Pitting Corossion

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Pitting CorrosionG.S. Frankel, The Ohio State University

Fig. 1 Deep pits in a metal

MANY ENGINEERING ALLOYS, such asstainless steels and aluminum alloys, are usefulonly because of passive films, which are thin(nanometer-scale) oxide layers that form natu-rally on the metal surface and greatly reduce therate of corrosion of the alloys. Such passivefilms, however, are often susceptible to localizedbreakdown, resulting in accelerated dissolutionof the underlying metal. If the attack initiates onan open surface, it is called pitting corrosion; atan occluded site, it is called crevice corrosion.These closely related forms of localized corro-sion can lead to accelerated failure of structuralcomponents by perforation or by acting as aninitiation site for cracking. Figure 1 shows anexample of deep pits on a metal surface.

It should be noted that, whereas localized dis-solution following breakdown of an otherwiseprotective passive film is the most common andtechnologically important type of pitting corro-sion, pits can form under other conditions aswell. For instance, pitting can occur during ac-

tive dissolution if certain regions of the sampleare more susceptible and dissolve faster than therest of the surface. This section concentrates onthe better-known and widely studied phenome-non of pitting corrosion of passive metals.

Pitting corrosion is influenced by many dif-ferent parameters, including the environment,metal composition, potential, temperature, andsurface condition. Important environmental pa-rameters include aggressive ion concentration,pH, and inhibitor concentration. Other phenom-enological aspects of localized corrosion includethe stochastic nature of the processes and thestages of localized attack, including passive filmbreakdown, metastable attack, stable growth,and perhaps eventual arrest.

Phenomenologyof Pitting Corrosion

Environment and Development of LocalEnvironment. Classical pitting corrosion causedby passive film breakdown will only occur in thepresence of aggressive anionic species, and chlo-ride ions are usually, although not always, thecause. The severity of pitting tends to vary withthe logarithm of the bulk chloride concentration(Ref 1). The reason for the aggressiveness of

chloride has been pondered for some time, anda number of notions have been put forth. Chlo-ride is an anion of a strong acid, and many metalcations exhibit considerable solubility in chlo-ride solutions (Ref 2). Chloride is a relativelysmall anion with a high diffusivity; it interfereswith passivation, and it is ubiquitous as a con-taminant.

The presence of oxidizing agents in a chlo-ride-containing environment is usually ex-tremely detrimental and will further enhance lo-calized corrosion. It should be noted thatchromate is an oxidizing agent that typically in-hibits corrosion by reducing to form Cr(III) film.Most oxidizing agents enhance the likelihood of pitting corrosion by providing extra cathodic re-actants and increasing the local potential. Of course, dissolved oxygen is the most commonoxidizing agent. One of the reactions by whichoxygen reduction occurs is:

O H O 4 OH

pH vs. SHE

2 2

rev

+ +

= −

− −2 4

1 23 0 059

e

E

→

( ). . (Eq 1)

where SHE is standard hydrogen electrode.Removal of oxidizing agents, such as removal

of dissolved oxygen by deaeration, is one pow-erful approach for reducing susceptibility to lo-calized corrosion. The influence of potential onpitting corrosion is described subsequently.

Pitting is considered to be autocatalytic in na-ture; once a pit starts to grow, the local condi-tions are altered such that further pit growth ispromoted. The anodic and cathodic electrochem-ical reactions that comprise corrosion separatespatially during pitting (Fig. 2). The local pit en-vironment becomes depleted in cathodic reactant

(e.g., oxygen), which shifts most of the cathodicreaction (such as is given by Eq 1) to the boldlyexposed surface outside of the pit cavity, wherethis reactant is more plentiful. The pit environ-ment becomes enriched in metal cations as a re-sult of the dissolution process in the pit (writtenfor a generic metallic element, M):

M rMn ne (Eq 2)

