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).
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
2128 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2129
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
2130 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2131
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
Tafel slope (V decade�1);
i0 (A cm�2); iL (A cm
�2)
Parameters to synthesise cathodic curve for
O2 reduction
Menu 4:
Metal: active inputs
Tafel slope (V decade�1);
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
2132 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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.
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2133
2134 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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.
2136 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2137
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 ·
2138 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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.
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2139
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.
2140 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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.
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2141
2142 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2143
2144 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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
condensate at 30 �C (0.2 mg L�1 O2). (b) Deconvolution of synthesised polarisation curve for low-alloysteel in synthetic condensate at 30 �C (0.2 mg L�1 O2).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2145
2146 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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
condensate at 30 �C (7.9 mg L�1 O2). (b) Deconvolution of synthesised polarisation curve for low-alloysteel in synthetic condensate at 30 �C (7.9 mg L�1 O2).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2147
2148 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2149
2150 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2151
2152 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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).
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2153
2154 H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156
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.
H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 2125–2156 2155
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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