼ iLic=ðiL þ icÞ ð7ÞAppropriate versions of Eq. (6) are used to model the curves for H+ and O2 reduc-tion. The current densities at each potential are then summed.

The general anodic reaction for active metal dissolution is M!Mn+ + ne�. Con-

sider the corrosion of iron. This process is pH dependent, and reference to the well

known Pourbaix diagram [29] for the iron/water system at 25 �C (dissolved ion activ-ity <10�6 M) shows the following:

1. For pH < �4.2 as the potential of the iron (Ew) is made more positive the reactionis

FeðsÞ ! Fe2þðaqÞ þ 2e� ðactive corrosionÞ ð8Þ2. For pH � 4.2 to �9.4 as Ew is made more positive active corrosion (formation ofFe2+) is followed by passivation due to precipitation of hydrous oxide,Fe2O3 Æ nH2O. (Note: the precipitate is usually represented as Fe(OH)3.)

3. For pH � 9.4 to �12.2 as Ew is made more positive iron passivates to form

Fe(OH)2 then Fe(OH)3.

4. For pH > � 12.2 as Ew is made more positive iron is transformed to soluble

HFeO�2 ions followed by passivation due to Fe(OH)3.

For active dissolution of a metal, e.g., Fe (Eq. (8) above) the Tafel equation is used:

ia ¼ i0 expðf1� agnF gact;a=RT Þ ð9Þ

or

gact;a ¼ ba logðia=i0Þ ð10Þwhere ba = Tafel slope = 2.303RT/(1 � a)nF.In order to model the anodic curve for a transition from active to passive beha-

viour, i.e., from the potential where passivation commences (passivation potential,

Ep) to that value where passivation is complete (Ecp), Hines [9] assumed that the

metal surface consists of two independent regions—one where metal dissolution

MðsÞ !MnþðaqÞ þ ne�

occurs, and the other where a film deposits. Initially, metal dissolution is seen over

the entire surface, but as filming starts the area on which the anodic reaction pro-

ceeds unimpeded gradually decreases, reaching a minimum when the potential at

Ecp is reached. Suppose S is the fraction of metal area on which no film forms

and (1 � S) is the fraction filmed. The rate of the anodic reaction on the total surfaceitotal,a can now be expressed in terms of the anodic current densities (i) on the un-

filmed and filmed regions. Thus


itotal;a ¼ iuS þ ifð1� SÞ ð11Þwhere iu and if are the rates on the unfilmed and filmed regions, respectively. S will be

equal to unity at Ep and will reach a value of zero at Ecp.

Hines [9] suggested two physical models for the dependence of S on the applied

potential E. However, Eqs. (14) and (15) in his paper do not generate the S curve

depicted in his Fig. 3 [9]. We have corrected these equations and the variation of

S with applied potential according to Hines� second model is now given by

S ¼ 2½expð�AðEw � EpÞpÞ=½1þ expð�AðEw � EpÞpÞ ð12Þwhere p = constant used to shape the passivation peak (2 symmetrical; 2–3 asymmet-

rical) and A = constant (10�3–10�4) that determines the width of the passivation

peak. Both p and A are obtained empirically and appear to have no physical signi-

ficance [11].

Substitution in (11) gives the following for itotal,a

itotal;a ¼ iuf2½expð�AðEw � EpÞpÞ=½1þ expð�AðEw � EpÞpÞgþ iff1� 2½expð�AðEw � EpÞpÞ=½1þ expð�AðEw � EpÞpÞg ð13Þ

In summary, when S = 1 (no film) (11) reduces to itotal,a = iu and the Tafel relation-

ship applies. When S = 0, itotal,a = if = icp.

In the presence of certain anions (e.g., Cl�) the film is attacked and at points

where the film is thin metal dissolution may proceed (localised or pitting corrosion).

That part of the anodic curve from the point where pitting commences (Ebr) to themaximum potential reached (Em) is now modelled. It is assumed that the metal dis-

solution can be described by a linear logarithmic current density/potential relation-

ship. The following empirical expression is proposed for the dependence of the

anodic current density ia on the potential Ew

ia ¼ icpfðicp þ mÞ=mg ð14Þ

where

m ¼ expfln icp þ ð1=iron transpassive slopeÞ½Em � ðEw þ EbrÞg ð15Þ

with respect to (14) and (15) the following applies:

(1) when Ew is equal to or less than jEbrjm becomes large and ia = icp;

(2) when Ew > jEbrjm is small and ia > > icp.

At higher positive potentials film breakdown (in the absence of aggressive anions)

and oxygen evolution may be possibilities. Currently these aspects have not been fac-

tored into the programme.Resistance polarisation due to the presence of the passive film will also be present

and the recorded anodic potentials must be corrected for the IR drop. Sometimes an

ionically conducting but non-passive porous film (e.g., graphitic carbon) may form

on a metal and the IR drop across this film must also be taken into account. If a cur-

rent I is passed across a film whose resistance is RX there will be a potential drop


given by gX = IRX. Resistance polarisation has the effect of making the electrode po-

tential (Ew) for a corrosion system larger than the �true� value (Etrue). Thus

Etrue ¼ Ew � IR ð16ÞThis type of polarisation can be responsible for the anodic portion of an experimen-

tal curve (e.g., for mild steel in oxygen-free 0.5M sulphuric acid) exhibiting curvature

instead of the expected straight line indicative of Tafel behaviour. SYMADEC

allows for the insertion of different values of film resistance and subsequent

calculation of the true potential.

3. Synthesising and plotting polarisation curves using SYMADEC

SYMADEC contains the following series of drop-down menus (Table 1) to allow

coordinated entry of parameters required for synthesising polarisation curves. Guid-

ance as to the magnitude of certain parameters (Tafel slopes and exchange current

densities) can be obtained from the literature (see Refs. [24,25]) whilst others (tem-

perature; [H+] and [O2]) will be known either from the conditions of the experiment

or may be obtained directly from the experimental curve (Tafel slope; limiting cur-

rent density; primary and complete passivation potentials; pitting potential). Dueattention to the magnitude of parameters employed should minimise the possibility

of synthesising and matching a curve by the inclusion of inappropriate values.

4. Examples of analysed experimental polarisation curves

4.1. Case 1: Pure iron corroding in oxygen-free H2SO4 (active corrosion, no film

formation)

Data for the experimental polarisation curve shown in Fig. 1a was recorded

potentiostatically by one of the authors (DPS). Conditions for recording the experi-

mental curve were as follows: The working electrode (WE) was the cross-sectional

surface of a 5 mm diameter rod of 99.999% �specpure� polycrystalline iron (JohnsonMatthey) embedded in Teflon. The corrodent was nitrogen purged 0.5M H2SO4 at

30 ± 0.5 �C. The electrode assembly, electrochemical cell and associated apparatuswere similar to those described by Schweinsberg and Ashworth [30]. The referenceand counter electrodes were saturated calomel and Pt foil (1 cm2) respectively.

