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 with their 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 updated 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 deconstruction/deconvolution
*
Corresponding author. Tel.: +61 73 864 2111; fax: +61 73 864 1804. E-mail address:
[email protected] (D.P. Schweinsberg).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.10.002
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1. Introduction The generation of polarisation curves continues to be important in aqueous corrosion research. The time-consuming potentiostatic method has been largely replaced by the potentiodynamic approach where the potential (E) of the corroding metal 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 potential/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 xdirection, notwithstanding the convention that anodic current is positive and cathodic 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 compound to act as a corrosion inhibitor, relative corrosivity of process streams and corrosion rate (icorr) of the metal. Unfortunately, extracting any of the above from the experimental curve may be quite 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 experimental 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 cathodic 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 correspondingly 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 extrapolation 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 estimation 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
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corrosion rate evaluation is when corrosion-monitoring probes based on the polarisation resistance method are used. The reaction kinetics of the corrosion process must be established before installation as these devices again assume that at Ecorr the anodic and cathodic corrosion reactions are under activation control. A final example where the experimental curve can be difficult to interpret is when the metal spontaneously passivates/pits in the corrodent prior to polarisation. The anodic portion of the experimental curve may now exhibit straight line behaviour but, because localised 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 experimental curve 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 representations of experimental curves with their schematic true anodic and cathodic curves have been published [1,2]. Thus Liening [1] discusses nine possible experimental curves for the reaction M þ Hþ ! Mþ þ 1=2 H2 These 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 experimental 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 obtained 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 components. Various computer-based programmes have been devised to effect the calculations 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 synthesis, 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 kinetics 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 concept, but instead of employing schematic representations have selected for comparison purposes actual experimental curves (in this case for the corrosion of iron and carbon steels). Each curve has been synthesised, matched and then deconstructed to reveal 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 analysis is also emphasised. The experimental curves were obtained either from experiments carried out in our laboratories, or from examples published in the
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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 minimise the current/voltage set and optimise graphical representation. The curves chosen range in complexity, starting with the simple case indicative of one anodic and one cathodic reaction and undergoing activation polarisation only at Ecorr to corrosion 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 metal are 2Hþ ðaqÞ þ 2e ! H2 ðgÞ
ðequivalent 2H2 O þ 2e ! H2 ðgÞ þ 2OH ðaqÞÞ
and O2 ðgÞþ 2H2 O þ 4e ! 4OH ðaqÞ ðequivalent O2 ðgÞþ 4Hþ ðaqÞþ 4e ! 2H2 OÞ The relationship between the rate of each of the above reactions, expressed as cathodic 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 expðanF gact;c =RT Þ
ð1Þ
where a = transfer coefficient; n = number of electrons involved in the reaction; F = Faradays constant; gact,c = Ew Ereversible; R = 8.314 J K1 mol1; T = abs. temp. Rearranging gives the Tafel equation: gact;c ¼ bc logðic =i0 Þ
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 iL f1 expðnF gconc;c =RT Þg
ð3Þ
where iL = limiting current density. Rearranging gconc;c ¼ ð2:303RT =nF Þ logf1 ðic =iL Þg
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 =i0 Þ þ ð2:303RT =nF Þ logð1 ðic =iL ÞÞ
ð5Þ
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It follows [18,28] that the approximate value of the total cathodic current density is given by itotal;c ¼ ½i0 expðanF g=RT Þ =½1 þ fi0 expðanF g=RT Þg=iL