The concentration of an anionic species suchas chloride must also increase within the pit inorder to balance the charge associated with the

cation concentration and to maintain chargtrality. This enrichment of anions occurs btromigration from the bulk solution in reto the potential gradient that develops as aof the ohmic potential drop along the cpath between the inside of the pit and thodic sites on the boldly exposed surface. nal aspect of the local pit environment thabe considered is the pH, which decreases,to cation hydrolysis:

M H O M OH H O

M OH H

2 2e e

e

2

2

2

2

+ + +

+

+ + Η +

+

→ ( )

→ ( )

The common cathodic reactions that mcompany the dissolution occurring in tsuch as the oxygen reduction reaction (result in a local increase in the pH at the casites. The acidity developed in the pit is notralized by the cathodic reaction becausespacial separation of the anodic and catho

actions. In summary, the local pit envirois depleted in the cathodic reactant, such solved oxygen; enriched in metal cation anionic species, such as chloride; and acThis acidic chloride environment is aggrto most metals and tends to prevent repassand promote continued propagation of th



2 / Forms of Corrosion

Fig. 3 Schematic of a polarization curve showingcal potentials and metastable pitting regio

pitting potential; E R, repassivationpotential; E corr, corpotential. Source: Ref 1

Fig. 2 Autocatalytic process occurring in a corrosion pit. The metal, M, is being pitted by an aerated NaCl solution.Rapid dissolution occurs in the pit, while oxygen reduction takes place on the adjacent metal surfaces.

A detailed analysis of the influence of pitchemistry changes on pit growth and stability isprovided in Ref 2 and 3. The concentration of various ionic species at the bottom of a modelone-dimensional pit geometry was determined asa function of current density based on a materialbalance that considered generation of cations bydissolution, outward diffusion, and thermody-namic equilibrium of various reactions such ascation hydrolysis (Eq 1). It was found that a criti-cal value of the product x • i, where x is pit depthand i is current density, corresponded to a criticalpit acidification for sustained pit growth. Currentdensity in a pit is a measure of the corrosion ratewithin the pit and thus a measure of the pit pen-etration rate. This x • i value can be used to de-termine the current density required to initiate orsustain pitting at a defect of a given size.

As the pit current density increases, the ionicconcentration in the pit solution increases, oftenreaching supersaturation conditions. A solid saltfilm may form on the pit surface, at which pointthe ionic concentration would drop to the satu-

ration value, which is the value in equilibriumwith the salt layer. Under these conditions, thepit growth rate is limited by mass transport out

of the pit. Salt films are not required for pit sta-bility (although some have suggested that theyare) (Ref 4–9), but they enhance stability by pro-viding a buffer of ionic species that can dissolveinto the pit to reconcentrate the environment inthe event of a catastrophic event, such as the sud-den loss of a protective pit cover. Under mass-transport-limited growth, pits will be hemispher-ical with polished surfaces. In the absence of asalt film (at lower potentials), pits may be crys-tallographically etched or irregularly shaped insome other fashion.

Potential. Electrochemical studies of pittingcorrosion have found that characteristic poten-tials exist. Stable pits form at potentials noble tothe pitting potential, E P, and will grow at poten-tials noble to the repassivation potential, E R,which is lower than E P. The effect of potentialon pitting corrosion and the meaning of thesecharacteristic potentials can best be understoodwith the schematic polarization curve shown inFig. 3. This figure is a plot of the potential versusthe logarithm of the current density. Potential is

measured versus a reference electrode, com-monly a saturated calomel electrode (SCE), anda potentiostat is used, along with an auxiliary or

counter electrode, to make such measuremAs mentioned previously, current densitymeasure of the rate of reaction. Common tice for measuring such curves involves potiodynamic polarization or automatic scanof the potential from a low value, such acorrosion potential, to higher values (Ref 1

The schematic polarization curve in Fshows the case of a spontaneously passiveterial, meaning that a protective passive fipresent on the metal surface at the open cior corrosion potential, E corr. During upscanning, breakdown occurs, and a stablstarts growing at the pitting potential E P, wthe current increases sharply from the pacurrent level and, on reversal of the scan dtion, repassivates at E R, where the current dback to low values representative of passivesolution. Corrosion experts generally conthat materials exhibiting higher values of E