Two hundred and fifty millilitre of nitrogen purged (1 h) corrodent was heated in

a 1 L RB flask to boiling under reflux. (High purity nitrogen gas was further purified

by passing it through alkaline pyrogallol solution. Under these conditions the purged

corrodent was considered to be oxygen-free.) The contents, after cooling to ambient

temperature, were introduced into the N2-flushed cell under positive N2 pressure.

Gas was then passed continuously over the corrodent. The WE was abraded manu-

ally with 1200 grade SiC paper, polished on filter paper saturated with MgO slurry,degreased with warm AR grade acetone, washed with water and immediately placed

Table 1

Menus incorporated in SYMADEC

Drop-down menus Parameters Notes

Menu 1: Redox inputs pH; [O2] (mg L�1); T (K);

[Mn+] (0.056 mg L�1)

Parameters for calculation of Erev for reactions:

M(s)!Mn+(aq) + ne�

2H+(aq) + 2e� !H2(g)

(or 2H2O + 2e� ! H2(g) + 2OH

�(aq))

O2(g) + 2H2O + 4e� ! 4OH�(aq)

(or O2(g) + 4H+(aq) + 4e� ! 2H2O)

Menu 2:

Hydrogen inputs

Tafel slope (V decade�1);

i0 (A cm�2); iL (A cm

�2)

Parameters to synthesise cathodic curve for

H+ reduction

Menu 3:

Oxygen inputs


i0 (A cm�2); iL (A cm

�2)

Parameters to synthesise cathodic curve for

O2 reduction

Menu 4:

Metal: active inputs


i0 (A cm�2)

Parameters to synthesise anodic curve up to Ep

Menu 5:

Metal: passivation to

film breakdown inputs

icp (A cm�2); Ep (V);

Ecp (V); Ebr (V); p; A;

Tafel slope after film

breakdown (V decade�1)

Parameters to synthesise anodic curve from

Ep to Em

Menu 6:

Plotting synthesised

curve

(a) Displays synthesised anodic curve

(b) Displays synthesised cathodic curve(s)

(c) Combines (a) and (b) to display

complete synthesised curve

Menu 7:

Matching and

deconvoluting

synthesised complete

polarisation curve

The experimental polarisation curve is plotted.

Alternatively a printed curve is scanned/

digitised and plotted. The synthesised

polarisation curve is overlaid on the

experimental one and the former is adjusted

(by varying parameters) until it matches the

experimental curve. The matched curve is then

deconvoluted into its anodic and cathodic

components. All curves are plotted with

potential (versus either SHE or SCE) as

ordinate and the logarithm of the current

density in the positive x-direction


whilst wet in the corrodent. The Luggin capillary was adjusted adjacent to (about

1 mm from) the WE. After 10 min immersion the WE was pre-polarised at

�756 mV (SHE) for 40 min to remove residual oxide film. The used corrodent wasthen transferred from the cell under positive N2 pressure to a waste bottle and imme-

diately replaced under pressure with fresh corrodent. Gas was passed over the

solution.

The potential of the WE was monitored with a chart recorder and reached a

steady state after 90 min. This was selected as the corrosion potential (Ecorr). TheWE was then polarised cathodically (20 mV steps) to �576 mV (SHE) (currentwas recorded after 1 min intervals). After cathodic polarisation the WE was allowed

to rest for 15 min. Over this period the potential of the WE either returned to its

(a)

(b)

Pot

entia

l vs

SH

E (

mV

)P

oten

tial v

s S

HE

(m

V)

Fig. 1. Case 1. (a) Experimental and synthesised polarisation curves for pure iron in O2-free 0.5M H2SO4at 25 �C. (b) Deconvolution of synthesised polarisation curve for pure iron in O2-free 0.5M H2SO4 at

25 �C.



previous steady state value or was within about 2 mV. Anodic polarisation was

commenced (10 mV steps) concluding at �206 mV (SHE).Parameters and data required to synthesise and match the experimental polarisa-

tion curve (shown in Fig. 1a) are listed in Table 2. Case 1 represents a very simple

corrosion system in that a pure metal is employed and there is only one oxidant,H+ ion. The pH of the solution is approximately 0 and the reversible potential (Erev)

for the H2/H+ system is accordingly zero. Since the corrodent was prepared using

pure water and AR grade acid, the concentration of dissolved iron (as Fe2+) will

be negligible, and a value of 0.056 mg L�1 (10�6 M) may be used to calculate Erevfor the Fe/Fe2+ system (�621 mV (SHE)).As a guide to the corrosion behaviour, reference can be made to the iron Pourbaix

diagram [29]. The diagram shows the corrosion susceptibility for the pure metal in

pure water at 25 �C. Thus, for Fe2+ ion activity = 10�6 M and pH � 0, at the meanEcorr (here about �320 mV (SHE) from the experimental curve), and for the morepositive potentials applied in the experiment), Fe reacts to form Fe2+ ions.

By deconstructing the matched synthetic curve the anodic and cathodic compo-

nents are revealed (Fig. 1b). The shape of the experimental curve indicates that over

the potential range employed corrosion is uniform, and both the metal dissolution

and H2 evolution reactions experience activation polarisation only. The high acidity

delays the onset of concentration polarisation and also ensures that polarisation due

to solution IR drop is negligible. It should be noted that concentration polarisationcan be reduced also by stirring the solution. Tafel behaviour is well defined on both

portions of the experimental curve and extrapolation will give an accurate value of

icorr at Ecorr (0.16 mA cm�2).

Uniform corrosion was confirmed by examination of the WE after polarisation.

In this case, due to the purity of the material, the nature of the corrodent and the

experimental conditions, there is little need to deconstruct the experimental curve

in order to understand the corrosion process. The ease of interpretation, together

with the ability to accurately evaluate corrosion rate by Tafel extrapolation fromthe curve, makes this system suitable for studies on the inhibition efficiency of

organic compounds for iron corrosion [30].

