inﬂuence of cold working on the pitting corrosion resistance of...

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Corrosion Science xxx (2007) xxx–xxx

www.elsevier.com/locate/corsci

Influence of cold working on the pittingcorrosion resistance of stainless steels

L. Peguet a,b,*, B. Malki a, B. Baroux a

a Institut National Polytechnique de Grenoble, ENSEEG/LTPCM, 1130, rue de la piscine,

Saint Martin d’Heres 38402, Franceb UGINE&ALZ Research Center (Arcelor), rue Roger Salengro, Isbergues 62330, France

Received 26 May 2006; accepted 4 August 2006

Abstract

This paper addresses the influence of cold rolling and tensile deformation on the pitting corrosionresistance of AISI 304 and AISI 430 stainless steels, investigated using some electrochemical tech-niques specifically designed for the different pitting stages to be analyzed separately. Cold work isshown to act differently depending on the pitting stage under consideration. (i) The pit initiation fre-quency shows a maximum after 20% cold-rolling reduction or 10% tensile deformation. This maxi-mum is also observed on the ferritic grade, contradicting the hypothesis of a direct effect of strain-induced martensite, and is more likely related to the dislocations pile-ups. (ii) The pit propagationrate increases monotonously with cold rolling reduction, and pit repassivation ability decreases(leading to a larger number of stable pits), suggesting that the overall dislocation density is the con-trolling factor in these stages. Last, the significance of pitting potential measurements is discussed inthe light of the effect of the cold-rolling reduction on the measured values.� 2006 Published by Elsevier Ltd.

Keywords: A. Stainless steel; C. Pitting corrosion; C. Effects of strain

0010-938X/$ - see front matter � 2006 Published by Elsevier Ltd.

doi:10.1016/j.corsci.2006.08.021

* Corresponding author. Address: Institut National Polytechnique de Grenoble, ENSEEG/LTPCM, 1130, ruede la piscine, Saint Martin d’Heres 38402, France. Fax: +333 21 63 20 56.

E-mail address: [email protected] (L. Peguet).

Please cite this article in press as: L. Peguet et al., Influence of cold working on the pitting ...,Corros. Sci. (2007), doi:10.1016/j.corsci.2006.08.021

mailto:[email protected]

2 L. Peguet et al. / Corrosion Science xxx (2007) xxx–xxx

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1. Introduction

Pitting corrosion of stainless steels (SS) results from a combination of electrochemicaland metallurgical factors including the effect of alloying elements and the nature and dis-tribution of the non-metallic inclusions, which have already been extensively studied [1].However, the metallurgical aspect of the question has been insufficiently taken intoaccount, especially for industrial steels. Accordingly, this paper aims to contribute tothe understanding of pitting mechanisms as related to material structure by focusing onthe influence of cold working.

Various metallurgical variables are likely to be affected by cold working: first of all, thecreation and slipping of dislocations, which lead to plastic deformation of metals [2]. Sec-ondly, austenitic grades can also be sensitive to martensitic transformation induced bycold working at room temperature [3], according to the sequence c(!e)! a 0 [4,5]. Lastly,plastic deformation may produce inclusions, elongation, or fractures at the interface withthe matrix [6–8]. The detrimental effect of cold working on carbon steel or iron corrosionresistance is well known. It was evidenced by weight loss in acidic media [9–16] but wasalso noted in chloride-containing media [17], including tests in real environments[18–21]. On the other hand, only a few results are available on the localized corrosionof cold-worked SS, and such highly scattered data do not permit scientifically reliableconclusions.

Both the thickness and composition of passive films are likely to be modified in manyways by cold working [22–24]. Moreover, transfer resistance and interfacial capacitance donot appear to change linearly as a function of cold reduction [25]. The related passive cur-rent in acidic media is found to increase with deformation by several orders of magnitude[26,27], in agreement with release tests [28], but an opposite result has been reported in achloride-containing medium [22]. Regarding the time to pit nucleation as a function ofcold reduction, it is found either to decrease [29,30] or to be unaffected [31], while the sta-bilization of metastable pits initiated on cold-worked SS is enhanced [32].

