Origins of pitting corrosion

Origins of pitting corrosion G. T. Burstein, C. Liu, R. M. Souto and S. P. V ines

Corrosion of metals and alloys by pitting constitutes one of the very major failure mechanisms. Pits cause failure through perforation and engender stress corrosion cracks. Pitting is a failure mode common to very many metals. It is generally associated with particular anions in solution, notably the chloride ion. The origin of pitting is small. Pits are nucleated at the microscopic scale and below. Detection of the earliest stages of pitting requires techniques that measure tiny events. This paper describes techniques designed to do this and discusses the measurements that result. Some metals show preferential sites of pit nucleation with metallurgical microstructural and microcompositional features de ning the susceptibility. However, this is not the phenomenological origin of pitting per se, since site speci city is characteristic only of some metals. A discussion is presented of mechanisms of nucleation; it is shown that the events are microscopically violent. The ability of a nucleated event to survive a series of stages that it must go through in order to achieve stability is discussed. Nucleated pits that do not propagate must repassivate. However, there are several states of propagation, each with a nite survival probability. Several variables contribute to this survival probability. Examples are shown of several metals and some common features of their behaviour are discussed. It is shown that for some systems, the pit sites can be deactivated. CEST/2124 Keywords: pitting, stainless steel, chloride solution At the time this work was carried out the authors were in the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK ([email protected]).Dr Souto is now in the Department of Physical Chemistry, University of La Laguna, La Laguna, Tenerite, Spain and Dr Vines is now with UK Nirex Ltd, Curie Avenue, Harwell, Didcot, Oxon, UK. Revised version of a presentation at Eurocorr 2003, organised in Budapest, Hungary on 28 September – 2 October 2003 by the EFC; accepted 19 January 2004. # 2004 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.

INTRODUCTION Pitting corrosion is one of the most dif cult forms of corrosion to manage reliably. Chloride is the most commonly encountered aggressive agent which causes pitting of many metals.1 ,2 Resistance to pitting corrosion is a major engineering design feature of many structures and components, and despite this, and the wealth of surrounding knowledge, chloride induced pitting remains a major form of failure. Many, many variables are involved in the phenomenon; almost every feature of a metal/environment system has an effect on pitting. For example, the well known phenomenon of chloride induced pitting of many metals is naturally a function of the chloride concentration and the temperature, quite apart from the identity, the composition, and microstructural characteristics of the metal. In addition to the chloride concentration, however, the presence of most other components of the environment has an effect on the pitting characteristics as well. Even those ions which have no DOI 10.1179/147842204225016859

obvious chemical effect in the system, can have a marked effect on pitting.3 – 5 One of the consequences of the sensitivity to the many variables is the dif culty in classifying true susceptibility or resistance to pitting. Another consequence is the fact that to de ne the pitting characteristics, one really needs to specify every possible component and feature of the system; if this is not done, it stands to reason that irreproducibility will result. Pitting in chloride solutions is often characterised by the so called pitting potential, and sometimes by the related repassivation or protection potential. There is a wealth of literature on these characteristic potentials and they form an important component of the knowledge of pitting. However, an important characteristic of pitting, at least in chloride solution, is the fact that pits can and do nucleate, and can even grow, at potentials below the pitting potential.5 – 1 3 The pitting potential, so often used to characterise pitting of metals and alloys, does not mark the boundary between pitting and no pitting at all. Convention frequently has it that passivity breaks down at or above the pitting potential; the recent evidence shows that this must be regarded as erroneous, as discussed below.