4.2. Case 2: Carbon steel corroding in oxygen-free H2SO4 (active corrosion,

non-passive film formation)

The scanned experimental polarisation curve shown in Fig. 2a was originally re-

corded galvanostatically by Bandy and Jones [31] for 1080 carbon steel (nominalcomp. 0.75–0.88% C; 0.60–0.90% Mn; 0.04 max P; 0.05 max S) immersed in oxy-

gen-free 0.5M H2SO4. Conditions for recording the experimental curve were as fol-

lows: A glass cell was used for the electrochemical experiments and the laboratory

temperature was 25 ± 1 �C. The corrodent was placed in the cell, deaerated by bub-bling oxygen-free H2 before, and then continually during the experiment. The WE

was fashioned from rod (exposed surface area 2.5 cm2). The reference and counter

electrodes were saturated calomel and Pt foil respectively. The exposed face of the

WE was abraded with emery paper (final finish 00 grade), degreased with detergent,

Table 2

Parameters and other data relevant to the synthesis of polarisation curves

Parameters Case 1 Case 2

filmed

Case 2

unfilmed

Case 3a Case 3b Case 4a Case 4b Case 4c Case 4d Case 5 Case 6

pH 0 0 0 4.9 5.52 9 9 9 12.3 8.8 7

[O2] (mg L�1) – – – 2 3 0.01 0.2 7.9 8 0.01 8

[Fe] (mg L�1) 0.056 0.056 0.056 0.056 0.056 0.056 0.056 0.056 0.056 0.056 0.056

Temperature (K) 303 298 298 308 316 303 303 303 298 313 313

H2 TS (mV dec�1) 98 98 98 130 134 151 101 169 120 30 130

H2 ECD (A cm�2) 8.48E�8 2.43E�6 2.43E�6 3.86E�8 1.00E�5 1.00E�8 1.00E�7 1.00E�7 1.31E�6 5.77E�6 4.18E�6

H2 LCD (A cm�2) – 7.58E�2 7.58E�2 1.00E�4 2.27E�3 – 1.24E�5 – – – –

O2 TS (mV dec�1) – – – 133 127 151 171 152 153 159 180

O2 ECD (A cm�2) – – – 3.00E�13 1.00E�11 4.58E�14 1.43E�10 3.92E�11 1.35E�11 5.06E�10 5.00E�11

O2 LCD (A cm�2) – – – 9.61E�5 3.51E�4 2.14E�5 3.41E�5 5.09E�5 2.56E�5 8.55E�6 6.50E�5

Fe TS (mV dec�1) 48 39 39 64 39 142 98 168 159 144 30

Fe ECD (A cm�2) 8.84E�11 1.67E�12 1.67E�12 1.94E�7 7.11E�9 3.49E�7 4.55E�7 5.56E�7 7.72E�7 8.38E�7 1.25E�7Fe PPP (mV) – – – – – �525 �575 �420 �732 �480 �580Fe CPP (mV) – – – – – �250 �164 �210 �494 �148 �410Fe BP (mV) – – – – – 79 90 �100 – 138 �152Fe TTS (mV dec�1) – – – – – 114 65 158 – 150 60

Fe CPC (A cm�2) – – – – – 3.00E�6 2.92E�6 1.43E�6 1.41E�6 4.01E�6 3.01E�6Res (X) 0 1 0 90 0 543 3228 4900 0 321 4000

Fe Exp – – – – – 2 2 2 2 2.01 2.14

Fe Lin – – – – – 1.00E�4 1.00E�4 1.70E�4 1.00E�4 1.00E�4 3.97E�4Fe R (mV) �621 �618 �618 �623 �629 �621 �621 �621 �618 �627 �627O2 R (mV) – – – 876 819 589 609 642 447 580 761

H2 R (mV) 0 0 0 �300 �346 �541 �541 �541 �727 �547 �435Notes: TS = Tafel slope; ECD = exchange current density; LCD = limiting current density; PPP = primary passivation potential; CPP = complete passivation

potential; BP pitting potential; TTS = transpassive Tafel slope; CPC = complete passivation current density; Res = resistance; Exp = exponential constant p;

Lin = linear constant A; R = reversible potential.

H.J.Flitt,

D.P.Schwein

sberg

/Corro

sionScien

ce47(2005)2125–2156

2135

(a) (b)

(c) (d)

Fig. 2. Case 2. (a) Experimental and synthesised polarisation curves for carbon steel in O2-free 0.5M

H2SO4 at 25 �C assuming presence of non-passive surface film. (b) Deconvolution of synthesised

polarisation curve for carbon steel in O2-free 0.5M H2SO4 at 25 �C assuming presence of non-passivesurface film. (c) Experimental and synthesised polarisation curves for carbon steel in O2-free 0.5M H2SO4at 25 �C together with synthesised curve assuming no surface film. (d) Deconvolution of synthesisedpolarisation curve for carbon steel in O2-free 0.5M H2SO4 at 25 �C assuming no surface film.


rinsed in distilled water, dried and then placed in the test solution. The potential of

the WE was monitored, becoming steady after 4 h. This potential was selected asEcorr. The current was then adjusted to give six increments per decade on a logarith-

mic scale. The cathodic and then anodic potentials were recorded after 3-min inter-

vals. Both cathodic and anodic portions of the polarisation curve were obtained.

Bandy and Jones [31] found that for repeated experiments Ecorr varied between

�260 and �275 mV (SHE). For the diagram illustrated in their paper [31, Fig. 9]the corrosion potential prior to cathodic polarisation was �268 mV (SHE). How-ever, no mention is made of Ecorr before anodic polarisation and it is not possible

to establish this potential from their Fig. 9.Parameters and data required to synthesise and match the experimental polarisa-

tion curve (synthesised curve shown in Fig. 2a) are listed in Table 2 (Case 2 filmed).


Again the corrosion system is relatively simple in that the WE consists essentially of

one metal (Fe) and there is only one oxidant, H+ ion. The pH of the solution is

approximately zero and Erev for the H2/H+ system is 0 mV (SHE). As for Case 1

the [Fe2+] was taken as 0.056 mg L�1. Erev for the Fe/Fe2+ system is now

�618 mV (SHE).The Pourbaix diagram for pure iron can be used also as a guide to the corrosion

behaviour of carbon steel. Thus at 25 �C (ion activity = 10�6 M and pH = 0) at Ecorr(�270 mV (SHE)) and for more positive potentials applied in the experiment, it canbe assumed that the anodic reaction is principally the dissolution of Fe to form Fe2+

ions.

By deconstructing the matched synthetic curve shown in Fig. 2a the anodic and

cathodic components are revealed (Fig. 2b). The shape of Bandy and Jones� curveindicates that on polarisation from Ecorr in the negative direction the hydrogen reac-tion is experiencing activation polarisation and Tafel behaviour is seen over about

one decade [31]. At more negative potentials the onset of concentration polarisation

is observed. Extrapolation of the linear portion of the experimental curve to their

mean Ecorr will give an accurate value of the corrosion rate.