The effect of plastic deformation on the pitting potential (Vpit) appears to be rather lim-ited compared to the influence of chloride concentration or addition of alloying elements.The variations in Vpit rarely exceed a few tens of millivolts and available data (Table 1) arehighly inconsistent. In addition, recent work has demonstrated non-monotonous varia-tions with cold reduction [25,33,34]. The number of pits counted by metallographic exam-ination has been reported either to increase with cold-working rate, including localextrema [31], or to be unaffected [35]. Some authors have even qualitatively linked pit siteswith deformation bands [33], or with interfaces between austenite and martensite inducedby cold working [26].

Regarding the propagation current density of pits, it is generally found to increase as afunction of cold working [31,36] but not in every case [36]. This dispersion is not surpris-ing, considering that SS dissolution in acidic media has been reported to be higher [37–39],lower [40], unaffected [41–43], and even non-monotonously modified [44] by plastic defor-mation. In the same way, the repassivation potential criterion appears very irregular as afunction of cold working [22,25,29,45]. Its trend slope depends both on grade composition[36] and the concentration of chloride in the medium [26].

The present study employs an approach based for the most part on AISI 304 SS, cold-rolled at various reductions, and using different electrochemical techniques supplementedby metallurgical characterizations. Such an approach makes it possible to examine the


Table 1Effect of plastic deformation on the pitting potential of stainless steels

Author Date Grade Deformation Reduction/strain Medium Vpit shift (comparedto annealed state)

B. Ravi Kumar [33] 2005 AISI 304L Cold rolling 10, 30, 50, 70, 90% NaCl 0.1 M #" (Minimum at 50%)S.V. Phadnis [22] 2003 AISI 304 Cold rolling 66% NaCl 3, 5% "

A. Barbucci [26] 2002 AISI 304 Cold rolling 35, 58% Na2SO4 0.3% + Cl� " (Low CCl� )(103! 5 · 104) ppm # (High CCl� )

U. Kamachi Mudali[33]

2002 AISI 316L Cold rolling 5, 10, 15, 20, 30,40%

NaCl 0.5 M "# (Maximum at 20%)316LN316LHN

V. A. C. Haanappel[25]

2001 AISI 304 Kitchen utensil – NaCl 35 g/L # (Lower deformation)" (Higher deformation)

D. Sinigaglia [72] 1983 AISI 316L Cold rolling 10, 30, 50% Physiological solution #16Cr–14Ni–2Mo !

G. Salvago [29] 1983 AISI 304L Cold rolling(�196 �C)

30, 50% HCl 0.1 M #

H. J. Torwie [73] 1981 Fe–19%Cr–(0, 15, 20,30%) Ni

Tensile test 0! 110% NaCl 100 ppm ! (ferritic 0% Ni)# (austenitic)

B. Mazza [45] 1979 AISI 304L Tensile test 0, 10, 15, 30% NaCl 3.5%, 40 �C #AISI316L Drawing 0, 10, 30, 50%

Cold rolling

B. Mazza [41] 1979 AISI 304L Tensile test 0–50% HCl 0.1 M #AISI316L Drawing

Cold rolling

C.J. Semino [35] 1979 AISI 304 Tensile test 30% (section) NaCl 0.5 M + NaHCO3

0.1 M#

AISI 316 #

B. C. Syrett [36] 1978 AISI 316L Cold rolling 0–37% Thyroid solution 37 �C "TRIP

A. Cigada [74] 1977 AISI 304L Tensile test 0–50% Physiological solution #B. Mazza [43] 1976 AISI 316L Drawing

Cold rollingA. Baghdasarian

[75]1975 TRIP Cold rolling 10, 20, 30% Artificial sea water "

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influence of cold working on the various pitting stages (initiation, propagation, andrepassivation) independently, and has proven efficiency [46]. In addition, both AISI 304and AISI 430 have been investigated after tensile testing so that the previous results couldbe compared to another deformation process as well as to a stable ferritic grade.

2. Experimental

2.1. Materials

The chemical compositions of the three investigated SS are shown in Table 2. AISI 304(Lab.) prepared in laboratory had undergone a cold-rolling deformation process. In addi-tion, AISI 304 and AISI 430 commercial grades (Arcelor) were scheduled for tensiletesting.