PROP AGATION OF PITS IN CHLORID E SOLUTION

For a long time1 4 ,1 5 it has been realised that the electrolyte content of a chloride induced pit must be either enriched in chloride, or of lower pH, or both, relative to the bulk environment. This is completely rational. Pitting occurs when the metal undergoes localised corrosion; because the corrosion is localised, it follows that the rest of the metal surface, namely that part which is not pitting at any particular time, must at that time, be passive. Irrespective of the origin of the event, in order to propagate a pit, some component must be present which allows growth to continue. Only in that way, can the active surface within the pit interior, continue to be active; otherwise it must passivate. There are two mechanisms by which a propagating pit can maintain this active state. One possibility is that the ohmic potential drop between the pit interior and the cathodic reaction is so large that the potential at the pit surface is lowered to a value in the active region of the metal; that way, the metal pit should maintain active propagation at a maximum rate controlled by the critical current density. The idea is a good one. However, current densities in the active region, even at the peak value, are rarely suf cient to account for the very rapid propagation rates of growing pits. These can be very high.1 ,1 6 ,1 7 In order to account for such high propagation rates, it is necessary to invoke a concentration of the chloride anion and/or a decrease in pH to a level that high propagation rates can indeed be sustained. It then remains to establish why the concentration of chloride should be high, and/or the pH low. In pure chloride solutions, the high chloride concentration and low pH must accompany each other, since for the pH to be low, an appropriate anion must be present, and that would necessarily be Cl . Similarly, if the chloride concentration were indeed high, then there must be an appropriate counter ion, and such a cation would be either the corroded metal cation, or H + (or both). The corroded metal cation, such as Fe2 + or Al3 + etc., would invariably hydrolyse anyway,

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so a low pH is a direct consequence of this, and this is observed.1 4 ,1 5 Returning now, to the question of how the chloride concentration would rise relative to the bulk, raises the question of the propagation rate of the pit. The dissolving metal cation requires a counter ion to enter the pit anolyte; in order to concentrate, its rate must be controlled by diffusion: this constitutes the second possibility for a controlling process.1 1 ,1 8 This is a major feature of pit propagation in chloride solution. Pit propagation is controlled by diffusion of metal cations out from the pit interior into the bulk electrolyte,1 1 this is the only way that the metal cation can concentrate. Concentration of the metal cation now causes the chloride concentration to rise within the pit anolyte, and this may reach saturation of the metal chloride. It has been shown that no passivation can occur if the metal salt is saturated, at least for stainless steels. Thus the pit can now propagate inde nitely provided the metal chloride solution within the pit remains saturated. Having rationalised this thus far, it is clear that an ohmic potential drop is not a controlling factor in pit propagation. Pits are propagated under diffusion control, the controlling diffusion rate being the egress of the dissolving metal cation from the pit surface to the bulk electrolyte. This does not of course, mean that there is no ohmic potential drop; far from it. Because pits propagate at a high local current density, there must be an ohmic potential drop, and the high current density is indeed required in order to generate the diffusion control. The process does not, however, require an ohmic potential drop at all; it is neither a necessary nor a suf cient condition for pit propagation. The phenomenon can be summarised by stating the following. (a) A pit can propagate provided the pit interior can sustain an electrolyte suf ciently aggressive that passivation is prevented. In chloride solution, this implies an anolyte saturated with the metal chloride salt, or nearly so. (b) A mechanism by which this saturated chloride solution is sustained is available when pits propagate at a high rate: diffusion control causes the rise in salt concentration. (c) The process requires a mechanism to nucleate it and kick it into action.

DETECTION OF THE EARLY STAGES OF PIT PROPAGATION The early stages of pit propagation require methods which enable the current from tiny pits to be recorded. This is not an easy task. Although pits propagate at a very high current density, the surface area from a pit growing in its early stages is very small, and even a high current density can result in a low value of the absolute current owing out of the pit. Because the pit grows on a surface that is otherwise passive, it is easy to see that the passive current from a specimen surface can well exceed the pit current from a small propagating pit because of the very large difference in surface areas between the two differently reacting regions of the surface. For example, a 1 cm2 passive surface showing a passive current density of 1 mA cm 2 would give a current of 1 mA. If a pit of diameter 1 mm, surface area 1.6610 – 8 cm2 (assumed to be hemispherical), were propagating at 0.1 A cm 2 the corresponding current would be 1.6 nA. The pit current would then be dif cult to detect unambiguously, and as a rapidly changing current transient, almost impossible. In fact a pit of 1 mm diameter is already in a fairly advanced state of propagation. One solution to the problem is to use small test electrodes: the microelectrode.1 2 ,1 3 ,1 8 – 2 1 The aim here is to make the electrode so small that the transient current due to the earliest stages of pit nucleation and growth can be detected unambiguously over and above the background passive current. In making the electrode tiny, another advantage is immediately obvious. When a metal, particularly stainless Corrosion Engineering, Science and Technology