ð6Þ
itotal;c ¼ iL ic =ðiL þ ic Þ
ð7Þ
or Appropriate versions of Eq. (6) are used to model the curves for H+ and O2 reduction. The current densities at each potential are then summed. The general anodic reaction for active metal dissolution is M ! Mn+ + ne. Consider 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 activity 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 =i0 Þ
ð10Þ
where ba = Tafel slope = 2.303RT/(1 a)nF. In order to model the anodic curve for a transition from active to passive behaviour, 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 ! Mnþ ðaqÞ 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 proceeds 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 surface itotal,a can now be expressed in terms of the anodic current densities (i) on the unfilmed and filmed regions. Thus
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itotal;a ¼ iu S þ if ð1 SÞ
ð11Þ
where 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 p
p
S ¼ 2½expðAðEw Ep Þ Þ =½1 þ expðAðEw Ep Þ Þ
ð12Þ
where p = constant used to shape the passivation peak (2 symmetrical; 2–3 asymmetrical) and A = constant (103–104) that determines the width of the passivation peak. Both p and A are obtained empirically and appear to have no physical significance [11]. Substitution in (11) gives the following for itotal,a itotal;a ¼ iu f2½expðAðEw Ep Þp Þ =½1 þ expðAðEw Ep Þp Þ g p
p
þ if f1 2½expðAðEw Ep Þ Þ =½1 þ expðAðEw Ep Þ Þ g
ð13Þ
In summary, when S = 1 (no film) (11) reduces to itotal,a = iu and the Tafel relationship 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 the maximum potential reached (Em) is now modelled. It is assumed that the metal dissolution can be described by a linear logarithmic current density/potential relationship. The following empirical expression is proposed for the dependence of the anodic current density ia on the potential Ew ia ¼ icp fðicp þ mÞ=mg
ð14Þ
where m ¼ expfln icp þ ð1=iron transpassive slopeÞ½Em ðEw þ Ebr Þ g
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 factored 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 current I is passed across a film whose resistance is RX there will be a potential drop
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given by gX = IRX. Resistance polarisation has the effect of making the electrode potential (Ew) for a corrosion system larger than the true value (Etrue). Thus Etrue ¼ Ew IR
ð16Þ
This type of polarisation can be responsible for the anodic portion of an experimental curve (e.g., for mild steel in oxygen-free 0.5 M 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. Guidance 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 (temperature; [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 current density; primary and complete passivation potentials; pitting potential). Due attention 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 experimental 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 (Johnson Matthey) embedded in Teflon. The corrodent was nitrogen purged 0.5 M H2SO4 at 30 ± 0.5 C. The electrode assembly, electrochemical cell and associated apparatus were similar to those described by Schweinsberg and Ashworth [30]. The reference and 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 manually 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
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Table 1 Menus incorporated in SYMADEC Drop-down menus
Parameters
Notes 1
Menu 1: Redox inputs
pH; [O2] (mg L ); T (K); Parameters for calculation of Erev for reactions: [Mn+] (0.056 mg L1) 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 decade1); i0 (A cm2); iL (A cm2)
Parameters to synthesise cathodic curve for H+ reduction
Menu 3: Oxygen inputs
Tafel slope (V decade1); i0 (A cm2); iL (A cm2)
Parameters to synthesise cathodic curve for O2 reduction
Menu 4: Metal: active inputs
Tafel slope (V decade1); i0 (A cm2)
Parameters to synthesise anodic curve up to Ep
Menu 5: Metal: passivation to film breakdown inputs
icp (A cm2); Ep (V); Ecp (V); Ebr (V); p; A; Tafel slope after film breakdown (V decade1)
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 was then transferred from the cell under positive N2 pressure to a waste bottle and immediately 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). The WE was then polarised cathodically (20 mV steps) to 576 mV (SHE) (current was 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
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Potential vs SHE (mV)
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Potential vs SHE (mV)
(a)
(b) Fig. 1. Case 1. (a) Experimental and synthesised polarisation curves for pure iron in O2-free 0.5 M H2SO4 at 25 C. (b) Deconvolution of synthesised polarisation curve for pure iron in O2-free 0.5 M H2SO4 at 25 C.