AISI 304 (Lab.) grade was cast as a 25 kg ingot under argon atmosphere. Next, plateswere obtained by hot and cold rolling with intermediate annealing, leading to 10%, 20%,30%, and 70% of final cold thickness reduction (Fig. 1). Disks with a diameter of 15 mmwere stamped from annealed and cold-rolled plates, to be investigated by electrochemicalmeasurements. The induced a 0-martensite content as well as mechanical properties includ-ing Vickers hardness and yield stress show a monotonous increase with cold-rolling reduc-tion (Table 3). A metallographic examination after potassium metabisulfite attack (10 simmersion in H2O 50 ml + HCl 10 ml + K2S2O5 0.3 g) reveals a homogeneous distribu-tion of martensite laths in cross section. Furthermore, examination after electro-nitricattack (30 s immersion in H2O 97 ml + HNO3 553 ml at 50 mA cm�2) shows an equiaxialstructure with a few residual primary ferrite islets in the annealed state, whereas the cold-rolling process induces the expected orientation of grains in the rolling direction. Neithermodifications of inclusions morphology nor evidence of decohesion between inclusionsand matrix were observed by scanning electronic microscopy. At a lower scale, disloca-tions structure arrangements analyzed by transmission electronic microscopy (TEM) arerepresentative of the transition through each cold-working stage [47,48]. Thus a planarstructure favouring dislocations pile-ups is seen at 20% of cold reduction, while at 70%the dislocations network evolves toward a cellular structure typical of a dynamic recoverystage (Fig. 2).

Regarding commercial grades, a tensile strain of 5 mm min�1 was applied to275 mm · 25 mm test specimens cut from plates 1 mm thick, from which 15 mm diameterdisks were stamped for electrochemical study. Engineering strains of 10%, 35%, and 60%for AISI 304 grade, and 10% and 20% for AISI 430 grade, were chosen, to take intoaccount the lower total elongation of the ferritic grade (25%) as compared to the austeniticone (70%). It was noted that tensile deformation of industrial grade AISI 304 induces alower a 0-martensite content and yield stress for a given strain compared to the equivalentcold-rolling reduction of AISI 304 (Lab.) grade (Table 4).

Table 2Chemical composition of the investigated materials in wt%

Grade Elaboration %C %Mn %Ni %Cr %Mo %Cu S (ppm) %N

AISI 304 Laboratory 0.026 1.45 8.57 17.86 0.20 0.20 51 0.036AISI 304 Industrial 0.037 1.42 8.66 18.18 0.25 0.22 12 0.038AISI 430 Industrial 0.042 0.38 0.16 16.26 0.03 0.04 21 0.027


Fig. 1. Cold-rolled plates processing of AISI304 (Lab.) grade.

Table 3a 0-Martensite fraction, Vickers harness, yield stress and chromium content in passive film of the AISI 304 (Lab.)grade as a function of cold-rolling reduction

Parameters Techniques Annealed 10% 20% 30% 70%

a 0-Martensite (%) Magnetization 2.9 14.7 21.5 44.3 58.0Hardness (HV) Microhardness 182 263 323 352 448Yield stress (MPa) Tensile test 268 657 784 995 1346Passive film Cr content (at.%) XPS analysis 23 – – 25 –

L. Peguet et al. / Corrosion Science xxx (2007) xxx–xxx 5

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Disks stamped from cold-rolled and strained SS were prepared following the same pro-cedure: specimens were polished with SiC paper to a 1200 grit finish and afterwards withdiamond paste up to 3 lm grade. They were degreased in an ultrasonic acetone/ethanolmixed bath, rinsed with distilled water, dried, and finally aged for 24 h in ambient air.It was noted that, following this procedure, the chromium content in passive film oxidelayers measured by XPS analysis was roughly the same for both 30% cold-rolled andannealed samples (Table 3).

2.2. Pitting potential measurements

Pitting potentials (Vpit) were measured in deaerated 0.5 M NaCl, pH 6.6 electrolyte at23 �C by leaving the sample at free potential for 15 min and then running a potentiody-namic scan at a constant scan rate (100 mV/min) until the current reached a value of10 lA, where Vpit was taken. The tests were repeated six times to provide a significantmean value. However, such a parameter may combine in a single criterion all the differentstages of the pitting process [46]. Accordingly, a few other selected electrochemical tech-


Fig. 2. TEM Images (white field) of AISI 304 (Lab.) grade (a) annealed, (b) 20% cold-rolled and (c) 70% cold-rolled.