E R are more resistant to pitting corrosioncyclic polarization experiments are commused for this purpose. In an oxidizing env

ment, or for a material that is very susceptibpitting, the open circuit potential, which itermined by the intersection of the polarizcurves associated with the anodic and cathpartial reactions, will be above E P, and theterial will spontaneously pit at open circuit

A correlation has been found such that mwith low experimentally determined pittingtentials have a higher tendency to form pitsurally at open circuit (Ref 1). If the E corr ibelow the E P, then there is a low likelihoodthe potential will ever go high enough toproach the E P and initiate a pit. Thereforedifference between the E P and E corr in a genvironment is the margin of safety and is

used as a measure of the susceptibility to lized corrosion (Ref 11–13). Because the resivation potential E R is typically lower thathe difference between E R and E corr is a mconservative measure of pitting susceptibilithe corrosion potential were to always rebelow the potential at which pits repassithen there is a very low likelihood that pi



Pitting Corrosio

will occur at all. A final measure of pitting sus-ceptibility is the difference between E P and E R,which is related to the extent of hysteresis in acyclic potentiodynamic polarization curve. Gen-erally, alloys that are susceptible to pitting cor-rosion exhibit a large hysteresis.

It should be noted that several other namesand subscripts have been used to describe thesecharacteristic potentials. For instance, it is com-mon to use the term breakdown potential ( E b)for the initiation potential, because one is notalways sure if the form of localized attack is pit-ting, crevice corrosion, or intergranular corro-sion, or if the current increase is the result of general transpassive dissolution. The pitting po-tential is sometimes referred to as the pit nucle-ation potential, E np, and the repassivation poten-tial is sometimes called the protection potential,

E prot. If creviced samples are used, the potentialsmight be referred to as crevice potential, E crev,and crevice repassivation potential, E r,crev.

The measures of susceptibility described pre-viously are useful for comparing the vulnerabil-

ity of various alloys to localized corrosion in agiven environment or for comparing the relativeaggressiveness of different environments. How-ever, there is abundant experimental evidencesuggesting that these interpretations of the char-acteristic potentials are simplistic and insuffi-cient for the development of a fundamental un-derstanding of the mechanism of pittingcorrosion. For instance, the potentiodynamicallydetermined pitting potential of many materialsexhibits a wide experimental scatter, of the orderof hundreds of millivolts. Furthermore, E P is, inmany cases, a function of experimental param-eters, such as potential scan rate. As is describedsubsequently, so-called metastable pits initiate

and grow for a period at potentials well belowthe pitting potential (Ref 14), which providesevidence in contradiction to the definition of thepitting potential as being the potential abovewhich pits initiate. The meaning of the repassi-vation potential has also been called into ques-tion. The E R of ferritic stainless steel decreases(i.e., moves in the active direction) with increas-ing values of the current density at which thepotential scan direction is reversed (Ref 12, 15).So, deeper pits apparently repassivate at lowerpotentials. In contrast, the repassivationpotentialfor pits in aluminum seems to be relatively in-dependent of the extent of prior pit growth for alimited number of experiments (Ref 16). A simi-lar lack of dependence of E

Ron prior growth has

been found for pits in stainless steel and othercorrosion-resistant alloys but only after the pas-sage of large charge densities (Ref 17). Further-more, pits did not initiate at potentials below thislimiting E R, even after very long times (up to 38months), which validates the use of the repassi-vation potential as a design criterion (Ref 18).

Alloy composition and microstructure canhave strong effects on the tendency for an alloyto pit (Ref 19). Chromium concentration playsthe dominant role in conferring passivity to fer-rous alloys. The pitting potential was corre-spondingly found to increase dramatically as the

chromium content increased above the critical13% value needed to create stainless steel (Ref 20). Increasing the concentration of nickel,which stabilizes the austenitic phase, moderatelyimproves the pitting resistance of iron-chromium(Ref 20). Small increases in certain minor alloy-ing elements, such as molybdenum in stainlesssteels, can greatly reduce pitting susceptibility(Ref 19). Molybdenum is particularly effectivebut only in the presence of chromium. Smallamounts of other elements, such as nitrogen andtungsten, also have a strong influence on the pit-ting resistance of stainless steels (Ref 21, 22).