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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 polarisation 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 L1 (106 M) may be used to calculate Erev for 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 = 106 M and pH 0, at the mean Ecorr (here about 320 mV (SHE) from the experimental curve), and for the more positive potentials applied in the experiment), Fe reacts to form Fe2+ ions. By deconstructing the matched synthetic curve the anodic and cathodic components 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 polarisation can 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 cm2). 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 from the 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 recorded galvanostatically by Bandy and Jones [31] for 1080 carbon steel (nominal comp. 0.75–0.88% C; 0.60–0.90% Mn; 0.04 max P; 0.05 max S) immersed in oxygen-free 0.5 M H2SO4. Conditions for recording the experimental curve were as follows: 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 bubbling 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 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 [O2] (mg L1) [Fe] (mg L1) Temperature (K) H2 TS (mV dec1) H2 ECD (A cm2) H2 LCD (A cm2) O2 TS (mV dec1) O2 ECD (A cm2) O2 LCD (A cm2) Fe TS (mV dec1) Fe ECD (A cm2) Fe PPP (mV) Fe CPP (mV) Fe BP (mV) Fe TTS (mV dec1) Fe CPC (A cm2) Res (X) Fe Exp Fe Lin Fe R (mV) O2 R (mV) H2 R (mV)
0 – 0.056 303 98 8.48E8 – – – – 48 8.84E11 – – – – – 0 – – 621 – 0
0 – 0.056 298 98 2.43E6 7.58E2 – – – 39 1.67E12 – – – – – 1 – – 618 – 0
0 – 0.056 298 98 2.43E6 7.58E2 – – – 39 1.67E12 – – – – – 0 – – 618 – 0
4.9 2 0.056 308 130 3.86E8 1.00E4 133 3.00E13 9.61E5 64 1.94E7 – – – – – 90 – – 623 876 300
5.52 3 0.056 316 134 1.00E5 2.27E3 127 1.00E11 3.51E4 39 7.11E9 – – – – – 0 – – 629 819 346
9 0.01 0.056 303 151 1.00E8 – 151 4.58E14 2.14E5 142 3.49E7 525 250 79 114 3.00E6 543 2 1.00E4 621 589 541
9 0.2 0.056 303 101 1.00E7 1.24E5 171 1.43E10 3.41E5 98 4.55E7 575 164 90 65 2.92E6 3228 2 1.00E4 621 609 541
9 7.9 0.056 303 169 1.00E7 – 152 3.92E11 5.09E5 168 5.56E7 420 210 100 158 1.43E6 4900 2 1.70E4 621 642 541
12.3 8 0.056 298 120 1.31E6 – 153 1.35E11 2.56E5 159 7.72E7 732 494 – – 1.41E6 0 2 1.00E4 618 447 727
8.8 0.01 0.056 313 30 5.77E6 – 159 5.06E10 8.55E6 144 8.38E7 480 148 138 150 4.01E6 321 2.01 1.00E4 627 580 547
7 8 0.056 313 130 4.18E6 – 180 5.00E11 6.50E5 30 1.25E7 580 410 152 60 3.01E6 4000 2.14 3.97E4 627 761 435
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Parameters
Notes: 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. 2135
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(a)
(b)
(c)
(d)
Fig. 2. Case 2. (a) Experimental and synthesised polarisation curves for carbon steel in O2-free 0.5 M H2SO4 at 25 C assuming presence of non-passive surface film. (b) Deconvolution of synthesised polarisation curve for carbon steel in O2-free 0.5 M H2SO4 at 25 C assuming presence of non-passive surface film. (c) Experimental and synthesised polarisation curves for carbon steel in O2-free 0.5 M H2SO4 at 25 C together with synthesised curve assuming no surface film. (d) Deconvolution of synthesised polarisation curve for carbon steel in O2-free 0.5 M 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 as Ecorr. The current was then adjusted to give six increments per decade on a logarithmic scale. The cathodic and then anodic potentials were recorded after 3-min intervals. 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). However, 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 polarisation curve (synthesised curve shown in Fig. 2a) are listed in Table 2 (Case 2 filmed).