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steel, is subject to a pitting environments, the pitting events often occur with high frequency, and on a larger specimen surface, they become dif cult to separate for analysis. This frequency is naturally reduced when the surface area is small, and allows individual events to be identi ed, separated and analysed. Microelectrodes have been made by encasing ne wires of the specimen material in an epoxy resin sleeve, and then grinding the end back to reveal a circular test electrode surface. In this fashion, electrodes of diameter down to 25 mm have been made routinely. This method works well, and the electrode surface can be ground down in the usual way and polished if required. Electropolishing is more dif cult because during electropolishing, the metal surface dissolves at a high rate, and thereby recesses into the epoxy resin sleeve. For a microelectrode this recession can limit transport to and from the metal surface. Because the currents from such an electrode can be very small, and the current transients due to pitting can be both fast and exceedingly small, it is desirable to minimise the noise generated by normal electronic instruments and to use highly sensitive techniques to read the current. On this basis, a two electrode circuit operating as a potentiostat has been used in the following way. The counter electrode was made from a large sheet of silver (21 cm2 sheet size, 42 cm2 total surface area) which was anodised in dilute HCl solution to produce a thin lm of AgCl over the surface. This, when equilibrated with a chloride solution, formed an excellent reference electrode, of reference potential determined only by the chloride concentration and the temperature. Because polarisation of the test microelectrode produced only small currents, use of the Ag/AgCl electrode as a counter electrode did not change its potential; the polarising current (absolute value) was well below the exchange current (absolute value) of the Ag/AgCl system. Thus the counter electrode maintained a constant potential, irrespective of the fact that it carried the polarising current, and was used successfully as a simultaneous reference electrode. With this combination, an electrode potential could then be preset and applied simply by connecting the working microelectrode and the combination reference/counter electrode together with a DC battery, linked in series with a voltage divider. This set the applied cell voltage, which was automatically also the electrode potential. The system employed a set of dry cells linked in parallel to one another to provide current from a maximum voltage of 1.5 V for a long period of time, via the voltage divider. If a higher potential was required, the battery system could be doubled in series to give 3 V, and more if necessary. The system obviated the necessity for a potentiostat, with the electronic noise that such instruments can engender. In many cases a commercial saturated calomel electrode could also be used as the combined counter/reference electrode, and this proved useful for examining the behaviour in chloride-free electrolytes. Detection of the current transients required the use of a current ampli er, AC powered, connected into the circuit. The device used could allow recording of very tiny currents because of its high ampli cation; an output of up to 101 0 V A 1 was used routinely, enabling currents of 51 pA to be recorded. This device was used as a zero resistance ammeter in series with the two electrodes. To minimise background noise, background electric elds, and background magnetic elds, the cell with the electrodes and polarising source were mounted into a pair of concentric Faraday cages (one of mild steel and one of aluminium), both short circuited and earthed. The current ampli er was also mounted into a single, separate, adjacent Faraday cage, short circuited to the other two. The output was then fed to digital storage equipment. The experiment was also sensitive to vibration. Thus the equipment was mounted on No. 1

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1 Current stemming from Type 304L stainless steel microelectrode of diameter 50 l m in a solution containing 0.025 M HCl and 0.075 M HClO4 (upper trace) as well as that obtained from a chloride free 0.1 M HClO4 solution (lower trace), both measured at 0.2 V(SCE). (adapted from Ref. 18)

aluminium and wood platforms, each layer separated by blocks of foam rubber, the entire assembly being situated on a concrete bench. Experiments consisted of presetting the potential to a desired level, and polarising the specimen surface for periods of time which depended on the rate of data acquisition. It turns out that metal surfaces display early stages of pitting at potentials below the so called pitting potential. This phenomenon was usefully employed in experiments because above the pitting potential, the current becomes quite high once a pit has been formed. Below the pitting potential, while pits are still formed, none can achieve a state of stable growth, and the pits must repassivate. Thus many events can be observed in one experiment.