Various measures have been developed to de-scribe the beneficial effects of steel compositionon resistance to localized corrosion. The pittingresistance equivalent number (PREN) was orig-inally developed as a pitting index for stainlesssteels (Ref 22):

PREN Cr 3.3Mo 16N (wt%) (Eq 4)

The multiplier value for nitrogen could be ashigh as 30. The PREN has been correlated tovarious other measures of corrosion resistancefor stainless steels, such as the critical pittingtemperature, which is described in the next sec-tion.

Because aluminum is a very active and reac-tive metal, the homogeneous addition of almostany metal (except zinc, lead, and magnesium)into aluminum alloys results in an increase inpitting potential (Ref 23–28). In order for thisalloying to be beneficial, it is essential that thestructure remain single phase. The pitting poten-tial of binary aluminum-copper alloys increasedwith copper concentration as long as the copper

was in solid solution (Ref 29).Pits almost always initiate at some chemical

or physical heterogeneity at the surface, such asinclusions, second-phase particles, solute-segre-gated grain boundaries, flaws, mechanical dam-age, or dislocations (Ref 19). Most engineeringalloys have many or all such defects, and pitswill tend to form at the most susceptible sitesfirst. Pits in stainless steels are often associatedwith MnS inclusions, which are found in mostcommercial steels. The role of MnS inclusionsin promoting the breakdown and localized cor-rosion of stainless steels has been recognized forsome time (Ref 30, 31). Recent improvementsin alloy production have led to steels with lowersulfur content to improve pitting resistance.

Pits in aluminum alloys are typically associ-ated with intermetallic particles (Ref 32, 33). Asdescribed previously, copper additions to alu-minum resulted in improvements in pitting re-sistance when the copper was in solid solution.However, when particles of the intermetallic h

phase (Al2Cu) formed, the resistance to pittingdecreased back to the range of aluminum alloyedwith little copper (Ref 29). The decrease in pit-ting potential with formation of h phase was ex-plained by the existence of a copper-depleted re-gion near the particles (Ref 29). This regionwould have a lower pitting potential, so pits

would tend to form there first. Anotherfound that microsegregation of copper animpurities at nodes in high-purity aluminusufficient to increase the tendency for pittirosion at open circuit (Ref 34).

Temperature is also a critical factor in corrosion, because many materials will noa temperature below a certain value, whicbe extremely sharp and reproducible (R41). This effect can be seen either by varytemperature at a range of fixed applied potor by varying the potential for a range of cotemperature experiments. Figure 4 is a ppitting and repassivation potentials for thrferent stainless steels in 1 M NaCl as a fuof solution temperature (Ref 40). At lowperatures, extremely high breakdown potare observed, corresponding to transpassisolution, not localized corrosion. Just abocritical pitting temperature (CPT), pittinrosion occurs at a potential that is far beltranspassive breakdown potential. This vaCPT is independent of environmentalpara

and applied potential over a wide range ameasure of the resistance to stable pit prtion (Ref 35). At higher temperatures, the potential decreases with increasing tempand chloride concentration. The CPT cused, similar to pitting potential, as a meranking susceptibility to pitting corrosiohigher the CPT, the more resistant the allopitting (Ref 35). If crevice corrosion is tmary concern, creviced samples can be udetermine a critical crevice temperature which is typically lower than the correspCPT. Aluminum alloys do not exhibit a Caqueous chloride solutions at temperatureto 0 C (32 F) (Ref 42).

Surface Condition. The exact conditiosurface can have a large influence on the behavior of a material. In general, samplpared with a rough surface finish are moceptible to pitting and exhibit a lower pittitential. For example, the pitting potential o302 stainless steel with a 120-grit finisshown to be approximately 150 mV lowethat for the same material with a 1200-gritover a range of chloride concentrations (RThe effect of surface roughness on pittinglated to the stabilization criteria describesequently. Rougher surfaces have more ocsites, which can sustain the conditions refor active dissolution at lower current deand thus lower potentials because of the diffusion path length and slower rate ofsion.