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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 L1. 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 = 106 M and pH = 0) at Ecorr (270 mV (SHE)) and for more positive potentials applied in the experiment, it can be 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 curve indicates that on polarisation from Ecorr in the negative direction the hydrogen reaction 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, conducting 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 values 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 behaviour. In their paper both cathodic and anodic Tafel lines are seen to intersect approximately 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 values. 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 Joness experimental curve shown in Fig. 2a. The anodic portion of the experimental polarisation curve (assuming no film and therefore no ohmic resistance) can also be synthesised 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 intersecting 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 SYMADEC is an alternative approach to that employed by Bandy and Jones. Their estimated corrosion rate and cathodic and anodic Tafel slopes were 1.18 ·
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103 A cm2, 98 mV dec1 and 38 mV dec1, respectively, whilst the calculated corrosion rate and corresponding parameters used by SYMADEC for curve synthesis and matching were 1.39 · 103 A cm2, 98 mV dec1 and 39 mV dec1, 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 that these steels behave like the pure metal and that in strong acidic solution Tafel behaviour 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 recorded 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 crushed cane. Preliminary experiments showed that mild steel, on exposure to MJ, becomes coated with a grey/black, porous, non-passivating film which was found to consist mainly 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) respectively. The WE was abraded with 600 grade SiC paper, degreased with AR grade acetone and then immediately exposed to the MJ contained in a 1 L glass cell. The dissolved oxygen concentration was 2 mg L1 and Ecorr (445 mV (SHE)) was steady after 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 the WE 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 min1. Parameters and data required to synthesise and match the experimental polarisation curve (shown in Fig. 3a) are listed in Table 2. For mild steel the dominant anodic 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 L1 and calculated Erev for the O2/H2O system is +876 mV (SHE). The total dissolved iron (ICP analysis) in the MJ was 25 mg L1 but because of the possibility of complexing with organics and other species in the MJ the concentration of dissolved iron as Fe2+ is probably much less than 25 mg L1. The exact value of [Fe2+] was not established and for curve synthesis 0.056 mg L1 (106 M) was used. This value can be justified in that it is accompanied by an acceptable i0 of 1.94 · 107 A cm2 for the Fe/Fe2+ system. Erev for the Fe/Fe2+ system is 623 mV (SHE)).
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(a)
(b)
(c)
(d)
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Fig. 3. Case 3a. (a) Experimental and synthesised polarisation curves for mild steel in dormant MJ at 35 C (100 min exposure; 2 mg L1 O2) assuming presence of non-passive surface film. (b) Deconvolution of synthesised polarisation curve for mild steel in dormant MJ at 35 C assuming presence of non-passive surface film. (c) Experimental and synthesised polarisation curves for mild steel in dormant MJ at 35 C (100 min exposure; 2 mg L1 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 C and here a higher temperature (35 C) and carbon steel is involved, the diagram can 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 applied 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 reaction, 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 synthesised curve seen in Fig. 3a are shown. The deconvolution reveals that at Ecorr the dominant cathodic reaction driving the corrosion is oxygen reduction.
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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 required to 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) can also be synthesised and this, a straight line exhibiting Tafel behaviour, is shown together 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 C was 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 tubing) and ground so that they were flush with the internal wall of the tubing. A commercial 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 min1 (2 m s1). 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), dissolved oxygen concentration (3 mg L1) and Ecorr (440 mV (SHE)) were steady after 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 deposited on the WE during the exposure period). This was followed by the cathodic scan when Ecorr had returned to within ±5 mV of its previous value. The scan rate was 60 mV min1. A black film was deposited on the WE during establishment of the corrosion potential 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. The experimental 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 corrosion rate can now be estimated by extrapolating the anodic Tafel line to the corrosion 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 polarisation curve are listed in Table 2. The dominant anodic reaction is again iron dissolu-
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(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 L1 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 L1 O2) assuming no surface film.
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tion driven by H+ ion and O2 reduction. The pH of the MJ was 5.52, and the calculated Erev for the H2/H+ system is 346 mV (SHE). The O2 concentration was 3 mg L1 and the calculated Erev for the O2/H2O system was +819 mV (SHE). As in Case 3(a) an [Fe2+] = 0.056 mg L1 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 positive potentials applied in the experiment) the anodic reaction is principally the dissolution 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 Ecorr the 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 reactions contribution to the total cathodic current becomes 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 corrosion they 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 OtienoAlego 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 synthetic 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 saturated calomel electrode (SCE) connected to a Luggin capillary was used as the reference electrode. The temperature was 30 C and the solution pH = 9.0. Bottled nitrogen gas (containing traces of oxygen) was passed continuously through the corrodent and this resulted in a dissolved oxygen level of approximately 0.01 mg L1. 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) for 10 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
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Potential vs SHE (mV)
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Potential vs SHE (mV)
(a)
(b) Fig. 5. Case 4a. (a) Experimental and synthesised polarisation curves for low-alloy steel in synthetic condensate at 30 C (0.01 mg L1 O2). (b) Deconvolution of synthesised polarisation curve for low-alloy steel in synthetic condensate at 30 C (0.01 mg L1 O2).