The anodic portion of the experimental curve in Fig. 2a might also be expected to

show Tafel behaviour. However, marked curvature is seen, and Bandy and Jones [31]

attribute this to a number of factors including a change in the nature of the metal

surface as liberated corrosion products deposit to form a non-passivating, conduct-ing surface film. They show how the anodic current density ianodic can be calculated

from iapplied = ianodic � icathodic in the potential region near Ecorr where iapplied does

not equal icathodic. The extrapolated Tafel line gives icathodic and the data points give

iapplied. Substituting these values into the above expression gives corresponding val-

ues of ianodic at a number of potentials [31]. A straight line can now be drawn through

these values of ianodic which is now representative of reasonable anodic Tafel beha-

viour. In their paper both cathodic and anodic Tafel lines are seen to intersect approxi-

mately at the mean value of Ecorr.The current authors (Flitt and Schweinsberg) have also observed anodic curvature

for carbon steel polarised in oxygen-free sulphuric acid. The WE was covered with a

black film, probably graphitic carbon which will impart a resistance to the WE. It

follows that the values of the recorded anodic potentials are greater than the true val-

ues. The effect of this resistance polarisation can be calculated using SYMADEC and

1 X was required to synthesise and match the anodic portion of Bandy and Jones�sexperimental curve shown in Fig. 2a. The anodic portion of the experimental polar-

isation curve (assuming no film and therefore no ohmic resistance) can also be syn-thesised and this, a straight line exhibiting Tafel behaviour, is shown together with

the matched cathodic portion in Fig. 2c. The corresponding deconvoluted anodic

and cathodic curves (assuming no surface film) exhibiting Tafel behaviour and inter-

secting at the mean value of Ecorr (approx. �270 mV (SHE)) are also shown in Fig.2d.

Compensating for anodic curvature due to resistance polarisation using SYMA-

DEC is an alternative approach to that employed by Bandy and Jones. Their esti-

mated corrosion rate and cathodic and anodic Tafel slopes were 1.18 ·


10�3 A cm�2, 98 mV dec�1 and 38 mV dec�1, respectively, whilst the calculated cor-

rosion rate and corresponding parameters used by SYMADEC for curve synthesis

and matching were 1.39 · 10�3 A cm�2, 98 mV dec�1 and 39 mV dec�1, respectively.

In conclusion it should be noted that pure iron is expensive, and for studies on

iron corrosion the WE is often fabricated from carbon steel. It is often assumed thatthese steels behave like the pure metal and that in strong acidic solution Tafel beha-

viour will be seen on both the cathodic and anodic portions of the experimental

polarisation curve. However, as discussed above, this is not necessarily so: the Tafel

region of the anodic portion can be obscured due to film resistance polarisation.

4.3. Case 3(a): Mild steel corroding in dormant, mixed cane sugar juice (active

corrosion, non-passive film formation)

The scanned experimental polarisation curve shown in Fig. 3a was originally re-

corded potentiodynamically by Cash [24] for a mild steel WE (typical of pipeline

steel used in a cane sugar mill) in dormant, mixed cane-sugar juice (MJ), open to

air at 35 �C. Mixed juice contains about 13% by weight of sucrose together with

Na � 52 ppm, K � 1300 ppm, Ca > 113 ppm, Mg � 109 ppm, Al � 25 ppm,Fe � 25 ppm, Si � 73 ppm, Cl� � 1200 ppm, sand and fine fibre from the crushedcane. Preliminary experiments showed that mild steel, on exposure to MJ, becomes

coated with a grey/black, porous, non-passivating film which was found to consistmainly of organic material [24]. In the sugar mill under flow conditions the thickness

of this film increases with both increasing flow rate and exposure time.

The working electrode employed by Cash [24] was the cross-sectional surface of

10 mm diameter rod of mild steel embedded in chemical resistant epoxy resin. The

reference and counter electrodes were saturated calomel and Pt foil (1 cm2) respec-

tively. The WE was abraded with 600 grade SiC paper, degreased with AR grade ace-

tone and then immediately exposed to the MJ contained in a 1 L glass cell. The

dissolved oxygen concentration was 2 mg L�1 and Ecorr (�445 mV (SHE)) was steadyafter approx. 30 min. After approx. 100 min exposure to the mixed juice the WE was

polarised anodically from Ecorr (in order to least disturb any film deposited on theWE

during the exposure period). This was followed by the cathodic scan when Ecorr had

returned to within ±5 mV from its previous value. The scan rate was 60 mV min�1.

Parameters and data required to synthesise and match the experimental polarisa-

tion curve (shown in Fig. 3a) are listed in Table 2. For mild steel the dominant ano-

dic reaction is iron dissolution and this is driven by H+ ion and O2 reduction. The pH

of the MJ was 4.9, and calculated Erev for the H2/H+ system is �300 mV (SHE).

The dissolved O2 concentration was established as 2 mg L�1 and calculated Erev

for the O2/H2O system is +876 mV (SHE). The total dissolved iron (ICP analysis)

in the MJ was �25 mg L�1 but because of the possibility of complexing withorganics and other species in the MJ the concentration of dissolved iron as Fe2+ is

probably much less than 25 mg L�1. The exact value of [Fe2+] was not established

and for curve synthesis 0.056 mg L�1 (10�6 M) was used. This value can be justified

in that it is accompanied by an acceptable i0 of 1.94 · 10�7 A cm�2 for the Fe/Fe2+

system. Erev for the Fe/Fe2+ system is �623 mV (SHE)).

(a) (b)

(c) (d)

Fig. 3. Case 3a. (a) Experimental and synthesised polarisation curves for mild steel in dormant MJ at

35 �C (100 min exposure; 2 mg L�1 O2) assuming presence of non-passive surface film. (b) Deconvolutionof synthesised polarisation curve for mild steel in dormant MJ at 35 �C assuming presence of non-passivesurface film. (c) Experimental and synthesised polarisation curves for mild steel in dormant MJ at 35 �C(100 min exposure; 2 mg L�1 O2) assuming no surface film. (d) Deconvolution of synthesised polarisation

curve for mild steel in dormant MJ at 35 �C assuming no surface film.


Although the Pourbaix diagram for pure iron corresponds to equilibria a 25 �Cand here a higher temperature (35 �C) and carbon steel is involved, the diagramcan be used as an approximate guide to the corrosion susceptibility of the mild steel.

Thus at pH = 4.9 and Ecorr = �445 mV (SHE) (and for more positive potentials ap-plied in the experiment) the anodic reaction is principally the dissolution of Fe to

form Fe2+ ions.

The cathodic portion of the experimental curve (Fig. 3a) has some appearance of

linearity but this does not indicate a Tafel region. Tafel behaviour refers to one reac-tion, and in this case the cathodic portion of the experimental curve is actually the

sum of two curves (oxygen reduction and hydrogen evolution). This is made clear

in Fig. 3b where the deconvoluted anodic and cathodic components of the synthes-

ised curve seen in Fig. 3a are shown. The deconvolution reveals that at Ecorr the

dominant cathodic reaction driving the corrosion is oxygen reduction.


The black film will impart a resistance to the WE and, as in Case 2, the values of

the recorded anodic potentials will be greater than the true values. The effect of this

resistance polarisation can be calculated using SYMADEC and 90 X were requiredto synthesise and match the anodic portion of the experimental curve shown in Fig.