PITTING AT POTENTIALS BELOW THE PITTING POTENTIAL Figure 1 shows the current from a Type 304L stainless steel microelectrode polarised in a solution containing 0.025 M Cl with added perchloric acid of pH 1. (Testing showed that this alloy was not sensitised.) The rst 150 s are shown of a long run. It is plain that the metal passivates continuously with time, since the overall current decays continuously in an approximately exponential fashion. The potential shown is well below the pitting potential for this particular stainless steel, by some 0.4 V. The decaying anodic current is interrupted frequently by brief spurts of current surge, all in the anodic direction. These transient surges in current are the nucleation, and in some cases the propagation, of pits. Because the metal is well below the pitting potential, those events that propagate do not do so inde nitely, and therefore constitute metastable pits; that is to say, they propagate brie y, but then repassivate. It is easy to distinguish the events that propagate metastably from those that nucleate only. A transient due to metastable propagation is shown in Fig. 2. There is an initiating surge in current which is sharp, followed by a partial decay as the nucleated pit tries to repassivate. This is the pit nucleation event. Repassivation is, however, incomplete, and a much slower current rise follows. This slower current rise is the propagation of the metastable pit: the pit is growing during this period by fast rate anodic

Origins of pitting corrosion 27

Current transient due to propagation of a single metastable pit on Type 304L stainless steel in a solution containing 0.025 M HCl and 0.075 M HClO4, at 0.2 V(SCE). The pit nucleates with a sharp anodic current surge, followed by partial decay then some propagation before the current returns to the original background level because of repassivation

dissolution. Only propagation can cause this slow rise in current with time. Finally, after a brief period of propagation, the current suddenly decays and the pit repassivates. Many events show only the nucleation process, with no evidence of propagation at all, and an example of this is shown in Fig. 3. The transient is characterised only by a sharp current surge similar in every way to the initiating part of Fig. 2. In Fig. 3, the current then decays continuously back to the background level, with no succeeding rise at all. This continuous decay is evidence that the pit nucleated here does not propagate, but repassivates immediately. In point of fact, it is indeed possible that some metal dissolution occurs here too, but since the current decays, we have no evidence of accompanying metal dissolution, and we de ne this as nucleation only. Many metals display events of this type in chloride solution. An example is shown in Fig. 4, this time with titanium in chloride solution well below the pitting potential (by more that 1 V). The event plotted in Fig. 4 represents nucleation only, but metastable pit propagation can also be observed in this system. We can thus identify two main types of event in chloride solution that lead to pitting at potentials below the pitting

3

Current transient due to nucleation only of a pit on 304L stainless steel in a solution containing 0.025 M HCl and 0.075 M HClO4 , at 0.2 V(SCE). The initial sharp current surge is followed by a continuous decay back to the steady state


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Current transient due to nucleation only of a pit on commercially pure titanium in a solution containing 1.0 M HCl at a potential of 0.5 V(SCE)

potential. One is pit nucleation only, and the other is pit nucleation followed by metastable propagation. The initiating event is fast and sharp, and appears to be the same in both cases. When viewed under high resolution, a further re nement of the observations can be made. Figure 5 shows an anodic current transient generated on stainless steel which displays an initiating fast rise, succeeded by a brief current plateau before repassivation ensues. This event clearly is a pit initiation followed by a very brief period of propagation over the plateau current, before the event dies through repassivation. The fact that the current is constant at the plateau is evidence that the metal dissolves here, since the anodic charge evolved must either form passivating oxide, which causes current decay, or it must form dissolved ions; there is no alternative. Thus we can de ne another step in this process. Following the sharp nucleation of the pit, a brief period of propagation may occur directly and then repassivate, or it may lead to more extended propagation into a metastable pit.

5

Anodic current transient measured at high resolution showing a current plateau (with some superimposed noise) immediately after nucleation followed by rapid repassivation. Type 304L stainless steel microelectrode of diameter 50 l m measured in a solution containing 0.025 M HCl and 0.075 M HClO4 , at 0.2 V(SCE) (adapted from Ref. 19)


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6 Fraction of pit nucleation events that propagate into metastable pits plotted against nucleation amplitude. Nucleation amplitude has been divided into logarithmic ranges each span covering half a decade, as follows. A: 1.6 ± 5.056 pA. B: 5.056 ± 16 pA. C: 16 ± 50.56 pA. D: 50.56 ± 160 pA. E: 160 ± 505.6 pA. Data for a Type 304L stainless steel microelectrode of diameter 50 l m measured in a solution containing 0.025 M HCl and 0.075 M HClO4 , at 0.2 V(SCE). The two graphs show data measured for the same steel ® nished by grinding to 1200 grit and to 4000 grit (adapted from Ref. 18)

