Localized Dissolution Kinetics, Salt Films and Pitting Potentials

8/13/2019 Localized Dissolution Kinetics, Salt Films and Pitting Potentials

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CorrosionScience,Vol. 39, No. l&l 1, pp. 1771-1790, 9970 1997Elsevier Science LtdPrinted in Great Britain. All rights reserved001&938X/97 17.00+0.00

PII: soolo-938x 97)

LOCALISED DISSOLUTION KINETICS, SALT FILMS ANDPITTING POTENTIALSN. J. LAYCOCK and R. C. NEWMAN

UMIST, Corrosion and Protection Centre, PO Box 88, Manchester M60 IQD, U.KAbstract-Dissolution kinetics within artificial pit electrodes have been studied and related to the transition in realpits from metastability to stability. For 302 and 316 stainless steel, two regimes of growth were identified; mixedactivation/ohmic control was found at lower potentials, but at higher potentials growth was under diffusion controlwith a metal salt film present on the electrode surface. The transition potential, E-r, between these two regimes wasfound to increase linearly with the log of the limiting current density, ilim. A mode1 for the anodic dissolutionkinetics is proposed which accounts for this observation and predicts that real pits will also grow in either active orsalt filmed states depending on their current density. Pitting potentials, Epi,, and transition potentials, ET, weremeasured as a function of chloride concentration. The differences in Epit between 302 and 316, or between differentchloride concentrations for one alloy, were equal to the corresponding differences in ET measured at currentdensities in the range l-5 A cmm2, typical of real pits. This is consistent with the idea that E,i, is the potential abovewhich pits are able to progress from metastable to stable growth, and therefore depends on the kinetics of pitdissolution rather than the breakdown resistance of the passive film. Metastable pitting occurs at potentials wellbelow Epi,. The variation of E,,, with chloride concentration and molybdenum alloying is completely explained bythe effects of these variables on kinetics within the pit environment as manifested in the measurement of ET. On anabsolute scale, ET (at l-5 A cmm2) falls within the range of Epit values measured for surface finishes from 120 to1200 grit. 0 1997 Elsevier Science LtdKeywords: A. stainless steel, B. potentiostatic, C. pitting corrosion.

INTRODUCTIONThe initiation and propagation of pits can be considered separately, with the initiation stageinvolving breakdown of the passive film and the development of an aggressive localchemistry at the corroding site, whilst propagation involves the continued stable growth ofthe corroding area. Perhaps the most commonly measured quantity in pitting studies is thepitting potential, Epit, above which stable pitting is discernible for a given alloy andenvironment.-3 It was at first thought that this was the lowest potential at which pits couldinitiate, but the later observation of short lived pitting events at lower potentials,7 nowcommonly referred to as metastable pitting,4 shows that Epit is really the potential abovewhich pits can stably propagate.

The propagation of a pit involves sustained, high rate anodic dissolution within anoccluded cavity at potentials where the majority of the metal surface is passive. Even at highanodic potentials, local potential and pH conditions mean that cathodic hydrogen evolutiondoes occur within the pit, but less than 5% of the anodic pit current is supplied by cathodicreactions within the pit879and pitting models often consider the pit entirely as an anode. In

Manuscript received 1 October 1996; in amended form 4 November 1996; accepted 29 March 1997.1771


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1772 N. J. Laycock and R. C. Newman

near-neutral chloride solutions7 anodic polarisation of stainless steels shows no significantactive loop, but does show pitting processes, so a local chemistry significantly different fromthe bulk solution is clearly required within an incipient pit; this is coupled with a variation inthe electrode potential between the pit bottom and the passive surface. Localizedacidification within a pit cavity was first suggested by Hoar, as early as 1937, as amechanism for the pitting of tin in near neutral solutions. Later, direct measurements ofthe pH within artificial pit cavities of stainless steels in near neutral bulk solutions,yielded pH values as low as - 0.13. I3

Pickering and Frankenthal4. used a microprobe to measure the potential drop withingrowing pits on iron and stainless steel and found that potential differences between the pitmouth and the pit bottom could be greater than 1 V. They proposed a model for pitpropagation based on the idea that, even at high applied potentials, the IR drop within thegrowing pit ensured that the potential of the metal at the pit bottom was always in the regionof active metal dissolution, but no consideration was given to the necessary development ofan aggressive local chemistry within the pit. Galvele16 used a one-dimensional pit nucleusgeometry to represent an imperfection in a passive film and assumed that metal dissolutionat the pit bottom was followed by hydrolysis reactions leading to acidification. Byconsidering the transport of ionic species in and out of the pit, he was able to calculatevalues of a critical quantity ix, where i is the pit current density and x is the pit depth, abovewhich the pH at the pit bottom was low enough to maintain the metal in the active state andtherefore allow continued pit propagation.

Mankowski and Smialowska7 simulated the local pit environment using acidified bulksolutions of FeCl2, whilst Hakkarainen18.19 dissolved 304 and 3 16 SS in 10 N HCl toproduce saturated, or near saturated solutions. It was found that solutions greater thanabout 80% saturated in metal ions were necessary to prevent passivation and allow a directtransition from active dissolution to diffusion controlled dissolution. Using artificial pit(lead in pencil) electrodes, Isaacs and Newman2 and then Gaudet et ~l.,~ obtained similarresults showing the existence of steady states for active dissolution on salt free surfaces, butonly for solutions more than 60% saturated in dissolution products and above a criticalpotential.