For stainless steels, heat treatment, griand abrasive blasting have been reporteddetrimental to pitting resistance, wherealing in HNO3 HF scales or passivatHNO3 is beneficial (Ref 22). Heat treatmair generate a chromium oxide scale and amium-depleted region under the scale. This typically removed mechanically, and thmium-depleted region is removed by pi(Ref 22). Other common surface defects iheat tint from welding, embedded iron pa




Fig. 4 Pitting (filled symbols) and repassivation (open symbols) in 1 M NaCl as a function of temperature for difgrades of stainless steel. SCE, saturated calomel electrode. Source: Ref 42

from machining, and MnS inclusions. The det-rimental effects of these defects are minimizedand the overall surface condition improved bypassivation in nitric acid, which increases thechromium content of the surface oxide film.

The effects of surface condition on localizedcorrosion are significant enough that care mustbe taken to not apply experimental data collectedon samples with special preparation to a real ap-plication without taking the surface conditioninto account.

Inhibitors. Pitting can be inhibited by thesame approaches that are commonly used to re-duce corrosion in general. All of the factors de-scribed previously can be used to mitigate pittingcorrosion: environment, alloy composition andstructure, potential, and temperature. As men-tioned previously, oxidizing agents acceleratepitting by increasing the potential, so removal of oxidizing agents, for instance, by deaeration, re-duces the tendancy for pitting corrosion.

Various chemicals, when added to corrosivesolutions, will inhibit pitting (Ref 19). Common

inorganic inhibitors include sulfates, nitrates,chromates, and molybdates. Some, such as sul-fate, may act simply by providing supportingelectrolyte that reduces the migration of chlorideions into the pit. It was suggested that nitratemight reduce inside pits in aluminum, consum-ing protons and thereby reducing pH (Ref 44).Others might adsorb at active sites or reduce pitgrowth kinetics.

High-strength aluminum alloys, which aresusceptible to pitting, owing to the influence of copper-containing intermetallic particles, are of-ten protected using a system of coatings. Thestandard coating system uses a chromate con-version layer covered by organic paint coats.The

primer coat might contain chromatepigments forfurther corrosion protection. The chromate con-version layer is formed by immersion into anacidic bath containing dichromate, fluoride, andferricyanide. The fluoride destabilizes the alu-minum oxide, allowing the following reaction tooccur (Ref 45, 46):

Cr O H Al

2Al Cr OH H O

2 72

32

− +

+

+ +

+ +

8 2

23

→ ( ) (Eq 5)

Chromate conversion layers also containsomeamount of unreduced chromate ions as a resultof adsorption of chromate onto a CrIII x(OH) ybackbone (Ref 47). The resulting coating is aCrIII-CrIV mixed oxide with an approximately 3to 1 CrIII to CrIV molar ratio. The chromate re-tained in the coating is critical for providing aself-healing capability (Ref 45, 46). Chromate-coated samples scratched to the substrate and ex-posed to a corrosive environment such as a saltspray will usually not exhibit severe corrosion atthe scratch. Chromate in the conversion coatingcan be released into solution, where it is mobileand migrates to exposed areas on the aluminumalloy surface. Even at very dilute concentrations,chromate in solution adsorbs on active sites onthe alloy and is reduced to form a monolayer of

CrIII species by a reaction similar to the one thatoccurs when the conversion coating forms (Eq5). This layer is effective at reducing the activityof both anodic and cathodic sites on the alloysurface. Chromate pigment in primer should actthe same way to protect a scratched area. Owingto the carcinogenic nature of chromate, consid-erable effort has been put into developing anequally effective and environment-friendly re-placement system. However, nothing developedto date is as effective as chromate for reducingthe corrosion of high-strength aluminum alloys(Ref 48).