It is important to establish what characteristic of the nucleation event leads to metastable propagation. The rst and most immediately obvious issue is to consider the amplitude of the initiating event, on the basis that the higher the initiating current transient, the more likely is the nucleated pit to propagate. Thus, in Fig. 6 this is analysed for Type 304L stainless steel in chloride solution at a single potential by plotting the fraction of nucleation events that propagate into metastable pits as a function of the amplitude of the nucleating current spike. To construct this graph, many current transients were analysed by classifying their nucleation amplitudes into a set of amplitude ranges. Because of the wide spread of recorded nucleation amplitudes, the sets were incremented logarithmically. It is clear from Fig. 6 that the likelihood of propagating a metastable pit from a nucleation event does indeed increase with increase in the amplitude of the nucleation current spike. In other words, large initiating events are more likely to cause pit propagation. This is however, not the only parameter that determines the ability to propagate, since the graph shown in Fig. 6 tends to atten off at high amplitudes. One possibility for this is that the incidence of large amplitude nucleation events decreases as the amplitude rises; that is to say, bigger events are rarer than small ones, and so the statistical signi cance of the data at higher amplitude would be degraded. However, the effect seen in Fig. 6 has proved to be fairly reproducible, and statistical signi cance is an unlikely explanation. It is deduced, therefore, that larger nucleation events are indeed more likely to lead to pitting, but there are other factors involved as well. Finally, Figs. 7 and 8 show the frequency of nucleation events as a function of time, these being plotted for a single chloride concentration and a single potential for Type 304L stainless steel and for titanium. An important issue is displayed. The frequency of nucleation on stainless steel decays continuously with time of exposure, ultimately falling to zero. On titanium, however, the frequency does not decay, and may indeed show a rise with time of exposure. This means that the nucleation events on the steel arise from discrete sites. The sites for pitting on stainless steel are commonly regarded as sulphide inclusions. Once these sites are exhausted, either through metastable pit propagation, or No. 1

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7 Frequency of nucleation of corrosion pits on Type 304L stainless steel of diameter 50 l m in 0.025 M HCl and 0.075 M HClO4, at 0.2 V(SCE) as a function of time of polarisation. The error bars show the standard deviation in the data after taking the mean of many identical runs (adapted from Ref. 18)

through nucleation, no further events are possible as long as the conditions are not changed. On titanium there is no evidence that discrete sites are involved, and it appears that the events can carry on for extended periods.

DISCUSSION The data shown above indicate that a common event describes pit nucleation on metals, and it is sharp and microscopically violent. The model proposed for this is as follows. Passivation is caused by oxide lm growth, which requires ion migration across the existing lm. If the migrating ion is the oxide anion, new oxide lm forms at the metal/oxide interface. The steady state is of course reached when the rate of formation of oxide lm at that interface equals the rate at which the oxide lm dissolves from the lm/solution interface. Assuming that oxide ions do indeed migrate in that way, it is only reasonable to allow chloride anions to migrate in parallel with the oxide. The much lower concentration of chloride in an aqueous environment (water is effectively an in nite source of oxide anions.), coupled with the larger size of the Cl anion means that chloride ion migration is more dif cult and rarer. Chloride anions that migrate across the oxide can also react with the metal at the metal/oxide interface, and in this case must form metal chloride. At the (probably) rare sites where suf cient metal chloride forms that a salt nucleus exists, its large molar volume causes expansion of the oxide lattice above, and passive lm rupture occurs. It is to be noted that many metal chlorides have a larger molar volume than the corresponding metal oxides. This is the violent nucleation event characterised by the fast rise time (Figs. 2 – 5). Upon rupture of the passivating lm, the nucleus of metal salt is immediately exposed to the chloride solution, and becomes instantaneously saturated locally. This can cause propagation for the period taken by the dissolving salt to disperse into the bulk environment, which would be expected to be very short. This is believed to be the origin of the event shown in Fig. 5. The embryonic pit cannot sustain its existence beyond the simple dissolution and accompanying dispersion of the local metal chloride; the pit repassivates as soon as the salt concentration falls below saturation. Alternatively, if the dissolving metal chloride lies in such a position that its exit from the site is restricted, then it may remain microscopically saturated, and establish a diffusion gradient to the exterior bulk solution. In this case, propagation can proceed in a more extended fashion, and the pit may grow for a more prolonged period: this is metastable pit growth (Fig. 2). The basic model accounts

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Origins of pitting corrosion 29