Real pits on stainless steel have often been found to grow, at least initially, withremnants of the undermined passive film providing a partial cover over the pit cavity.7,22.2Indeed, it has been suggested that in this case, pitting can be considered a special case ofcrevice corrosion,24 although other authors prefer to think of crevice corrosion as a specialform of pitting.25 A further common observation is that many pits, particularly at relativelylow potentials, are crystallographic in nature with flat wails and etched interiorsurfaces,4.24,26 but pits at higher potentials tend to be approximately hemispherical andpossess polished interiors.8,27.28 For iron-base metals, Sato2 distinguished between etchingpits and polishing (or brightening state)pits. Etching pits formed at less noble potentials werecrystallographic pits and Sato believed that a critical pH must be reached in the pit cavity forthis type of dissolution to occur; a combination of this low pH and the IR drop in the pitkept the metal in the active state. Polishing pits had bright (polished) internal surfaces,formed at more noble potentials and required the maintenance of a critical aggressive anionconcentration within the pit. Sato noted that the critical local chloride concentration for thetransition from etching to brightening pits was close to the critical concentration for stablepitting of stainless steel. Later, Sato2s showed that pits initiated above Epit, grew ashemispherical, polishing state pits, but if the potential was then reduced, the pits either


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Salt films and pitting potentials 1773

repassivated or propagated as active state (etching) pits. These active state pits often grew asdeep, non-hemispherical holes or into crevice like geometries.

According to Vetter and Strehblow,2g a thin resistive layer of metal salt was supposed toexist at a corroding pit surface, although the pit solution was not saturated with this salt.This layer was postulated since their calculations suggested that pH changes and ohmicdrops could not be significant enough to explain the sharp active/passive transitionoccurring at the pit edges, although the presence of an overhanging passive film was notconsidered and could equally explain this observation. Larger pits are often found to bedish-shaped rather than perfectly hemispherical, because without a pit cover, the pit edgeshave lower associated solution or diffusional resistances and dissolve more rapidly. Incontrast to Vetter and Strehblow, Beck and Alkire3 calculated that pit solutions couldindeed become saturated with metal salts within 10-g-10-4 seconds of nucleation. The saltlayers then precipitated within the pit were presumed to play a significant role in pit stability.Reviewing the literature, Beck3 concluded that precipitation of a non-porous barrier saltlayer was essential during the early growth of a pit. Significantly, he noticed thatVermilyea32 had shown that the pitting potential for several metals, including iron, nickeland magnesium, was close to the standard potential of formation for the relevant metalchloride. In common with Vetter and Strehblow,2g Beck thought that crystallographic etchpits could grow with a salt layer present, provided the film was thin enough (approximately10 nm), but at higher applied potentials, thicker films would form and result inhemispherical, electropolished pits as have frequently been observed.

It is generally accepted that the growth of a metustable pit is essentially the same as theearly growth of a stable pit.33 However, the lifetime of a metastable pit is cut short byrepassivation when the pit fails some critical test of its stability. Boehni and co-workerscarried out a great deal of work on the metastable pitting of austenitic stainless stee1.3b36With a similar idea to that described by Beck,3 they believed in a quasi-thermodynamic saltfilm formation potential, Esr, below which metastable pitting was impossible. Above &, atspecific weak spots, the passive film was believed to be less stable than the salt film, but theprecipitation of the salt film was a kinetically hindered process so that early pit growthoccurred under mixed ohmic/charge transfer control until the salt precipitated and diffusioncontrol was established. This mode1336 gave an important role to the pit cover incontrolling pit stability. According to Vetter and Strehblow,2g a high resistance layer wasrequired between the passive surface and the active pit bottom. Franke136 suggested that thishigh resistance was initially provided not by a salt layer, but by the porous pit cover with thepores acting as resistors in parallel. It was proposed that only pits which survived longenough to precipitate a salt film became stable, whilst other pits repassivated when theircover ruptured and the ohmic barrier was lost. If a salt film is present when the cover breaks,the film is presumed to thicken and accommodate the extra potential so that pit stability ismaintained.

Pistorius and Burstein also believed in an important role for the pit cover, but as adiffusion barrier rather than a resistive one. As established earlier by Galvele16 the criticalquantity in pit growth is the product, i x which Pistorius and Burstein termed the pitstability product. For 304 SS, taking a minimum metal ion concentration for stable growthof 3 M and a maximum of 6 M (allowing for supersaturation), equation (1) was obtainedshowing the possible range of values of i u for stable pit growth.

0.3 A m- < i a -c 0.6 A m- (1)


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Incidentally, Frankel et a/.34 calculated a critical pit stability product of 0.4 A m- , anda value of 0.6 A m ~ can be inferred from the work of Williams et ~1.~~However, Pistoriusand Burstein found that all metastable pits, even those which went on to become stable,initially grew with pit stability products less than 0.3 A m-l. They believed this was possiblebecause the diffusion barrier provided by the pit cover during metastable growth enabled theconcentrated local chemistry to be maintained. Since metastable pits in their experimentsgrew with an essentially constant current density, ia increased as the pit grew and if itreached 0.3 A m- then the pit was able to survive the complete loss of its cover and becomestable. In contrast to the model of Frankel et a1.3436 Pistorius and Burstein believed that allpits grew under diffusion control, with a salt film on their surfaces, even before the cover waslost. By growing single, real pits on microelectrodes and scanning the potential downwardsduring growth, they showed that pits at high potentials (700 mV (SCE)), were indeed underdiffusion control.