Stochastics. Because pitting events are rela-tively rare and unpredictable, pit initiation maybe considered to be random in nature. Stochasticor probabilistic approaches have been developedto handle this randomness and the large scattertypically observed in measurements of pittingpotential and induction time (which is the timefor a stable pit to form following a sudden in-crease in potential into the pitting range, or fol-lowing the injection of chloride into a nonag-

gressive solution). A large ensemble of pittingpotential values follows a normal distribution,suggesting random variation (Ref 49). The prob-ability for pitting (P) can be determined by:

P( E ) n /(1 N ) (Eq 6)

where N is the total number of samples studied,and n is the number of samples that had pittedat a potential of E or lower (Ref 49). The poten-tial at P 0.5 is a representative value for agiven material and surface preparation. Induc-tion times at a given potential can also be mea-

sured and the survival probability, P(t ), dmined using Eq 6, except that n is the numof samples that initiate pits by time t afteplication of the potential. The pit-generationk, can then be given by (Ref 49):

λ t d

dt P t ( ) ( )= − ln

(

The value of the pit-generation rate can alused as a measure of susceptibility to pittin

Stages of Pitting

Pitting can be considered to consist of vastages: passive film breakdown, metastableting, pit growth, and pit stifling or death. Anthese stages may be considered to be the critical. For instance, once the passive breaks down and a pit initiates, there is a pbility that a stable pit will grow. On the ohand, pits will not initiate if they cannot groleast for a short while. The passive state iquired for pitting to occur, but some researcbelieve that details of the passive film comsition and structure play a minor role in theting process. This view is supported by thethat many observations of pitting tendencybe fully accounted for by growth consideratFurthermore, pit growth is critical in pracapplications of failure prediction. Finallymetastable pitting stage may be thought to bmost important, because only pits that surthis stage become stable growing pits. Metble pits exist on the edge of stability. Studi



Pitting Corrosio

metastable pits can therefore provide insight intofundamental aspects of pitting, because both ini-tiation and stability are key factors in metastablepitting.

Pit Initiation and Passive Film Breakdown.The breakdown of the passive film and the de-tails of pit initiation comprise the least under-stood aspect of the pitting phenomenon. Break-down is a rare occurrence that happensextremely rapidly on a very small scale, makingdirect observation extraordinarily difficult. Thepassive film is often drawn schematically as asimple inert layer covering the underlying metaland blocking access of the environment to themetal. The reality is, of course, much more com-plicated. Depending on alloy composition, en-vironment, potential, and exposure history, thisfilm can have a range of thickness, structure,composition, and protectiveness. Typical passivefilms are quite thin and support an extremelyhigh electric field (on the order of 106 to 107 V/ cm). The passage of a finite passive current den-sity is evidence of continual reaction of the

metal, to result in film thickening, dissolutioninto the environment, or some combination of the two. The view of the passive film as being adynamic structure, rather than static, is criticalto the proposed mechanisms of passive filmbreakdown and pit initiation.

Theories for passive film breakdown and pitinitiation have been categorized into three mainmechanisms that focus on passive film penetra-tion, film breaking, or adsorption (Ref 50, 51).As with most such situations, different mecha-nisms or combinations of these mechanisms maybe valid for different metal-environment sys-tems. These mechanisms have been consideredin terms of pure metal systems. However, pits in

real alloys are most often associated with inclu-sions or second-phase particles, and these factorsmust also be taken into consideration.

Metastable Pitting. Metastable pits are pitsthat initiate and grow for a limited period beforerepassivating. Large pits can stop growing for avariety of reasons, but metastable pits are typi-cally considered to be those of micron size, atmost, with a lifetime on the order of seconds orless. Metastable pits are important to understandbecause, under certain conditions, they continueto grow to form large pits. Metastable pits canform at potentials far below the pitting potential(which is associated with the initiation of stablepits) and during the induction time before theonset of stable pitting at potentials above the pit-ting potential. These events are characterized bypotential transients in the active direction at opencircuit or under an applied anodic current, or an-odic current transients under an applied anodicpotential. Such transients have been reported instainless steels (Ref 14, 52–58) and aluminum(Ref 59, 60) for many years. Individual metasta-ble pit current transients can be analyzed for pitcurrent density, and stochastic approaches can beapplied to groups of metastable pits. It has beenargued that when stable pits are small, they be-have identically to metastable pits and, in fact,are metastable (Ref 14). Stable pits survive the

metastable stage and continue to grow, whereasmetastable pits repassivate and stop growing, forsome reason.