3a. The anodic portion (assuming no film and therefore no ohmic resistance) canalso be synthesised and this, a straight line exhibiting Tafel behaviour, is shown to-

gether with the matched cathodic portion (and also the experimental curve) in Fig.

3c. The corrosion rate can be estimated by extrapolating this line to the corrosion

potential. The corresponding deconvoluted anodic and cathodic curves are shown

in Fig. 3d.

4.4. Case 3(b): Mild steel corroding in flowing, mixed cane sugar juice (active

corrosion, non-passive film formation)

An experimental polarisation curve originally recorded potentiodynamically by

Cash [24] for a mild steel WE in mixed cane sugar juice (MJ), open to air at 43 �Cwas scanned. In contrast to Case 3(a) the MJ was flowing through a laboratory

flow-rig and conditions for recording the experimental curve were as follows. The

flow-rig was constructed from black polyethylene tubing (18 mm i.d.). The WE

and CE were mild steel discs (0.95 cm2) mounted in the electrode assembly (PVC tub-

ing) and ground so that they were flush with the internal wall of the tubing. A com-mercial Ag/AgCl reference electrode with the tip mounted as close as possible to the

WE was used. In this experiment the MJ flow rate was 24 dm3 min�1 (2 m s�1).

The WE was abraded with 600 grade SiC paper, degreased with AR grade acetone

and then immediately exposed to the flowing MJ. The juice temperature (43 �C), dis-solved oxygen concentration (3 mg L�1) and Ecorr (�440 mV (SHE)) were steadyafter approx. 30 min. After approx. 100 min exposure to the flowing juice the WE

was polarised anodically from Ecorr (as in Case 3(a) to least disturb any film depos-

ited on the WE during the exposure period). This was followed by the cathodic scanwhen Ecorr had returned to within ±5 mV of its previous value. The scan rate was

60 mV min�1.

A black film was deposited on the WE during establishment of the corrosion po-

tential and its resistance acts to make the values of the recorded anodic potentials

greater than the true values. As in Case 3(a) the film resistance can be calculated using

SYMADEC and the curved anodic portion of the experimental curve can be

matched. The anodic portion can also be synthesised assuming no film (and therefore

no ohmic resistance) and the result is a straight line exhibiting Tafel behaviour. Theexperimental curve in Fig. 4a is shown as it would appear if there was no film. The

matched cathodic portion of the experimental curve is also shown in Fig. 4a. The cor-

rosion rate can now be estimated by extrapolating the anodic Tafel line to the corro-

sion potential. Because of the increased temperature and movement of the corrodent

iL is now greater than that seen in Case 3(a). The corresponding deconvoluted anodic

and cathodic curves are shown in Fig. 4b.

Parameters and data required to synthesise and match the experimental polarisa-

tion curve are listed in Table 2. The dominant anodic reaction is again iron dissolu-

(a)

(b)

Fig. 4. Case 3b. (a) Experimental and synthesised polarisation curves for mild steel in flowing MJ at 43 �C(100 min exposure; 3 mg L�1 O2) assuming no surface film. (b) Deconvolution of synthesised polarisation

curve for mild steel in flowing MJ at 43 �C (100 min exposure; 3 mg L�1 O2) assuming no surface film.



tion driven by H+ ion and O2 reduction. The pH of the MJ was 5.52, and the calcu-

lated Erev for the H2/H+ system is �346 mV (SHE). The O2 concentration was

3 mg L�1 and the calculated Erev for the O2/H2O system was +819 mV (SHE).

As in Case 3(a) an [Fe2+] = 0.056 mg L�1 was used and the calculated Erev for the

Fe/Fe2+ system was �629 mV (SHE).Using the Pourbaix diagram for pure iron at 25 �C as a guide, it is a reasonable

assumption that at 43 �C, pH = 5.52, and Ecorr = �440 mV (SHE) (and for more po-sitive potentials applied in the experiment) the anodic reaction is principally the dis-

solution of Fe to form Fe2+ ions.

As in Case 3(a) the cathodic portion of experimental curve appears to show some

linearity, but again this does not indicate a Tafel region as the cathodic portion of

the experimental curve is the sum of two curves (oxygen reduction and hydrogen

evolution). The deconvoluted cathodic curve seen in Fig. 4b shows that at Ecorrthe dominant cathodic reaction driving the corrosion is again oxygen reduction.

Fig. 4b also clearly indicates that when the potential is made more negative than

Ecorr the hydrogen evolution reaction�s contribution to the total cathodic current be-comes increasingly important. At a sufficiently negative potential this curve will also

come under complete diffusion control.

Cases 3(a) and 3(b) are good examples of situations in which the experimental

curve does not provide a Tafel region which in turn can be used to estimate corrosion

rate. Although the anodic portions of the curves are indicative of active corrosionthey are curved due to deposition of non-passive films. As for the cathodic portions

they are the sum of two reactions. The presence of a straight-line region is simply

fortuitous.

4.5. Case 4(a): Low-alloy steel corroding in oxygen-containing, simulated steam

turbine condensate (active corrosion, induced passivation and pitting)

The experimental curve (Fig. 5a) was recorded potentiodynamically by Otieno-Alego et al. [32] for A-470 turbine rotor disc steel (0.24% C, 1.8% Cr, 3.68% Ni,

0.46% Mo, 0.3% Mn, 0.12% V, 0.0004% S, 0.0004% P, 0.05% Si) immersed in a syn-

thetic steam turbine condensate containing 2 ppm NaCl, 2 ppm Na2SO4, 2 ppm

NaOH and 5 ppm SiO2.

A single compartment Perspex cell (800 cm3) fitted with a Perspex lid was used.

The WE (10 mm dia.) and Pt counter electrode (1 cm2) were mounted in chemical

resistant epoxy resin and immersed in the test solution using a Perspex holder. A sat-

urated calomel electrode (SCE) connected to a Luggin capillary was used as the re-ference electrode. The temperature was 30 �C and the solution pH = 9.0. Bottlednitrogen gas (containing traces of oxygen) was passed continuously through the cor-

rodent and this resulted in a dissolved oxygen level of approximately 0.01 mg L�1.

The WE was abraded with 1200 grade SiC paper, degreased with AR grade acetone,

inserted in the solution and then immediately pre-polarised at �756 mV (SHE) for10 min to remove any air-formed oxide film. After reaching a steady Ecorr (approx.