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polarisation when Ecorr had returned to within ± 5 mV of the previous value. The polarisation scan rate was 10 mV min1. In this case there are two oxidants (H+ ion and small amount of O2) driving corrosion. Ecorr was 539 mV (SHE). Although low-alloy steel is corroding, the material is approximately 93% Fe, and the Pourbaix diagram for pure iron is a reasonable guide 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 corroding 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 corrosion 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 at more positive potentials by a rapid increase in current density suggesting in the presence of Cl pitting corrosion. Otieno-Alego et al. [32] reported that pits were observed 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 L1, 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 relatively 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 portion of the experimental curve before onset of passivation is also curved and cannot be used 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 L1 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 passivates 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
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(a)
(b)
Fig. 6. Case 4b. (a) Experimental and synthesised polarisation curves for low-alloy steel in synthetic condensate at 30 C (0.2 mg L1 O2). (b) Deconvolution of synthesised polarisation curve for low-alloy steel in synthetic condensate at 30 C (0.2 mg L1 O2).
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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 passivation 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 L1, Erev for the Fe/Fe2+ system is 621 mV (SHE). The deconvoluted anodic and cathodic components of the synthesised curve are shown 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 passivation 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 experimental curve. The alloy, on exposure to a synthetic steam turbine condensate in which the oxygen concentration is 0.20 mg L1 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 the tip 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 increased to 7.9 mg L1. Ecorr was 80 mV (SHE) and the Pourbaix diagram shows that at this potential at pH 9.0 pure iron spontaneously passivates with the formation 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), suggests that the low-alloy steel on immersion in the corrodent may have undergone spontaneous passivation followed by localised (pitting) corrosion. Thus there is now sufficient 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 experimental curve are listed in Table 2. Erev for the H2/H+ system is 541 mV (SHE) with
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(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 L1 O2). (b) Deconvolution of synthesised polarisation curve for low-alloy steel in synthetic condensate at 30 C (7.9 mg L1 O2).
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Erev for the O2/H2O system +642 mV (SHE). Using a minimum value of [Fe2+] = 0.056 mg L1, 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 portions are shown in Fig. 7b and this reveals that at Ecorr the localised corrosion is dominated 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 corrosion 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 oxygen) can determine whether an alloy on immersion in the corrodent experiences active 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 L1; 25 C). The solution was open to air ([O2] 8 mg L1) and the pH = 12.3. The WE (abraded with 1200 grade SiC paper and degreased with AR grade acetone) was immediately placed in the test solution 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 min1 to approximately +740 mV (SHE). In this case at pH = 12.3 and dissolved oxygen is the main oxidant driving the corrosion. Ecorr is apparent from the experimental curve (345 mV (SHE)). The Pourbaix diagram for pure Fe shows that at pH = 12.3, and as the potential is made more positive, the metal oxidises to form firstly soluble HFeO 2 ions, followed by passivation 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 immersed 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 also shown 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 L1, 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 experimental 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
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(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 L1 O2). (b) Deconvolution of synthesised polarisation curve for mild steel in distilled water containing commercial detergent at 25 C (8 mg L1 O2).