Stable Pitting and Pit Growth. Pits grow ata rate that depends on material composition, pitelectrolyte concentration, and pit-bottom poten-tial. The mass-transport characteristics of the pitinfluence pit growth kinetics through the pit elec-trolyte concentration. Pit stability depends on themaintenance of pit electrolyte composition andpit-bottom potential that are at least severeenough to prevent repassivation of the dissolvingmetal surface at the pit bottom.

In order to understand pit growth and stability,it is essential to ascertain the rate-determiningfactors. Pit growth can be controlled by the samefactors that can limit any electrochemical reac-tion: charge-transfer processes (activation),ohmic effects, mass transport, or some combi-nation of these factors. Pit growth at low poten-tials below the range of limiting pit current den-sities is controlled by a combination of ohmic,charge transfer, and concentration overpotential

factors. At high potentials, mass transport maybe rate controlling. Ultimately, however, masstransport determines the stability of pits even atlower potentials, because the local environmentcontrols passivation. The rate of pit growth de-creases with time for pitting controlled by eitherohmic or mass-transport effects. The pit growthrate often varies with t n, where n is approxi-mately equal to 0.5.

Pits often grow with a porous cover. Thiscover can make visual detection extremely dif-ficult, so that the awareness of the severity of attack is overlooked and the likelihood of cata-strophic failure is enhanced. The pit cover mightbe a thick, precipitated product layer that forms

as the concentrated and acidic pit solution meetsthe bulk environment, which might be neutral orlimited in water, as in the case of atmosphericcorrosion. Small pits in stainless steels oftenhave a pit cover that is a remnant of the under-mined passive film (Ref 14). Larger pits in stain-less steel can be covered by a layer with a con-siderable thickness of metal that is detachedfrom the rest of the metal sample (Ref 61). Thesecovers make optical detection extremely diffi-cult, because they remain reflective. A short ex-posure to ultrasonic agitation, however, removesthe cover and reveals the whole pit diameter.

Death and Pit Arrest. Despite the autocata-lytic nature of pitting, large pits, which wouldbe considered to be stable by any criterion, canstop growing or die. As mentioned previously, if the conditions (environment and potential) at thedissolving wall of a pit are not sufficiently ag-gressive, the pit will repassivate. The potentialat the pit bottom is lower than that at the outersurface as a result of the ohmic potential dropassociated with current flow out of the pit. Asthe pit deepens, the ohmic path length and ohmicresistance increase. This tends to cause an in-crease in the ohmic potential drop, a decrease inthe local potential, and a decrease in the pit cur-rent density. The environment tends to be acidicand rich in chloride, owing to hydrolysis of the

dissolved metal cations and electrolytic tion of chloride into the pit. The high contion in the pit is depleted by transport outpit but is replenished by continued dissoluthe pit bottom. As the pit deepens, the rtransport out of the pit decreases, so the be stable with a lower anodic current denplenishing the environment. As mentioneviously, the pit current density tends to dewith time, owing to an increase in the pitand ohmic potential drop. Repassivationoccur if a sudden event, such as loss ocover, caused a sudden enhancement of traand dilution of the pit environment to thethat the rate of dissolution at the pit bwould be insufficient to replenish the logressive environment.

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Agrawal, Ed., Localized Corrosion,

NACE-3, NACE, 1974● H.-H. Strehblow, Mechanisms of Pitting

rosion, Corrosion Mechanisms in Theory

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● Z. Szklarska-Smialowska, Pitting Corroof Metals, NACE, 1986

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