Table 4a0-Martensite fraction and yield stress for the AISI 304 commercial grade as a function of strain

Parameters Techniques Annealed 10% 35% 60%

a0-Martensite (%) Magnetization 2.3 2.5 6.2 17.1Yield stress (MPa) Tensile test 322 471 657 700


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niques were employed, so that the various steps (i.e., pit initiation, propagation, andrepassivation) could be studied independently (Table 5).

2.3. Pitting transients at open circuit potential

Pitting transient measurement is carried out using a three-electrode setup operating atopen circuit potential [49,50]. This system includes two identical electrodes connectedthrough a zero resistance ammeter (ZRA) and a reference electrode (SCE), allowing simul-taneous measurement of both intensity and potential transients in aerated (0.1 M

Table 5Experimental techniques and related parameters used to characterize the pit evolution steps

Pit evolution step Experimental technique Measured parameters

Initiation Transients analysis Number of transients for 24 hInterfacial capacitance/Resistance

Propagation ‘‘PPR’’ Current density from pits

Stabilization/repassivation Acid injection ‘‘PPR’’ Free potential evolutionRepassivation potential



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NaCl + 2 · 10�4 M FeCl3) of (1 M NaCl + 10�3 M FeCl3). The variation of current signalwith time was first measured over 24 h periods. The typical signal shows a succession ofcurrent fluctuations or transients. Each of these was considered to be the signature ofthe appearance, propagation, and repassivation of a metastable pit [51–53] (i.e., a pit thathas repassivated after a few seconds of propagation). A sampling frequency of 18.75 Hzwas selected, to provide sufficient resolution for these events to be studied. In addition,the capacitance C and the transfer resistance R of the interface were determined for simul-taneous current and potential transients. The method of estimating C and R, including ananalysis of the electrical charge balance in all three of the system’s electrodes, is welldetailed elsewhere [50].

2.4. Pit propagation rate test

The pit propagation rate (PPR) test is a method first proposed by Syrett [54] in whichthe specimen is subjected to a potential cycle. In this adaptation, the potential is firstpotentiodynamically scanned in a deaerated 0.5 M NaCl, pH 6.6, 23 �C electrolyte at ascan rate of 10 mV/min from OCP to a given potential between free potential Vcorr andVpit. It is held at this potential for 10 min to obtain a steady-state of the current densityin the passive condition. The potential scan is then continued to potentials more noblethan Vpit until the current reaches a value of 10 mA, corresponding to the growth ofnumerous pits (whose diameters may reach a few tens of lm). The potential is thendecreased in a single step to a value between Vcorr and Vpit and held at this value for10 min. Since no new stable pits appear at potentials below Vpit, the recorded current isa measure of the rate of existing pit growth. After the potential is left at free potentialin order to repassivate the pits, the potential is scanned to the selected value once againto ensure that passivity is achieved. The average value of current during the 10-min pitgrowth period is determined by graphic integration of the current vs. time recording.The total area actually pitted (total projected pit area) is determined by microscopic exam-ination (Fig. 3), and the pit propagation rate is calculated by dividing the average currentby the area actually pitted.

Fig. 3. Example of a sample surface submitted to ‘‘PPR’’ test after image analysis giving the number of pits andtheir mouth area.



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2.5. Depassivation test

The ‘‘depassivation test’’, based on free potential measurements during acid injection,was carried out in order to assess the passive film stability. Samples were left for 15 minin a 1 L cell of water at pH 4.5 before injecting concentrated H2SO4 doses according tothe following sequence (C being the reference concentration of a 95% H2SO4 solution):

(i) 1 ml dose of 10�2 C H2SO4 solution every minute for 5 min.(ii) 1 ml dose of 10�1 C H2SO4 solution every minute for 5 min.

(iii) 1 ml dose of C H2SO4 solution every minute for 5 min.(iv) 5 ml dose of C H2SO4 solution every minute for 5 min.(v) 10 ml dose of C H2SO4 solution every minute until depassivation.

Injections are continued until a sharp decrease of free potential is achieved, correspond-ing to overall surface depassivation (Section 3.3).