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compared with Case 4(b). The size of the passivation peak is markedly reduced because; 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 the experimental 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 L1 NaCl and 150 mg L1 of an oxygen scavenger (activated hydrazine hydrate (LEVOXINTM)). The oxygen concentration during polarisation was measured as 0.01 mg L1 and the pH of the solution was 8.8. The WE (abraded with 1200 grade SiC paper and degreased with AR grade acetone) was placed in the test solution and pre-polarised at 580 mV (SHE) for 5 min to remove any residual oxide film. The electrode was then immediately polarised in the positive direction (20 mV min1) through to approximately +300 mV (SHE). The activated hydrazine hydrate, in addition to reducing the oxygen concentration, 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 the H+ 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 potential 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 be partly resolved by referring to Lienings schematic diagrams [1]. He shows that such a curve will arise when the concentration (diffusion) controlled portion of the true cathodic curve intersects the true anodic curve at two points on the active/passive nose, and the activation-controlled portion intersects the passive region. There is only 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 experiment (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 assumed that at this potential and for pH = 8.8 and in the presence of the dissolved oxygen the 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 results in an active/passive transition followed by a cathodic loop. At even higher applied 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
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(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 L1 O2). (b) Deconvolution of synthesised polarisation curve for mild steel in NaCl salt solution plus O2 scavenger (LEVOXIN) at 40 C (0.01 mg L1 O2).
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Erev for the O2/H2O system +580 mV (SHE). Using a minimum value of [Fe2+] = 0.056 mg L1, 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 transition from the experimental curve. The deconvoluted anodic and cathodic components of the synthesised/matched curve are shown in Fig. 9b. This clearly shows 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 reduction 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 undergoing combined activation and concentration polarisation when it intersects with the actual anodic curve in the passive region (where a stable passive film has formed) and at a more negative potential (where the film is unstable). These points of intersection are responsible for the cathodic loop with the current density 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 L1 NaCl and 100 mg L1 of a commercial inhibitor for iron (zinc phosphinocarboxylic acid (ZnPCA)). An extra 15 mg L1 of zinc was added (as zinc sulphate) 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 L1. 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 remove any residual oxide film. The electrode was then polarised in the positive direction (20 mV min1) to approximately +100 mV (SHE). In this case at pH = 7.0 and [O2] = 8 mg L1 the main oxidant driving the corrosion is dissolved oxygen. Although Ecorr was not measured after the cathodic, prepolarisation 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 to Fe2+ 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]. Inhibition 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 proposed that Zn(II) reacts with the PCA and the resulting zinc complex (ZnPCA)
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(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 L1 O2). (b) Deconvolution of synthesised polarisation curve for mild steel in NaCl salt solution plus Zn-augmented ZnPCA at 40 C (8 mg L1 O2).
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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 an active/passive transition, and the current density of the concentration controlled portion of the cathodic curve is just greater than that at the tip of the active/passive nose. Evidence for the Zn-augmented ZnPCA promoting the formation of a passive 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 corrosion 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 passivation/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 L1, Erev for the Fe/Fe2+ system is 627 mV (SHE). It is again impossible to estimate the values of parameters for 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 with the 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 shows the corrosion potential more positive than the pitting potential resulting in spontaneous pitting.
5. Conclusions • Experimental polarisation curves for the corrosion system Fe/H2O/H2/O2 can be synthesised 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 anodic and cathodic components operative in the following corrosion systems: active corrosion; active corrosion and non-passive film formation; active corrosion followed by induced passivation and induced pitting; spontaneous passivation and induced pitting; spontaneous passivation and spontaneous pitting. Curves exhibiting either a cathodic loop or a cathodic dip can also be analysed. • The accurately analysed curves replace schematic representations and are a valuable reference source for the interpretation of experimental curves for the aqueous corrosion of pure iron/carbon/low-alloy steels.