This work concentrated on the kinetics of pit growth in order to establish whether activeand salt filmed dissolution were both possible in growing pits under different conditions, ornot. In addition, the transition from metastable to stable pit growth was studied todetermine the critical factors that control pit stability and so gain a more completeunderstanding of the pitting potential and the factors which control it.

EXPERIMENTAL METHODAll electrochemical experiments used a potentiostat made by ACM Research together

with a sweep generator made by Thompson Instruments. Data were recorded digitally usinga 386 DX computer fitted with a Keithley DAS 8 A/O data acquisition card used as ananalog to digital converter in conjunction with Keithley Easyest LX software. In metastablepitting tests, experiments were designed such that currents were below 2 uA and could bemeasured using a Keithley model 614 electrometer giving low background noise levels. Asaturated calomel electrode (SCE) was always used as the reference electrode and allpotentials quoted here refer to this scale. An approximately 10 mm length of 1 mm diameterplatinum wire was used as the counter electrode. All test solutions were made fromanalytical grade chemicals and de-ionized water. Artificial pit electrodes were made bymounting narrow steel wires in Araldite Rapid epoxy resin so that one end of the wire wasexposed as the electrode surface. AISI 302 and 316 stainless steel (SS) wires of 10, 50 and500 urn diameters were used, and the electrode surface faced vertically upwards in allexperiments. To begin a series of experiments, an electrode was wet abraded with coarse Sicpaper and immersed immediately in the test solution. The applied potential was thenstepped to + 750 mV (SCE) so that stable pitting occurred; coalescence of these pits led togeneral dissolution of the surface and the formation of a one-dimensional artificial pitcavity. Experiments were carried out in 1 M, 0.3 M, 0.1 M and 0.01 M NaCl solutions atroom temperature (20 i 2C) with the cell open to the air.

At regular intervals during the lifetime of a given pit, the potential was decreased at10 mV s- whilst the current and potential were recorded. During each of these scans thecurrent remained potential independent for some time before showing a small peak and adecrease indicating loss of the salt film present during diffusion controlled dissolution. Thekinetics of active dissolution were recorded down to a current value about 80% of thelimiting current, then the scan direction was reversed and the salt was precipitated again.Further pit growth was carried out at 250 or 350 mV before repeating the backscan


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procedure. The transition potential, ET, between diffusion controlled and activation/ohmiccontrolled growth was defined for a particular limiting current density, irimas shown in Fig.1. The pit anode resistance was taken to be the reciprocal of the slope of the current vspotential graph (e.g. Fig. I), measured at ET on the increasing potential part of the scan.

The pitting potentials of 302 and 316 SS wires were determined as a function of bulksolution chloride concentration at room temperature. The working electrodes were lengthsof 500 pm diameter wire positioned so that a surface area of 0.5 cm2 was immersed in thesolution, and a waterline was present on the sample. Wires were longitudinally abraded toeither a 120 or 1200 grit finish and rinsed with de-ionized water immediately before theywere placed in the cell. The test solution volume was approximately 200 cm3 and thesolution was thoroughly de-aerated using nitrogen prior to the samples being introducedinto the cell. Once the samples were introduced, the cell was sealed, except for nitrogen inletand outlet lines, and the flow of nitrogen was reduced to a low level to avoid any effects offlow on the pitting characteristics. Each experiment was performed on a freshly preparedelectrode, and each length of wire was discarded after 3 or 4 experiments. Once in thesolution, the sample potential was held at - 500 mV and then swept anodically at 1 mV s- .Polarisation was continued until the anodic current exceeded 1 mA cmU2 and the pittingpotential, Epit, was defined at a sustained current density of 10 uA cmp2 For each testcondition (alloy, chloride concentration, surface finish) three measurements were carriedout.Some potentiostatic experiments were also carried out on the metastable pittingbehaviour of 302 SS in 1 M NaCl at room temperature. So that low noise data could beobtained, and individual pitting transients could be easily resolved over a wide range ofpotential, a small electrode surface area was used. For experiments at up to 80 mV, a 2 cm

100 200 300E mV vs SCE)

Fig. I. Current density vs potential during potential sweep experiment for a 50 pm diameter, 302 SSartificial pit in 0.1 M NaCI, showing definitions of ia,,, and ET


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the current vs time curve and application of Faradays Second Law, assumingstoichiometric dissolution of Fe, Cr and Ni with valencies of 2, 3 and 2, respectively. Thecharacteristic time, z, for diffusion out of the pit is approximately L2/D, where L is the pitdepth and D is the ionic diffusivity, and for a 100 urn deep pit this would give r = 10 s. Atthis depth, the metal is dissolving at approximately 0.3 urn s- (Fig. 3) and so the depthwould increase by 3% during t. For a 1 mm deep pit, z is 1000 s and the dissolution rate is0.03 urn s- . which again gives a 3% depth increase during r. This justifies the use of steadystate calculations such as Ficks First Law to describe the growth of these pits, as wasimplied by the good linear fit shown in Fig. 2. It is also interesting that for any given pitdepth, the limiting current density was slightly higher for 316 than for 302 SS (Fig. 2).Current oscillations due to periodic passivation and reactivation beneath the salt film3 weremore pronounced in the 3 16 pits which consequently spent more time in the supersaturatedstate and therefore had slightly higher average dissolution rates.

The transition potential, ET, between activation/ohmic controlled growth and diffusioncontrolled growth can be plotted against the log of the limiting current density, iiim,as in Fig.3 for 302 SS in 1 M NaCl solution. Results are shown for 10, 50 and 500 urn diameterartificial pits. A good linear fit is obtained for both the 10 and 50 urn plots, although closerexamination of the 50 urn results reveals that the data are beginning to curve upwards at thehigher current densities. The small number of tests with 500 urn diameter wire show a moreextreme version of this effect so that a linear fit for the data was not appropriate.