3. Results

3.1. Measuring pitting potentials and transients

The values of the pitting potential (Vpit) of cold-rolled AISI 304 were experimentallydetermined in deaerated 0.5 M NaCl, pH 6.6, at 23 �C (Table 6). Vpit is found to decreaseslightly in a monotonous manner up to 70% cold-rolling reduction. To investigate thisfurther, the pitting corrosion initiation stage was, studied by focusing on the numberof metastable pitting transients initiated at open circuit potential in NaCl 0.1 M + FeCl32 · 10�4 M (Fig. 4). The pit initiation frequency increases with the cold-rolling rate.However, the number of metastable pits shows a maximum for 20% cold reduction,where it is four timer, higher than in the annealed state. Both interfacial capacitance(C) and transfer resistance (R) also exhibit a non-monotonous trend: C shows amaximum for 20% while R seems to reach a minimum at around 10% cold reduction(Table 6).

This rather surprising maximum in initiation frequency was confirmed by studying theinfluence of tensile deformation on both the AISI 304 and AISI 430 experimental grades.

Table 6Indicators related to AISI 304 laboratory grade deduced from electrochemical techniques as a function of cold-rolling reduction

Measured parameter Experimentaltechnique

Annealed 10% 20% 30% 70%

Vpit (mV/SCE) in 0.5 M Potentiodynamic 325 300 289 283 272

C (lF/cm2) Transients analysis 74 84 89 83 71R (kX cm2) 6043 2118 2835 4168 4005

Vrep (mV/SCE) PPR 200 200 100 50 50Number of pits after 200 mV/SCE

polarization78 146 165 189 212


Fig. 4. Number of metastable pits initiated during 24 h on AISI 304 grade at free potential in NaCl0.1 M + FeCl3 2 · 10�4 M as a function of cold reduction.


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Indeed, a similar counting of metastable pitting transients at open circuit potential during24 h in NaCl 1 M + FeCl3 10�3 M showed a significant rise in the initiation frequency fora 10%-elongation (Table 7).

3.2. Analysis of transients and pit propagation rate

The pitting propagation stage was investigated both at mesoscopic and macroscopicscales. The mesoscopic propagation stage (i.e., corresponding to a dissolved charge onthe order of lC) was assessed by statistical analysis of metastable pits initiated at open cir-cuit potential during 24 h in NaCl 0.1 M + FeCl3 2 · 10�4 M. The lifetime of each tran-sient, its maximum intensity and the corresponding dissolved charge with maximumintensity exceeding a threshold of 235 nA are the main parameter considered in our anal-ysis. Then, using a power law of the anodic current [55]: I = k · ta where t is the tune, anda and k are constants deduced from the mean statistical values of previous parameters, anaverage transient shape was reproduced in Fig. 5 for 0%, 20%, and 70% cold-rolling reduc-tions. The highest propagation rate is again observed for 20% cold reduction. Neverthe-less, this does not lead to a higher intensity at the top of the transient; it is insteadlinked to a faster repassivation. Moreover, a significant increase of pit lifetime is seen inthe 70% cold-rolled sample. Finally, the resulting charge dissolved from the overall

Table 7Number of metastable pits initiated during 24 h on industrial AISI 304 and AISI 430 grades at free potential inNaCl 1 M + FeCl3 10�3 M as a function of strain

Annealed 10% 20% 35% 60%

AISI 304 116 297 – 64 40AISI 430 371 515 309 – –



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metastable pits at the end of the 24-h test monotonously increases as a function of cold-rolling reduction (Fig. 6).

The macroscopic propagation stage was investigated under applied potential by apply-ing the PPR test to annealed,, 20%, and 70% cold-rolling reductions. In this case, a signi-ficant and monotonous increase of the dissolution current density with cold-rolling ratewas found (Fig. 7).

Fig. 5. Average transient shapes for cold-rolled AISI 304 deduced from statistical analysis of current transientsrecorded during 24 h in NaCl 0.1 M + FeCl3 2 · 10�4 M.

Fig. 6. Total dissolved charge by metastable pitting during 24 h for AISI 304 in NaCl 0.1 M + FeCl3 2 · 10�4 Mas a function of cold rolling rate.


Fig. 7. Propagation current density of macroscopic pits measured by ‘‘PPR’’ test on AISI 304 as a function ofcold-rolling reduction and applied potential.

Fig. 8. Free potential evolution for AISI 304 submitted to H2SO4 injection as a function of cold-rollingreduction.