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Acknowledgments The authors wish to thank the School of Physical and Chemical Sciences for providing 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 polarisation curves. References [1] E.L. Liening, in: B.J. Moniz, W.I. Pollock (Eds.), Process Industries Corrosion, NACE, 1986, p. 85. [2] O.W. Siebert, in: G.S. Haynes, R. Baboian (Eds.), Laboratory Corrosion Tests and Standards, ASTM STP 866, ASTM, Philadelphia, 1985, p. 65. [3] O.F. Devereux, Corrosion 35 (1979) 125. [4] O.F. Devereux, K.Y. Kim, Corrosion 36 (1980) 262. [5] J.G. Hines, J.H. Cleland, Proc. 8th Int. Congr. Metall. Corros. Mainz. 2 (1981) 1959. [6] S.O. Berhardsson, R. Mellstrom, in: F. Mansfeld, U. Bertocci (Ed.), Electrochemical Corrosion Testing, ASTM STP727, 1981, p. 352. [7] R.S. Munn, Mater. Perform. 22 (August) (1982) 29. [8] O.F. Devereux, K.Y. Kim, K.S. Yeum, Corros. Sci. 23 (1983) 205. [9] J.G. Hines, Br. Corros. J. 18 (1983) 10. [10] J.H. Cleland, C. Edeleanu, Br. Corros. J. 18 (1983) 15. [11] P.A. Brook, J.S.L. Leach, B.R. Pearson, in: Proc. 166th Meeting of the Electrochem. Soc., Louisiana, USA, 1984, p. 243. [12] H.E.H. Bird, B.R. Pearson, P.A. Brook, Corros. Sci. 28 (1988) 81. [13] K.S. Yeum, O.F. Devereux, Corrosion 45 (1989) 478. [14] B.R. Pearson, P.A. Brook, Corros. Sci. 32 (1991) 387. [15] K.R. Trethewey, J.S. Keenan, Corros. Prev. Control. 89 (August) (1991). [16] K.R. Trethewey, J.S. Keenan, I. Wilson, Corros. Prev. Control. 115 (October) (1991). [17] K.R. Trethewey, J.S. Keenan, in: R.S. Munn (Ed.), Microcomputer-based Corrosion Modelling Applied to Polarisation Curves, ASTM STP 1154, ASTM, Philadelphia, USA, 1992, p. 113. [18] S. Nesic, J. Postlethwaite, S. Olsen, Corrosion 52 (1996) 280. [19] A. Anderko, P. McKenzie, R.D. Young, Corrosion 57 (2001) 202. [20] D.W. Shoesmith, in: CorrosionMetals Handbook, vol. 13, ASM International, Metals Park, OH, USA, 1987, p. 29. [21] H.J. Flitt, in: Proc. 7th RACI Electrochemistry Conf., Australia, 1988, p. 287. [22] H.J. Flitt, G.A. Cash, D.P. Schweinsberg, in: Proc. 7th European Symp. on Corrosion Inhibitors, Ann. Univ. Ferrara, Italy, 1990, p. 1435. [23] V. Otieno-Alego, G.A. Hope, H.J. Flitt, G.A. Cash, D.P. Schweinsberg, Australasian Corrosion Association Conference No. 31, Sydney, paper F09, 1991. [24] G.A. Cash, Ph.D. Thesis, Griffith University, Brisbane, Queensland, Australia. [25] V. Otieno-Alego, G.A. Hope, H.J. Flitt, G.A. Cash, D.P. Schweinsberg, Corros. Sci. 33 (1992) 1719. [26] V. Otieno-Alego, G.A. Hope, H.J. Flitt, D.P. Schweinsberg, Corros. Sci. 34 (1993) 1289. [27] V. Otieno-Alego, G.A. Hope, H.J. Flitt, D.P. Schweinsberg, Corros. Sci. 37 (1995) 509. [28] V.S. Bagotzky, Fundamentals of Electrochemistry, Plenum Press, New York, 1993. [29] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, 1974. [30] D.P. Schweinsberg, V. Ashworth, Corros. Sci. 28 (1988) 539. [31] R. Bandy, D.A. Jones, Corrosion 32 (1976) 126; see alsoD.A. Jones, Principles and Prevention of Corrosion, Macmillan, New York, 1992.
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[32] V. Otieno-Alego, G.A. Hope, H.J. Flitt, D.P. Schweinsberg, Corros. Sci. 35 (1993) 103. [33] Personal Communication from Dr. H.J. Flitt. [34] (a) A. Harris, A. Marshall, Corros. Prev. Control. (June) (1980) 18; (b) A. Harris, A. Marshall, Corros. Prev. Control. (August) (1980) 17.