1 h) the corroding WE was polarised cathodically. This was followed by anodic

(a)

(b)

Pot

entia

l vs

SH

E (

mV

)P

oten

tial v

s S

HE

(m

V)

Fig. 5. Case 4a. (a) Experimental and synthesised polarisation curves for low-alloy steel in synthetic

condensate at 30 �C (0.01 mg L�1 O2). (b) Deconvolution of synthesised polarisation curve for low-alloysteel in synthetic condensate at 30 �C (0.01 mg L�1 O2).



polarisation when Ecorr had returned to within ± 5 mV of the previous value. The

polarisation scan rate was 10 mV min�1.

In this case there are two oxidants (H+ ion and small amount of O2) driving cor-

rosion. Ecorr was �539 mV (SHE). Although low-alloy steel is corroding, the mate-rial is approximately 93% Fe, and the Pourbaix diagram for pure iron is a reasonableguide to corrosion behaviour and subsequent anodic polarisation. The diagram

shows that at pH = 9.0 and for an Ecorr � �539 mV (SHE) pure iron is actively cor-roding to form Fe2+ ions. Further, if the WE is made more positive iron passivates

with the formation of precipitated Fe2O3 Æ nH2O (or Fe(OH)3).The shape of the experimental polarisation curve (Fig. 5a) supports the use of the

iron Pourbaix diagram to predict corrosion behaviour. The curve suggests active cor-

rosion at Ecorr and indicates that polarisation in the positive direction (by means of

the potentiostat) results in a classical active/passive transition. This is followed atmore positive potentials by a rapid increase in current density suggesting in the pres-

ence of Cl� pitting corrosion. Otieno-Alego et al. [32] reported that pits were ob-

served on the WE after anodic polarisation.

Parameters and data required to synthesise and match Otieno-Alego et al.�s [32]experimental polarisation curve based on the above assumptions (Fig. 5a) are listed

in Table 2. Erev for the H2/H+ system is �541 mV (SHE) with Erev for the O2/H2O

system +589 mV (SHE). Again, using a minimum value of [Fe2+] = 0.056 mg L�1,

Erev for the Fe/Fe2+ system is �621 mV (SHE).

The deconvoluted anodic and cathodic components of the synthesised curve are

shown in Fig. 5b. This shows that at Ecorr the corrosion is driven mainly by reduction

of the small amount of oxygen in solution (the reduction of H+ ion contributes rel-

atively little to the total cathodic current density at this potential). Further, both

cathodic reactions are under complete activation control at the corrosion potential.

The cathodic portion of the experimental curve is a composite one and it is futile

searching for a linear �Tafel� region to ascertain corrosion rate. The anodic portionof the experimental curve before onset of passivation is also curved and cannot beused to estimate corrosion rate.

An estimation of the corrosion current density may be ascertained (Fig. 5b) from

the intersection of Ecorr with the synthesised anodic and oxygen curves.

4.6. Case 4(b): Low-alloy steel corroding in oxygen-containing, simulated steam

turbine condensate (spontaneous passivation and induced pitting)

The experimental polarisation curve (Fig. 6a) was recorded by Otieno-Alego et al.[32] as for Case 4(a) except that the oxygen concentration was increased from 0.01 to

0.20 mg L�1 by passing a nitrogen/air mixture through the corrodent. Again there

are two oxidants (O2 and H+) driving the corrosion and the iron Pourbaix diagram

shows that at pH � 9.0 and Ecorr = �141 mV (SHE), pure iron spontaneously pass-ivates with the formation of Fe(OH)3.

The more positive Ecorr (�141 mV (SHE) versus �539 mV (SHE) for Case 4(a))and the shape of the experimental curve (Fig. 6a) and suggests that the higher oxygen

level has been instrumental in passivating the low-alloy steel WE upon its immersion

(b)

(a)

Fig. 6. Case 4b. (a) Experimental and synthesised polarisation curves for low-alloy steel in synthetic




in the corrodent. The experimental curve also suggests that subsequent polarisation

with the potentiostat in the positive direction from Ecorr results in localised corrosion

at approximately 90 mV (SHE). This behaviour was supported by the existence of

pits seen on the WE after anodic polarisation to �+250 mV (SHE) [32].Assuming spontaneous passivation in the corrodent followed by induced pitting,

the polarisation curve was synthesised and matched to the experimental one (Fig.

6a). Parameters and data required to synthesise and match the experimental curve

are listed in Table 2. In this case values of some parameters (e.g., the primary pas-

sivation potential for the active/passive transition) cannot be estimated from the

experimental curve. Erev for the H2/H+ system is �541 mV (SHE), with Erev for

the O2/H2O system +609 mV (SHE). Again, using a minimum value of [Fe2+] =

0.056 mg L�1, Erev for the Fe/Fe2+ system is �621 mV (SHE).

The deconvoluted anodic and cathodic components of the synthesised curve areshown in Fig. 6b and this shows that at Ecorr the corrosion is driven overwhelmingly

by oxygen reduction. The cathodic oxygen curve cuts the anodic one in the passive

region. This masks the active/passive portion of the anodic curve and, unlike in the

previous case, no estimate of the primary passivation potential and the complete pas-

sivation potential can be obtained from the experimental curve. The deconvolution

shows that at Ecorr oxygen reduction is under complete activation control.

Deconvolution clearly shows that there are no �Tafel regions� on the experimentalcurve. The alloy, on exposure to a synthetic steam turbine condensate in which theoxygen concentration is 0.20 mg L�1 spontaneously passivates, and the complete

passivation current density as seen in Fig. 6b may be taken as an estimate of the

its corrosion rate.

The small �step� at approximately �450 mV (SHE) arises from the closeness of thetip of the �passivation peak� to the cathodic oxygen curve.

4.7. Case 4(c): Low-alloy steel corroding in oxygen-containing, simulated steam

turbine condensate (spontaneous passivation and spontaneous pitting)

The experimental polarisation curve (Fig. 7a) was recorded by Otieno-Alego et al.

[32] as for Cases 4(a) and 4(b) except that the oxygen concentration was further in-

creased to 7.9 mg L�1. Ecorr was �80 mV (SHE) and the Pourbaix diagram showsthat at this potential at pH � 9.0 pure iron spontaneously passivates with the forma-tion of Fe(OH)3.

The shape of the anodic portion of the experimental curve (Fig. 7a), coupled with

the more positive corrosion potential (compared with the previous case), suggeststhat the low-alloy steel on immersion in the corrodent may have undergone sponta-

neous passivation followed by localised (pitting) corrosion. Thus there is now suffi-

cient dissolved oxygen to drive Ecorr to a value either equal to, or more positive than

the pitting potential. Further work by Otieno-Alego et al. [32] showed that pits

formed a few minutes after immersion.