A simple model of the kinetics within the artificial pit is proposed. At the point where ETwas measured in these tests, the pit was in the active (salt-free) state, but exactly at thelimiting current density. The overpotential at this point includes contributions from bothactivation effects, AE and solution resistance effects, qrn. If the corrosion potential in thesaturated pit environment is EC,,,, then equation (2) applies.

200

,,,(A cm -)Fig. 3. The transition potential, ET, as a function of limiting current density for IO,50 and 500 pm

diameter artificial pits of 302 SS in 1 M NaCI.


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ET = -&,,r + A&c, + QIR (2)The activation effect contribution is given by equation (3) where b, and icorr are the

anodic Tafel slope and corrosion current density within the pit environment. The totalsolution resistance, R,, is given by equation (4) where Rint is the solution resistance withinthe pit cavity and R,,, is the resistance of the external solution. The composition of the pitsolution varies with depth, but if the resistivity of this solution can be represented by anaverage resistivity, p, then a pit of radius r and length L has an internal resistance given byequation (5). The external solution resistance can be calculated by equation (6) which givesthe resistance for semi-infinite current flow from a disc to a counter electrode at infinity in asolution of resistivity p.

Ks = Rint + Rext (4)Riot = pL/nr' (5)

(6)For an artificial pit electrode, the active surface area (m2) is constant and the total IR

drop at the limiting current, /ii,,,, is therefore given by equation (7). For bulk solutions suchas the 0. l-l M NaCl used in these experiments, p is low and measurements were made withthe pit depth much greater than the pit radius (L > r). For example, assuming a 4.2 Msaturation concentration of metal ions, stoichiometric dissolution with an average metalion charge of 2.2, and an effective diffusivity39 of 0.86 x 10K5 cm2 s-, then, using FicksFirst Law, an iii, of 1 A cm- corresponds to a pit depth of approximately 80 urn. 1 MNaCl has a resistivity of 13.9 52 cm40 (at 25C) and therefore R,,, for a 10 urn diameter pitgrowing at 1 A cm 2 is 7 kR. Similarly, the concentrated pit solution resistivity can beestimated as 5 R cm, giving an Rint of 50 kR. In this case R,,, is less than 15% of Rint and thetotal IR drop is approximated by equation (8).

/I,, R, = ii,, p L + iii, pm/4 (7)hm R, = h, PL (8)

Applying Ficks First Law to the 302 SS data at a depth of 100 urn, iii, can be used tocalculate the saturation metal ion concentration as in equation (9) with the bulkconcentration of the dissolving ions taken to be zero. This gives a value for Csat, of 3.95 Mwhich is slightly lower than the literature result of 4.2 M.39 It could be that the presumeddiffusion coefficien? of 0.86 x lo- cm2 s- IS responsible for this discrepancy, or simplythat the 1 M Cl- present in the supporting electrolyte has slightly reduced the solubility ofthe metal ions.

Cqa, = i,,,L/nFD (9)It has been established that the IR drop during these tests is given by equation (8) whichsuggests that ZlimRs should be independent of pit depth. Diffusion is governed by Ficks FirstLaw and SO ili, z l/L but R, is directly proportional to L and therefore IlimR, is a constant


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for all pit depths. This result was verified experimentally by measurement of the total pitresistance, R,,, at various iiimvalues. Figure 4 shows the results of such an experiment for theconditions where the above assumptions are most likely to be correct, i.e. for the narrowestwire in the most conducting solution. Rtot was measured as the gradient of the Zvs E urve atET n the increasing potential part of the potential scan (Fig. 1 shows i vs E . At this point,Rtots made up of the charge transfer resistance, R,, zm2 and the solution resistance, R, las in equation (10). R,,an be defined as dE/di for the anodic line assuming Tafel kinetics, asshown in equations (11)-(12), and SO Rctx /iiimS n Fig. 4, the linear plot of R,,,gainst l/irim shows that R,must also be inversely proportional to iiim verifying that ZlimR,s aconstant.

Z-Got R, + tZWrr2) (10)E = a + b,logi (11)Ret bJ(2.3)i (12)

Equation (2) can now be rewritten more explicitly as equation (13) and since ZlimR,s aconstant, this equation explains the linear dependence of ET n log ilimas shown in Fig. 3.The displacement of ET o higher potentials for the 50 pm pit compared to the 10 l.trn pit iscaused by the relatively increased contribution of Z& to R, s described by equation (7). Atthe higher current densities (smaller L) this effect is more pronounced and accounts for theslight upwards curve of the data. For 500 pm diameter pits, the contribution of Z& meansthat Z,imR,s no longer constant and a linear plot is no longer obtained, although equation

+

700600

1 I ilim A-cm*)Fig. 4. Pit anode resistance, R,,,, as a function of l/iii,,, for a 10 p diameter artificial pit of 302 SS

in 1 M NaCl. Measurements made in the salt-free state at ET.


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I780 N J. Laycock and R. C. Newman

13) still valid.ET = Ecorr balOg 13)

The contribution of the IR rop outside the pit, IiimRext,can be calculated for any givencurrent density if the bulk solution resistivity is assumed to be constant and unaffected bydissolution products leaking from the pit. The resistivities of 1 M, 0.1 M and 0.01 M NaClare 13.9, 93.7 and 844 Acm, respectively4 (at 25C). Subtraction of calculated R,,,aluesfrom the data of Fig. 3 produces a better linear fit for the 50 and 500 urn pit lines and bringsthe absolute values closer to those measured for the 10 urn pit (Fig. 5).