Frequency of nucleation of corrosion pits on commercially pure titanium of diameter 50 mm in 1.5 M HCl at a potential of 0.5 V(SCE) as a function of time of polarisation. Data for one run only (adapted from Ref. 13)

for all types of observed current transient, at least qualitatively. Turning now to the propagation process, it is clear that in order to obtain a diffusion-controlled process, two features must be considered. The amplitude of the initiating event is clearly an important factor. As the nucleating current increases, it is more likely that diffusion control can be attained, and it is only under diffusion control that the saturated salt solution can exist to allow propagation. However, the geometry of the nucleated site is also important in this. For a given value of the nucleating current (the amplitude), a more occluded site is more likely to reach the diffusion-controlled limit than a more open site, simply because outward movement of the dissolving metal cations is more restricted in the more occluded site. Thus the form of Fig. 6 may be accounted for. The likelihood of propagation does rise with increasing nucleation amplitude, but the other factor here is the site geometry. This factor also accounts for the effect of surface roughness on pitting, an often neglected factor. Rougher surfaces show a greater tendency to pitting, because they show a greater tendency towards less open pit sites. In this connection it is noteworthy that the pitting potential of metal surfaces is also a function of the surface nish. Finally, the frequency of pit nucleation must be addressed. Stainless steels are known to undergo pitting corrosion from the sulphide inclusions, at least preferentially. Once these have gone, either through repetitive nucleation, or by nucleation and metastable propagation, then further pitting is reduced substantially or eliminated, provided the conditions do not change. This accounts for the frequency graph of Fig. 7. There are several reasons that can be invoked for the sulphide inclusions to be the susceptible sites. One could be, for example, that this is the only part of the passive oxide on stainless steels where the chloride can actually migrate through. Or perhaps these are the only sites where suf cient chloride can accumulate to allow oxide rupture to occur. Whatever the reason, it is clear that the chromium oxide rich lm that normally passivates stainless steel is very resistant to the passage of chloride. Not all metals show this behaviour however, and may undergo pitting at random positions over the surface. Certainly, weaker sites in the oxide may be preferential, but they need not be exclusive, and this appears to be the situation for titanium shown in Fig. 8. The fact that the frequency does not decay signi cantly is evidence that the only requirement for pit nucleation on titanium is the presence of the passivating oxide lm and the chloride anion.


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It should be noted that the pitting potential does not demarcate any of these processes. The sole interpretation that can be attached to the pitting potential, is that it demarcates the potential above which nucleated pits can propagate inde nitely to achieve a state of stable growth, and below which, stable growth cannot be achieved. This also accounts for the fact that the pitting potential decreases as the surface roughness increases, since this too, involves the ability of the metastably propagating pit, propagating under diffusion control, to transfer into stable pit growth. Once stable pit growth is achieved, the geometry of the pit itself is suf cient to sustain its own diffusion control, and the geometry of the rest of the surface becomes irrelevant to the propagation of that individual pit.

CONCLUSIONS Pitting corrosion occurs in a series of consecutive steps. The nucleation event is a sharp, microscopically violent event, attributed to chloride migration from the electrolyte to the metal lm interface. The pit nucleates when the volume expansion at that interface causes passive lm rupture. The nucleated event can repassivate immediately, or it can propagate through the formation of a locally saturated salt solution during dissolution of the chloride salt. This is the embryonic pit and its formation is the second step. If the current is high enough and/or the site is suf ciently occluded, the embryonic pit may develop into a metastable pit, in which stability is maintained by occlusivity as well as the surrounding vestiges of the ruptured oxide lm. In the metastable state, which is the third consecutive growth step, the pit is not of itself suf cient to sustain diffusion control; the geometry of the site and the surrounding passive lm are required to prevent wash-out and repassivation. If the pit propagates metastably to a suf cient size then it may reach a state where its growth is stabilised by its own size, geometry and depth and may then propagate inde nitely. This is stable pit growth and the potential at which this occurs de nes the pitting potential. The pitting potential characterises only the transition from metastable pit growth to stable pitting. Stable growth of the pit is the fourth and last step in the pit growth kinetic sequence. The pitting potential is not a characteristic breakdown potential of surface. This is an important distinction to


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recognise in circumstances where the surface integrity of a component is required.

ACKNOWLED GEMENTS We are grateful to the EPSRC, the British Council and the Acciones Integradas Programme for nancial support.

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