The synthesised/matched curve (assuming spontaneous passivation/pitting) is

shown in Fig. 7a. Parameters and data required to synthesise and match the exper-

imental curve are listed in Table 2. Erev for the H2/H+ system is �541 mV (SHE) with

(a)

(b)

Fig. 7. Case 4c. (a) Experimental and synthesised polarisation curves for low-alloy steel in synthetic




Erev for the O2/H2O system +642 mV (SHE). Using a minimum value of

[Fe2+] = 0.056 mg L�1, Erev for the Fe/Fe2+ system is �621 mV (SHE). Here it is

impossible to estimate the values of parameters for the active/passive transition

and pitting from the experimental curve. The iron breakdown/pitting potential

and Ecorr were taken as coincident. The deconvoluted anodic and cathodic portionsare shown in Fig. 7b and this reveals that at Ecorr the localised corrosion is domi-

nated by reduction of oxygen, and the cathodic reaction is under complete activation

control at the corrosion potential. Because the alloy is pitting the concept of corro-

sion rate (which applies to uniform corrosion) is meaningless.

The cathodic portion of the experimental curve exhibits (as in the previous case) a

small �step� at approximately �350 mV (SHE).Cases 4(a), 4(b) and 4(c) show how the oxidant concentration (here mainly oxy-

gen) can determine whether an alloy on immersion in the corrodent experiences ac-tive corrosion, spontaneous passivation, or spontaneous passivation/pitting.

4.8. Case 4(d): Low carbon steel corroding in oxygenated pure water also

containing a basic detergent (spontaneous passivation)

The experimental curve shown in Fig. 8a was recorded potentiodynamically by

one of the current authors (HJF) [33] for 1020 carbon steel immersed in distilled

water containing a commercial detergent (25 mg L�1; 25 �C). The solution was opento air ([O2] � 8 mg L�1) and the pH = 12.3. The WE (abraded with 1200 grade SiCpaper and degreased with AR grade acetone) was immediately placed in the test solu-

tion and pre-polarised at �756 mV (SHE) for 5 min to remove residual oxide film.The electrode was then polarised from this potential at 20 mV min�1 to approxi-

mately +740 mV (SHE).

In this case at pH = 12.3 and dissolved oxygen is the main oxidant driving the cor-

rosion. Ecorr is apparent from the experimental curve (�345 mV (SHE)). The Pour-baix diagram for pure Fe shows that at pH = 12.3, and as the potential is made morepositive, the metal oxidises to form firstly soluble HFeO�

2 ions, followed by passiv-

ation due to deposition of protective Fe(OH)3.

The shape of the experimental curve and the value of Ecorr suggest that the high

oxygen level polarises and then spontaneously passivates the WE when it is im-

mersed in the corrodent. Fig. 8a also indicates that induced anodic polarisation from

Ecorr to �+740 mV (SHE) was insufficient to result in pitting. In addition HJF [33]did not observe any pits on the WE after the experiment.

The synthesised and matched polarisation curve (assuming passivation) is alsoshown in Fig. 8a and the deconvoluted anodic and cathodic components are shown

in Fig. 8b. Parameters and data required to synthesise and match the experimental

curve are listed in Table 2. Erev for the H2/H+ system is �727 mV (SHE) with Erev

for the O2/H2O system +447 mV (SHE). Again, using a minimum value of

[Fe2+] = 0.056 mg L�1, Erev for the Fe/Fe2+ system is �618 mV (SHE). In this case

estimating the values of parameters for the active/passive transition from the experi-

mental curve is less difficult than in the previous two cases. Fig. 8b shows that at

Ecorr the corrosion is driven entirely by reduction of oxygen. This example can be

(a)

(b)

Fig. 8. Case 4d. (a) Experimental and synthesised polarisation curves for mild steel in distilled water

containing commercial detergent at 25 �C (8 mg L�1 O2). (b) Deconvolution of synthesised polarisationcurve for mild steel in distilled water containing commercial detergent at 25 �C (8 mg L�1 O2).



compared with Case 4(b). The size of the passivation peak is markedly reduced be-

cause; at the higher pH (12.3 versus 9) fewer HFeO�2 ions are required to precipitate

the hydrated oxide. Also, because the �nose� is very small the cathodic portion of theexperimental curve does not exhibit a step as seen in Cases 4(b) and 4(c).

4.9. Case 5: Low carbon steel corroding in oxygen-containing water (induced

passivation and induced pitting)

The experimental curve shown in Fig. 9a was recorded potentiodynamically by

one of the current authors (HJF) [33] for 1020 carbon steel immersed in distilled

water (open to air) at 40 �C containing 25 mg L�1 NaCl and 150 mg L�1 of an oxy-gen scavenger (activated hydrazine hydrate (LEVOXINTM)). The oxygen concentra-

tion during polarisation was measured as �0.01 mg L�1 and the pH of the solutionwas 8.8.

The WE (abraded with 1200 grade SiC paper and degreased with AR grade ace-

tone) was placed in the test solution and pre-polarised at �580 mV (SHE) for 5 minto remove any residual oxide film. The electrode was then immediately polarised in

the positive direction (20 mV min�1) through to approximately +300 mV (SHE).

The activated hydrazine hydrate, in addition to reducing the oxygen concentra-

tion, reacted with the water raising the pH of the solution to 8.8. Although the

amount of oxygen remaining in solution is small it will act in conjunction with theH+ ions to drive the corrosion.

At this point it should be noted that the procedure adopted for recording an

experimental curve can add to difficulties in its interpretation. Here the corrosion po-

tential was not established by letting the WE stabilise after pre-polarisation, and as a

result it might be thought that the experimental curve shown in Fig. 9a exhibits three

such potentials, and perhaps two �active/passive transitions�. This dilemma can bepartly resolved by referring to Liening�s schematic diagrams [1]. He shows that sucha curve will arise when the concentration (diffusion) controlled portion of the truecathodic curve intersects the true anodic curve at two points on the active/passive

�nose�, and the activation-controlled portion intersects the passive region. There isonly one active/passive transition, and what appears to be a second transition (at

more positive potentials) is actually a �cathodic loop�.In the current example the corrosion potential was established in a separate experi-

ment (HJF [33]) and corresponded to the most negative of the �three possibilities�(�503 mV (SHE)) seen in Fig. 9a. From the Pourbaix diagram it can be assumedthat at this potential and for pH = 8.8 and in the presence of the dissolved oxygenthe low carbon steel is actively corroding to Fe2+. It can also be assumed from the

shape of the curve that induced polarisation in the positive direction from Ecorr re-

sults in an active/passive transition followed by a cathodic loop. At even higher ap-

plied potentials film breakdown occurs at approximately +138 mV (SHE). (Note:

pits were observed by HJF [33] on the WE after induced polarisation to +300 mV

(SHE).)

Parameters and data required to synthesise and match the experimental curve

(Fig. 9a) are listed in Table 2. Erev for the H2/H+ system is �547 mV (SHE) with

(a)

(b)

Fig. 9. Case 5. (a) Experimental and synthesised polarisation curves for mild steel in NaCl salt solution

plus O2 scavenger (LEVOXIN) at 40 �C (0.01 mg L�1 O2). (b) Deconvolution of synthesised polarisationcurve for mild steel in NaCl salt solution plus O2 scavenger (LEVOXIN) at 40 �C (0.01 mg L�1 O2).