Equation (13) shows that the slope of ET vsog iiimplots should be equal to b the anodicTafel slope in the pit environment. The calculated values for the gradients of linear plots inFigs 2 and 5 are shown below in Table 1.

The IR,,,orrection for the 500 urn data is probably an over-correction since thesolution near the pit mouth would have been significantly more conductive than the nominalbulk conductivity due to metal ions transported out of the pit. The average value for 6, fromthe IR,,, corrected data for the two other datasets is 110 mV/decade. Gaudet et ~1.~reported a value of 54 mV/decade for 304 SS, whilst Newman and Isaacs2 measured60 mV/decade for an Fe-19Cr-1ONi alloy with a change to 75 mV/decade at potentialsbelow - 300 mV. For 304L SS, the effect of impurities precluded measurement at potentialsbelow - 250 mV, but above this a Tafel slope of 60 mV/decade was recorded. These resultssuggest that the average value obtained from Table 1 is a little high, although a possible

0. 1 1 IO,,,(A cm-)

Fig. 5. The transition potential, ET, as a function of limiting current density for IO,50 and 500 )tmdiameter artificial pits of 302 SS in 1M NaCI, with the contribution of the potential drop outside the

pit subtracted from the experimentally measured values.


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Salt films and pitting potentials 1781Table 1. Slopes of the linear regression lines for the & vs ilimplots shown

in Figs 2 and 5Pit diameter (pm) Experimental slope

(Fig. 3)mV/decade

ZkX1,corrected slope(Fig. 5)

mV/decade10 113 10550 148 115500 - 88

explanation is that these data were measured in an approximately 100% saturated pitsolution and the actual dissolution rate of stainless steels has been shown to decrease slightlyin the range from 85-100%.2 Equally possible is that the ZR correction used by otherauthors202 was overestimated, or that at the higher current densities used in this work thereis a genuinely increased Tafel slope.Comparison w it h eal pi t s

Equation (13) can be applied equally to real pits, so that the results of Fig. 3 shouldresemble those obtained if the same experiment could be performed with real pits, althoughcurrent densities would be slightly higher during metastable growth (e.g. l-10 A cme2). Inaddition, the form of the calculations for R, is different for hemispherical cavities than forlinear artificial ones.

Slight differences exist between literature expressions for the ohmic resistance of an openpit,8,3 ,34but in all cases R,w l/v and according to Pistorius and Burstein8 R, is given byequation (14), where p is the solution resistivity. This equation was derived andexperimentally verified for a constant solution resistivity, so for pits in relativelyconducting bulk solutions, it is reasonable to use the bulk solution resistivity and ignorethe effects of the concentrated pit solution,241 although this should be recognised as asimplification. Similarly, the diffusion limited current from an open, hemispherical pit isgiven by equation (15) Combination of these two results shows that, as for artificial pits,Zl imRss a constant for all pit sizes.

R, = p/3r (14)Zlim nF3rDCSat (151Zl imRs nFpD (16)

Obviously, the presence of the pit cover during metastable growth would complicate thecalculations. However, the effects of the cover would be to both increase R, and to decreasethe effective diffusion coefficient, D . These two effects would to some extent cancel eachother out with regard to the value of the transition potential, ET.Ohmic contr ol or diffusion control ?

It is commonly accepted that for 18Cr-8Ni stainless steels, such as 302 and 316, pitpropagation can only occur with the local environment > 75% saturated in metal ions i.e.approximately 3 M.8,9,21Supersaturation of the pit solution, possibly to a factor of 1.5, isrequired in order to precipitate a salt film, but as a first approximation it is reasonable to


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assume that pit growth could occur in the active state with solutions between 75 and 100%saturated, or with a salt film present and the concentration fixed at 100% ofsaturation. In thiscase, any given pit growing at an applied potential above the ET value for that pit will be saltfilmed and under diffusion control. At lower potentials, the pit would grow in the active state.

Pit growth in the active state is believed responsible for the etch pits found at lowpotentials by many authors,0,25*26 whilst the commonly observed polished andhemispherical pits are more likely to have grown with a salt film present on their surface.Sato26.27 made a precise distinction between these two forms of pitting and believed that thepolished pits could only form when a critical concentration of chloride ions was maintainedwithin the pit. Other authorsR*29.42 have stressed the importance of salt films in maintainingpit stability. Frankel et u1.34-36 suggested that metastable pit growth is stabilised by theporous pit cover, but that the transition to stable pit growth only occurs for pits which canprecipitate a salt film. In this model then, the criterion for stable pitting is the ability tomaintain a 100% saturated pit chemistry in an uncovered, hemispherical pit. Using thisconcept, it is possible to use equation (13) or Fig. 3, to predict the potential above whichstable pitting is possible. It is proposed that the defining characteristic of any pit is its currentdensity. Typical current densities measured in the first seconds of growth for pits on stainlesssteel are in the range from l- IO A cm _ *. and most studies of metastable pits below thepitting potential find values at the lower end of this scale.34,43 Since all pits must be growingwithin a narrow range of local chemistries (between 75 and 150% saturated), these currentdensities approximately represent the range of limiting current densities possible in pits.According to equation (13) the il value for a given pit determines the value of ET

Figure 6 shows a typical recording of metastable pitting transients from 302 SS at anapplied potentials of 0 mV. It was found that the number of observable metastable pitsincreases with potential as illustrated in Fig. 7, but the qualifying comment that these are

0 100 200 300time s)

Fig. 6. Current decay following a potential step to 0 mV showing metastable pitting on 302 SS in1 M NaCl with a 120 grit finish.