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3.3. Repassivation and passivity

The repassivation potential of AISI 304 as assessed by the PPR test is found to decreasewith cold rolling (Table 6). At the same time, an increase of macroscopic pits number isnoted for a given polarization (Table 6). In addition, the passive film growing on thecold-rolled surface is found to be less stable. Indeed, the higher the cold-rolling reduction,the earlier a sharp decrease of free potential is produced by acid injections (Fig. 8). Assum-ing a local acidity in the pits, the larger depassivation pH (the critical value of acidity caus-ing instability of the passive film followed by general corrosion) for cold-rolled samples isbelieved to prevent pit repassivation.



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4. Discussion

The present study has demonstrated two distinct effects of cold working on SS localizedcorrosion, depending on the stage and geometrical scale of the pitting process: the first is anon-monotonous trend as a function of deformation while the second is fairly linear. Inaddition, use of the pitting potential technique as a tool for measuring susceptibility topit initiation is re-examined.

4.1. Non-monotonous trend at mesoscopic scale

At the initiation stage, the main unexpected-indeed, relatively rare-behaviour [25,33,34]is the non-monotonous dependence on cold-working reduction. A maximum of metastablepits number is found at 20% of cold-rolling reduction or 10% of tensile deformation. Thisbehaviour, seen even in ferritic grades, contradicts the suggestion that strain-induced mar-tensite is the main factor governing this preliminary corrosion step. It also eliminates anycorrelation with the linear evolution of macroscopic metallurgical parameters such as yieldstress or hardness. Many authors acknowledge that the absolute amount of martensite hasvery little importance [26,33,37,45,56–58] and can hardly be separated from the influenceof dislocations and internal stresses. An understanding of the mechanisms involved wouldrequire a smaller-scale analysis owing to the fact that in stainless steels, pit initiation is clo-sely related to passive film stability and to non-metallic inclusions acting as triggers forfuture pits. First, the passive film is assumed either to be weakened [26,35,41], unaffected[31], or strengthened [22,25] by cold working, which is attributed in a recent study to ahigher Cr/Fe ratio [22]. Secondly, the shapes of inclusion are likely to affect the pitting[59,60], and cracks in the inclusions or at the inclusion-matrix boundary [6–8] are thoughtto be detrimental to initiation [45]. Nevertheless, neither sufficient morphological modifi-cations of the inclusions nor changes in the chemical composition of the passive film(Table 3) have been found to explain such a result in the present work. TEM analysis sug-gests that it may rather be related to dislocations features produced during the variouscold-working stages. This interpretation is supported by TEM images showing planardeformation structures for 20% cold rolling while development of cellular structures isobserved at higher reductions (Fig. 2). Multiplication of dislocations as well as theirarrangement into pile-ups induce high stress concentrations [2] and are likely to modifythe local deformation potential [61]. A mechano-electrochemical model has even beendeveloped, indicating that a maximum of dislocations pile-ups is likely to increase thepotential difference between inclusion and surrounding matrix [62], such local electro-chemical heterogeneity is expected to enhance pit initiation. Moreover, some workers havenoted that the specific combination of MnS inclusion and applied mechanical stress couldaffect the susceptibility to pitting [63]. Considering that martensitic transformation pro-ceeds by oriented rapid dislocations movements and multiplication [64], a possible indirecteffect of martensite cannot be ruled out to the extent that it favours planar structures andthe stability of dislocations pile-ups. For the highest cold-rolling reductions, the dynamicrecovery process of dislocations followed by rearrangement into cellular structures [47,48]could limit the previous effect and cause the decrease observed in the initiation frequency.Furthermore, deformation potential is associated with lattice distortion, which provokeselectron transfer to ensure a constant Fermi level [65]. The resulting change in the surfacecharge associated with the double-layer rearrangement has already been detected experi-



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mentally by an increase in the total interfacial capacitance (C) [66]. A similar result isfound in the present work: a dependence of C on cold-rolling reduction (Table 6). It istherefore assumed that the C-value maximum for 20% cold rolling is connected to themaximum of dislocations pile-up.