Erev for the O2/H2O system +580 mV (SHE). Using a minimum value of

[Fe2+] = 0.056 mg L�1, Erev for the Fe/Fe2+ system is �627 mV (SHE). In this case

it is again impossible to estimate values of parameters for the active/passive tran-

sition from the experimental curve. The deconvoluted anodic and cathodic com-

ponents of the synthesised/matched curve are shown in Fig. 9b. This clearlyshows how induced polarisation from the pre-polarisation potential (�580 mV(SHE)) results in a diminution in both the rate of H2 evolution and oxygen

reduction. At Ecorr the corrosion is seen to be driven mainly by the oxygen reduc-

tion reaction. At more positive potentials there is sufficient Fe2+ ion in solution to

induce passivation and this is followed at higher potentials by pitting in the

aggressive Cl� solution. Fig. 9b also shows that the actual oxygen curve is under-

going combined activation and concentration polarisation when it intersects with

the actual anodic curve in the passive region (where a stable passive film hasformed) and at a more negative potential (where the film is unstable). These

points of intersection are responsible for the cathodic loop with the current den-

sity for oxygen reduction exceeding the anodic current density between the upper

two intersection points.

4.10. Case 6: Low carbon steel corroding in oxygen-containing water (spontaneous

passivation and pitting)

The experimental curve shown in Fig. 10a was recorded potentiodynamically

[21,22] for 1020 carbon steel immersed in distilled water at 40 �C containing

25 mg L�1 NaCl and 100 mg L�1 of a commercial inhibitor for iron (zinc phosphi-

nocarboxylic acid (ZnPCA)). An extra 15 mg L�1 of zinc was added (as zinc sul-

phate) and the pH was adjusted to 7.0 with dilute KOH solution. The test

solution was open to air and the oxygen concentration was measured at �8 mg L�1.The WE (abraded with 1200 grade SiC paper and degreased with AR grade acetone)

was placed in the test solution and pre-polarised at �600 mV (SHE) for 5 min to re-move any residual oxide film. The electrode was then polarised in the positive direc-

tion (20 mV min�1) to approximately +100 mV (SHE).

In this case at pH = 7.0 and [O2] = 8 mg L�1 the main oxidant driving the corro-

sion is dissolved oxygen. Although Ecorr was not measured after the cathodic, pre-

polarisation step, its value is obvious from the experimental curve (�142 mV(SHE)). The low carbon steel can be expected to corrode similarly to pure iron

and from the Pourbaix diagram for Fe at pH = 7.0, and as the potential is made

more positive (from approximately �560 to +100 mV (SHE)), Fe is oxidised toFe2+ ions.

Phosphinocarboxylic acid (PCA), combining both the phosphino functional

group and the carboxylic functional group in one molecule, has been used as a

corrosion inhibitor for steel in cooling water and it is assumed that the molecule

is chemisorbed on the metal to act principally as an anodic inhibitor [34]. Inhi-

bition efficiency of PCA is markedly increased by the addition of zinc (optimum

inhibition in the range approximately 20–40% by weight of Zn). It has been pro-

posed that Zn(II) reacts with the PCA and the resulting zinc complex (ZnPCA)

(a)

(b)

Fig. 10. Case 6. (a) Experimental and synthesised polarisation curves for mild steel in NaCl salt solution

plus Zn-augmented ZnPCA at 40 �C (8 mg L�1 O2). (b) Deconvolution of synthesised polarisation curvefor mild steel in NaCl salt solution plus Zn-augmented ZnPCA at 40 �C (8 mg L�1 O2).



is also chemisorbed on the steel surface, reducing the rate of both the cathodic

and anodic corrosion reactions [34]. In the present case the cathodic portion of

the experimental curve (Fig. 10a) reveals a cathodic �dip� at approximately�500 mV (SHE). Liening [1] notes that such a �dip� can arise when there is anactive/passive transition, and the current density of the concentration controlledportion of the cathodic curve is just greater than that at the tip of the active/pas-

sive �nose�. Evidence for the Zn-augmented ZnPCA promoting the formation of apassive film is provided by the relatively positive value of Ecorr and by the shape

of the anodic portion of the experimental curve. The latter suggests active corro-

sion at Ecorr deriving from adsorption of aggressive Cl� ions and subsequent

localised corrosion. Under these conditions Ecorr is more positive than the pitting

potential. At the conclusion of the polarisation pits were observed on the steel

[21,22].It can be assumed therefore that the low-alloy carbon steel on immersion in the

corrodent in the presence of inhibitor and chloride ions undergoes spontaneous pas-

sivation/pitting. On this basis the synthesised/matched curve is shown in Fig. 10a

and parameters and data required for synthesis are listed in Table 2. Erev for the

H2/H+ system is �435 mV (SHE) with Erev for the O2/H2O system +761 mV

(SHE). Using a minimum value of [Fe2+] = 0.056 mg L�1, Erev for the Fe/Fe2+ sys-

tem is �627 mV (SHE). It is again impossible to estimate the values of parametersfor the active/passive transition and film breakdown from the experimental curve.Deconvolution (Fig. 10b) reveals the dominance of the oxygen reaction and shows

how as the potential is made more positive the rates of hydrogen evolution and

reduction of oxygen decrease. The figure also shows how the �dip� is generated withthe current density of the concentration controlled portion of the oxygen curve just

greater than that at the tip of the active/passive �nose�. Finally, Fig. 10b also showsthe corrosion potential more positive than the pitting potential resulting in sponta-

neous pitting.

5. Conclusions

• Experimental polarisation curves for the corrosion system Fe/H2O/H2/O2 can besynthesised using the appropriate mathematical relationships and kinetic and

thermodynamic data for the reactions involved in the corrosion process.

• Deconstruction of the synthesised, accurately matched curve reveals the true ano-dic and cathodic components operative in the following corrosion systems: active

corrosion; active corrosion and non-passive film formation; active corrosion fol-

lowed by induced passivation and induced pitting; spontaneous passivation and

induced pitting; spontaneous passivation and spontaneous pitting. Curves exhi-

biting either a cathodic loop or a cathodic dip can also be analysed.

• The accurately analysed curves replace schematic representations and are a valu-able reference source for the interpretation of experimental curves for the aqueous

corrosion of pure iron/carbon/low-alloy steels.


Acknowledgments

The authors wish to thank the School of Physical and Chemical Sciences for pro-

viding facilities for the writing of this paper. We would also like to acknowledge

those researchers whose results have been used in our analysis of experimental polar-isation curves.

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