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-100 -50 0 50 100 150E mV vs SCE)

Fig. 7. The number of metastable pits with > 5 nA peak current as a function of potential for302 SS in 1M NaCI. Three results are averaged for each data point and the error bars represent the

standard deviation. Also shown are selected ET values from the 10pm data in Fig. 3.

only the observable pits is important. The number of pit nucleation events may not beaffected by potentiaJM but the stability of metastable growth increases with potential. Thenumber of events counted and plotted in Fig. 7 is therefore only the number of events whichgrew to a peak current higher than the detection limit (approximately 5 nA). Currentdensities calculated for these metastable pits were in the range 0.5-2.5 A cmw2, whichcorresponds to ET values from 0 to 120 mV. Similar results to Fig. 7 have been obtained byother authors,45,46 and have often been considered to fit a smoothly increasing curve. Analternative suggestion is that a sudden increase in pit stability occurs as most pits becomesalt filmed, resulting in an apparent increase in the number of observable pits aboveapproximately 40 mV.

It is important to note that many metastable pits will grow with salt films presentunderneath the cover, but a pit can only become stable if it maintains the salt film when thecover ruptures. In fact, since ET is proportional to log (iI any stable pit must already havebeen salt filmed in its metastable stage. In practice, it may only be necessary for the pit tomaintain a 75% saturated solution when the cover ruptures, although a salt film couldactually help the pit to survive the locally violent event. When the cover breaks, increasedmass transport from the pit acts to dilute the solution. A salt layer at this time could act as areservoir of fresh solution; saturation would be maintained until all the salt had dissolved.As long as some salt remains, the solution must remain saturated and the fastest a salt layercould dissolve would be if the pit repassivated beneath it so that virtually no flux of metalions was produced by metal dissolution. Ignoring the effects of migration and convection,the time for the salt to dissolve, t,, is given by equation (17), where d is the initial salt filmthickness, m, is the molar volume of the salt, and iii,,.,s the limiting current density for theopen pit produced when the cover ruptures.


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I784 N. J. Laycock and R. C. Newman

t, = nFd/(m,i,i,) (17)For FeCl,, the density is 1.93 g cm 3.40 giving an m of 103 cm3 mall and an open pit

of 5 pm radius would have an illm of around 7 A cm --. Salt film thicknesses have been foundto be less than 1 um4 and for a very thin salt layer of only 10 nm, equation (17) gives adissolution time of 0.3 s. This time would be shortened by transiently increased convectioncaused by collapse of the cover, but the delay before dilution of the pit environment mayenable the pit to survive until the stability of the pit in the new steady state conditions can betested. Furthermore, when the cover breaks, the effective R, would decrease but the salt filmcould thicken to accommodate this extra applied potential in the self regulating mannerdescribed by Isaacs4 This view is in good agreement with those of Frankel et uI.,~~Sato2627 and Beck3 who all suggested that pit growth could occur in the active state, even ifonly as a transient stage prior to salt precipitation. This is, however, in complete contrastwith the opinions of Vetter and Strehblow29 or Pistorius and Burstein.8 These authorsproposed models in which all pits are necessarily covered with a salt film.

Vetter and Strehblow believed a highly resistive salt layer to be necessary in order toaccount for the potential difference between active and passive areas at the edge of pits.Pistorius and Burstein measured single pit polarisation curves and found these pits to beunder diffusion control, and Burstein44 went on to propose a pit nucleation mechanism inwhich the salt film existed before the pit itself. However, Pistorius and Burstein onlypresented single pit polarisation curves for pits on 304 SS grown at high potentials(700 mV). with current densities of approximately 8 A cm-*. Predicted ET values for thesepits are around 150 mV and so growth would be expected to be diffusion controlled. Pits onthis type of steel have been detected at potentials as low as - 200 mV45 and it is the growthof such pits which would be occurring in the active state as predicted by Fig. 3.

PITTING POTENTIALSAll pits initially grow metastably with a porous cover to protect the local environment,

but this cover eventually ruptures to produce an open, hemispherical pit. In order topropagate from now on, the pit must be salt filmed. ET for a given pit represents theminimum potential at which the salt film can be maintained and therefore is the minimumpotential at which this pit can become stable. In general, for a macroscopic stainless steelsample, E,,, will be the lowest ET value from the distribution of pits on the sample at theinstant they lose their cover. In some cases, lower pitting potentials will be measured if thepit cover is exceptionally strong, or if the inclusion at which the pit initiated was particularlylarge, so that the open pit can remain stable without a salt layer due to the size of the cavityalone.

Figure 8 shows EPlt values measured on 302 SS as a function of [Cll] and for twodifferent surface finishes, 120 and 1200 grit. Also included on this graph is the variation ofET for an iiim value of 2 A cmW2. (This value was chosen as being typical of the fastestgrowing metastable pits seen in this work.) These Er values were determined using 50 urndiameter artificial pits to produce ET vs log itim plots (Fig. 9) from which the data in Fig. 8were then taken.