A non-monotonous trend similar to the one governing the initiation frequency can beseen at the pit propagation stage. At mesoscopic scale, the maximum dissolution rateobserved for pitting transients at 20% cold rolling could also be related to the maximumin dislocations pile-ups on inclusions. Indeed, pile-ups have even been suggested to dis-solve preferentially [67]. If we then consider a micron-scale affected area where dissolutionis increased by pile-ups [62], the related dissolved volume can be compared with a currenttransient charge (lC). Only the mesoscopic propagation stage could be affected by thislocal dissolution activation. The very different result for the macroscopic propagationstage may support this hypothesis, as discussed in the following section.

4.2. Monotonous behaviour at macroscopic scale

The macroscopic pit propagation rate is found to increase monotonously with coldworking. We suggest that this is related more to the total density of dislocations inducedby cold working (macroscopic effect) than to local structures such as pile-ups (mesoscopiceffect). Plastic deformation is believed to increase the density of dislocations, which in turnenhances dissolution, owing to the presence of lower bonding energy points compared to‘‘perfect’’ crystals [13]. Such experimental evidence provides new insights into the validityof thermodynamic approaches for assessing the dissolution rates in metals containingstructural defects [61,62,68].

The repassivation ability, investigated with OCP technique, appears at first sight to benon-monotonous as a function of the cold-rolling reduction. Indeed, and early repassiva-tion of metastable pits is found for 20% cold rolling. Nevertheless, each current transientcorresponds to a potential transient [50], and the large potential drop related to the largedissolution rate for 20 may by itself induce an instantaneous repassivation. This trend is nolonger noted at applied potential, where a monotonic decrease of the repassivation poten-tial with cold working is observed. Furthermore, we find an increase in the lifetime ofmetastable pits with a high cold-rolling reduction (70%). These observations may probablybe attributed not only to the increase of pit dissolution rate but also to the linear degra-dation of passive film stability. The easier depassivation of steel as a function of cold-roll-ing reduction (Fig. 8) supports this point of view. Last, the plastic deformation of thesubstrate and especially the presence of dislocations is believed to weaken the protectivenature of the passive film [26,35,41].

4.3. Reappraisal of the so-called ‘‘pitting potential’’

Based on early descriptions the ‘‘breakdown potential’’ [71], the pitting potential (Vpit)is generally considered as a critical potential for the passive film breakdown, disregardingthe fact that passive ‘‘film breakdown and pit development are in fact two independentstages active at different scales [70]. An embryonic pit could be initiated at values lowerthan Vpit; the conditions for further stable pit development being met only if the potentialensures a sufficient metal dissolution rate in a given pit geometry [71]. From this view-point, Vpit measures better the ability of a pit embryo to grow up to an arbitrary intensity



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value (10 lA in the present work), believed to correspond to the onset of a stable micro-scopic pit. As the pit growth rate increases with cold working, it is not so surprising to findin this work that Vpit monotonously decreases with the cold rolling rate, whereas pit ini-tiation frequency exhibits a non-monotonous behaviour.

5. Conclusions

The present study has confirmed that the effects of cold working on stainless steel pit-ting phenomena are fairly complex, which may explain the inconsistencies in the literature.Nevertheless, it has made it possible to isolate the various results of cold reduction, whichdepend both on the stage and on the scale of the pitting process:

(i) At the initiation stage, a maximum of metastable pits is found for 10% tensile defor-mation or 20% cold-rolling reduction. This behaviour, seen even in ferritic grades,contradicts the theory that strain-induced martensite is the main factor governingsensitivity to corrosion. It is suggested, based on a mechano-electrochemicalapproach, that dislocations features resulting from the successive cold-workingstages better explain this trend.

(ii) Regarding the propagation stage at mesoscopic scale, the highest pit dissolution rateis obtained for 20% cold-rolling reduction, when dislocations piled up on inclusionsare believed to preferentially dissolve. On the other hand, the dissolution rateincreases monotonously at the macroscopic propagation stage, when the overall den-sity of dislocations is more probably the relevant factor.

(iii) Except some specific effects observed at rest potential, the repassivation ability alsodecreases when the cold-working rate increases, leading to a larger macroscopic pitdensity.

(iv) The pitting potential combines in a single criterion both the resistance to pit initia-tion, pit propagation, and pit repassivation. In the present work, Vpit more affectedby the propagation/repassivation stages than by pit initiation and then vary monot-onously with cold-rolling reduction.

References

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