The difference in Epit values between 120 and 1200 grit finishes reflects the nature of thesites available for pit initiation. Pits initiate at specific sites on the surface (probably MnS


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1785slop0 = 40 mVslopa = -93 mVslop = -100 mV

Salt filmsand piningpotentials

, 0 Epr (I= SW400 - A E, 12OOgrit)

*2

300 -

g 200 -1EQ, IOO-3Q

O-

-100 1 1 I ---I0.001 o.ojo 0.100 1.000 10[cl-l

Fig. 8. Pitting potentials (I&) for 302 SS as a function of log [Cl-] for 120 and 1200 grit surfacefinishes, compared with ET measured using a 50 pm diameter artificial pit for an ilim value of

2 A cn-.

inclusions) and rougher surfaces generally provide more occluded geometries around thesesites. Metastable growth is easier to maintain at these occluded sites and consequently moremetastable pits are seen on rougher surfaces. However, these pits grow at relatively lowcurrent densities because both ohmic and diffusional resistances are higher for these sites

-100 1 I0.1 1 10ilim A cme2)

Fig. 9. ET vs ilim or 50 pm diameter artificial pits of 302 SS in various chloride concentrations.


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(this is why they are favourable). Using a probabilistic argument, it is now easy to see whyE,i, is lower for the coarser surface finish; there are simply many more metastable pits43.47and the chances of one becoming stable are therefore increased.

The initiation of pitting occurs on length scales significantly less than the surfaceroughness in this work, and therefore metastable pitting was stabilised by the extraocclusion provided by the surface geometry. However, in terms of ET and its relationship tostable pitting, the relevant ET value is not for metastable growth, but for the open cavityproduced when the cover collapses. High current densities are generally required for theseopen pits to propagate. For instance, using equation (14) and dividing by the surface area ofa hemispherical pit (2nr2), ifi, for an open pit of radius 5 urn would be approximately7 A cm-2. From Fig. 3, the best estimate of Er is obtained from the 10 urn diameter pit data,and for an ii,,,, of 7 A cm -*, E-r is approximately 120 mV. Comparison with &ii for the1200 grit surface finish and 1 M [Cl _ ] (Fig. 8) shows that EJ- s indeed a reasonable estimateof Epit for this surface condition. For a lower current density metastable pit to survive, itmust either be additionally protected by the surface roughness, or grow to a relatively largesize before losing its cover, but Isaacs and Kisselz3 have shown that coarse surface finishesreduce the lifetime of metastable pits by weakening the pit cover. The size of repassivatedmetastable pits is usually in the region of l-10 urn, which can be compared with grit sizes ofapproximately 100 urn for a 120 grit finish. Therefore. it is not unreasonable to expect extraprotection for pits on coarsely finished surfaces, and the lower Epi,values are consistent withthe lower ET values for lower current density pits. In this case, the measured Epi, ofapproximately 70 mV for I M NaCl at the 120 grit finish is consistent with an iii, ofapproximately 1 A cme2, which is near the lower end of the current density distributionmeasured in these tests.

The effect qf chloride concentrationThe influence of chloride concentration on the pitting potential is shown in Fig. 8 for

302 SS and chloride concentrations from 0.01-l M. These data are in agreement withliterature results648 showing that Er,it follows the relationship of equation (18). Galvele16accounted for this behaviour by considering the effect of [Cl-] on the ZR drop within the pit.In this model, the value of B should be 59 mV; a value which was found experimentally forpitting of ironI but for stainless steel B has been measured as 90 mV.16*48 Newman et a1.49confirmed that the IR,,, drop in artificial pits followed a relationship identical to that ofequation (18) and found B to be 60 mV for iron and 90 mV for stainless steel.

Eplt = A - B log [Cl-] (18)Figure 8 shows experimentally determined values of B to be 90 and 100 mV for 120 and

1200 grit surface finishes, respectively, and that the value of ET also decreases with log [Cl-]with a slope of 93 mV. This similarity between the dependence of &it and ET on [Cl-] isdemonstrated again in Fig. 10 which shows ET vs log iiim plots for various chlorideconcentrations; as [Cl-] decreases from 1 M to 0.1 M, ET is displaced to higher potentials byapproximately 90 mV/decade. As suggested by Galvele,16 this can be explained by anincrease in the IiimR, term in equation (13) due to the decrease in average conductivity of thepit solution as [Cl-] decreases. For the 0.01 M NaCl solution the conductivity of the bulksolution is low and the IR drop outside the pit cavity becomes significant at high currentdensities, so that the curved plot in Fig. 9 is produced. A correction for this effect (similar to


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-1000 001 0 010 0 100 1 ooo 10

r Cl1Fig. 10. The pitting potential, &if, measured for 302 and 316 SS in various NaCl solutions with a120 grit surface finish.

that carried out for the 50 and 500 pm diameter pits in Fig. 5) was not successful, becauseenrichment of the solution outside the pit mouth by dissolved ions significantly affected thesolution conductivity in this initially dilute solution, but to an unknown degree.Alloying with molybdenum

Figure 10 shows the well known increase in Epi, caused by the addition of Mo to an alloy.316 SS is nominally identical to 302 SS except for the addition of 2.5% MO, and its pittingpotential is 7&100 mV more noble for all chloride concentrations. Many possible reasonsfor this effect have been proposed and can be split into two basic categories; those whichsuggest MO improves the properties of the passive film, and those which consider the effectof MO on the dissolution kinetics within a pit.

The model of pit dissolution described by equation (13) obviously takes no account ofthe passive film and this approach has already been shown to explain the effect of [Cl-] onEpi,. In Fig. 11 the variation of ET with iii,,,for 50 urn diameter artificial pits of both 302 and3 16 SS is plotted and, at the current densities relevant to pitting, the 3 16 data are displacedto higher potentials by - 100 mV (measured at iii,,,= 2 A cm-*). Given the definition of Epi,stated earlier, this effect on pit propagation accounts completely for the increase in Epit for316 relative to 302, and is consistent with the work of Newmano95 who showed that MOalloying caused the anodic Tafel line in the pit environment to be shifted to higherpotentials. In terms of ET, as given by equation (13), MO acts to increase EC,, and decreaseI,,,, in the pit environment by inhibiting the anodic reaction. In addition, the increased slopeof the 3 16 data in Fig. 11 suggests that there is also a slight increase in b,.

Ezuber43 found that the distribution of metastable pit current densities was shifted tohigher potentials for 3 16 in relation to 304. That is, the modal pit current density increasedwith potential in the same way for both steels, but a given value was reached at a potentialabout 100 mV more positive for 316. This is also consistent with the idea that MO simply


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I788 N. J. Laycock and R. C. Newman

0

1000. 1 1 10

&,.,,(A cmm2)F1g. I ET vs i for 50 pm diameter artificial pits of 302 and 3 16 SS in I M NaCl

hinders anodic dissolution within the pit such that higher potentials are required to reach thesame current density. Therefore, all other things being equal, higher potentials are requiredfor pits on 3 16 SS to precipitate a salt film and become stable, as shown by Fig. 11. Thisargument will apply for all MO additions that do not raise the critical pitting temperatureabove the ambient temperature. At some MO content, stable pitting at ambient temperatureceases altogether due to passivation within the saturated pit environment.52-55

CONCLUSIONSArtificial pit electrodes of 302 and 316 SS have been used to measure the transition

potential, ET, between active and salt covered dissolution, and this potential can bedescribed by the simple equation shown below;ET = EC,,, + b,log 2 + him

( >(19)

Metastable pit growth is initially stabilised by the presence of a pit cover which helps tomaintain the aggressive local chemistry within the pit. This pit cover will eventually collapseand a pit must survive this event in order to become stable. It is proposed that, in normalcircumstances, only pits which can maintain a salt film on their surfaces in the absence of acover will become stable. In effect, the ET value for pits at the instant they lose their coverdefines the pitting potential, EpIt.

Increasing solution chloride concentration has been shown to decrease Epi, by between90 and 100 mV/decade for 302 SS. The value of ET was also found to decrease by 90 mV/decade with increasing log[Cl-] which is consistent with the suggestion that ET defines E,,,,.


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This effect is due to a decrease in the Zt&2, term in equation (19) as reported by Galvele32and Newman et aLI

316 SS is essentially identical to 302 SS but for the addition of 2.5% MO. The effect ofthis MO is to raise the Z&i, of 316 SS by 70- 100 mV relative to 302 SS at all chlorideconcentrations. At the current densities typical of metastable pitting in these steels (1-5 Acmm2) ET was also found to be approximately 100 mV higher for 316 SS than for 302 SS.This is due to inhibition of anodic dissolution in the pit environment by Mo,~ such thatEC,,, is raised and i,,,, is lowered (equation (19)). Therefore, the beneficial effect of MO onpitting resistance is completely accounted for by its effect on the anodic kinetics within thepit.Acknowledgements-Financial support from the EPSRC and from Unilever Research is gratefully acknowledged.

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L. Nellmans and M. Warzu, Corros. Sci. 3,239 (1963).2. Z. Szklarska-Smialowska and M. Janik-Czachor, Br. Corros. J. 4, 138 (1969).3. K.K. Starr, E.D. Verink and M. Pourbaix, Corrosion 32, 47 (1976).4. L. Stockert, F. Hunkeler and H. Boehni, Corrosion 41, 676 (1985).5. P.H. Balkwill, C. Westcott and D.E. Williams, Muter. Sci . Forum 44/45, 299 (1989).6. D.E. Williams, C. Westcott and M. Fleischmann, J. Electrochem. Sot. 132, 1796 (1985).7. D.E. Williams, C. Westcott and M. Fleischmann, J. Electrochem. Sot. 132, 1804 (1985).8. P.C. Pistorius and G.T. Burstein, Phi l . Trans. R. Sot . L ond. A 341, 531 (1992).9. R.C. Alkire and K.P. Wong, Corros. Sci. 28, 411 (1988).

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Society, Pennington NJ, 1981, p. 101.28. N. Sato, Corros. Sci. 37, 1947 (1995).29. K.J. Vetter and H.H. Strehblow, in Locali zed Corrosion, ed. R.W. Staehle et al. NACE, Houston, 1974, p.

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33. H.S. Isaacs. Corros. Scl. 29, 313 (1989).34. G.S. Frankel, L. Stockert. F. Hunkeler and H. Boehni, Corrosion 43, 429 (1987).35. F. Hunkeler, G.S. Frankel and H. Boehni. Corrosion 43, 189 (1987).36. G.S. Frankel, in Adv ances i n Locali zed Corrosion, ed. H. Isaacs. U. Bertocci, J. Kruger and S. Smialowska.

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S. Smialowska. NACE. Houston, 1991, p. 271.51. R.C. Newman. Corros. Sci . 25. 341 (1985).52. R.J. Brigham and E.W. Tozer, Corrosion 29, 33 (1973).53. R. Qvarfort. Corros. Sci. 29, 987 (1989).54. N.J. Laycock, M.H. Moayed and R.C. Newman, in Cri fi cal Factors i n Locali zed Corrosion II , ed. R.G. Kelly,

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55. N.J. Laycock, M.H. Moayed, and R.C. Newman, J. Nectr ochem. Sot., in press.

Documents

Localized Dissolution Kinetics, Salt Films and Pitting Potentials