Marcus P. Corrosion mechanisms in theory and practice

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Indeed, numerous experimental studies have been performed to study the

evolution of the environment in a crevice. Most data were obtained in actively

corroding artificial crevices or pits, either by sampling the solution or by direct pH

and Cl

–

concentration measurements. Table 1 summarizes significant results that

confirm the foregoing trends. In crevice solutions, the drop of pH depends on the

hydrolysis constants of the metal cations. On stainless alloys, chromium and

molybdenum are considered to be the cause of the very low, sometimes negative, pH

observed. Iron, nickel, and aluminum exhibit much less acidic hydrolysis reactions

and the pH values in the crevices are higher: values of 3 to 5 are reported for iron,

values of 3 to 4 for the aluminum alloys.

It must be noted that the pH drop is due to the dissolution of the base material:

thus, the more resistant the material, the lower is the pH required for dissolution to

occur. Thus, the more acidic crevice pH values are observed on the more resistant

materials. This is shown, for example, in the studies of Bogar and Fujii (Fig. 14 [24])

on Fe-Cr ferritic stainless steels and Suzuki et al. [28] on 304L, 316L, and

18Cr-16Ni-5Mo austenitic stainless steels (Table 1).

The concentration of the ions in the crevices can reach very high values of the

order of several mol/L [28,33], and the solution may eventually become

saturated, which possibly limits the dissolution rate. In such concentrated solutions,

salt films may form on the dissolving metal surfaces: NiCl

has been identified in

nickel [34] and FeCl

in iron [35] and stainless steel [35] pits or crevices.

Several studies also outline the strong effect of the potential of the free surfaces

on the pH drop inside an actively corroding crevice: in particular, early work of

Pourbaix [36] on unalloyed steel showed (Fig. 15) that the higher the potential of the

external surfaces, the higher the potential and pH drops in the crevice. Similar results

were obtained by Zuo et al. [37] for stainless steel actively corroding in an artificial

crevice. Conversely, Pourbaix [36] and Turnbull and Gardner [38] on unalloyed

Crevice Corrosion of Metallic Materials 361

Figure 13 Sketch of the main processes leading to environment evolution in a crevice.

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Table 1 Some Experimental Measurements of Local pH and Composition in Crevices

For more details see Turnbull [32].

Anodic polarization.

steels and Peterson and Lennox [39] on 304 stainless steel showed that cathodic

polarization of the free surfaces induced an increase of pH and a cathodic

polarization of the surfaces inside the crevice (Figs. 15 and 16).

In the artificial crevice experiments, several other features are interesting. As

an example, for the same 304 stainless steel:

The relationship between the total amount of anodic charge passed in an artificial

crevice and the pH is not unique (Fig. 17),

The relationship between the chromium content (and more generally the composition)

of the crevice solution and the pH is not unique and does not agree with any

simple hydrolysis reaction (Fig. 18),

Crevice Corrosion of Metallic Materials 363

Figure 14 Effect of chromium content of the alloy on the pH in the crevice. (From Ref. 24.)

Figure 15 Effect of the potential of the external surfaces on the pH and potential inside an

artificial C-steel crevice [36]. Same numbers refer to the corresponding conditions on the free

surface and inside the crevice.

The pH of a synthetic solution made of reagent grade transition metals and sodium

chlorides does not have the same pH as the crevice solution of the same

composition (Fig. 19). In addition, this pH spontaneously decreases with time

[33,40] (Figs. 18 and 20) for periods of time of several weeks which should not

be neglected when modeling the crevice evolution. Indeed, Zhen [41] showed,

by ultraviolet (UV) spectroscopy, the presence of at least two different Cr

compounds in stainless steel crevice solutions and observed a spontaneous

evolution with time toward one of these species.

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Figure 16 Effect of cathodic polarization by zinc anode on the pH and potential inside

a 304 SS crevice [37].

Figure 17 Experimental relationships obtained by different authors between pH and

dissolution current 304 SS artificial crevice and comparison with simple hydrolysis

equilibria [40].

Crevice Corrosion of Metallic Materials 365

Figure 18 Experimental relationship obtained by different authors between pH and Cr

content of artificial crevice solutions [40].

Figure 19 Comparison of artificial crevice solution with synthetic solution of the same

composition [40].

Thermodynamics of the Crevice Environments for Fe-Ni-Cr Alloys

Thus it appears that the detailed chemistry of the crevice environments is not very

well known for the following reasons:

The ions and solid species formed may be more complex than the simple metal

hydroxides assumed in most models. For example, Tsuru et al. [22] and

Ogawa [42] have shown, in the case of stainless steel, the formation of

chloro-hydroxy complexes involving up to 20 chloride ions, although the

coordination number of chromium ions should not exceed 6. Tsuru et al.

[22] also showed that in iron and stainless steel crevices, iron is likely to

be complexed to chloride as [FeCl(H

]

(i.e., FeCl

), but Brossia et al.

[35] found that CrCl

is the most likely metallic chloride in stainless

steel crevices.

In concentrated solutions, the presence of large amounts of chloride has been

shown to decrease the pH [22,33,42] by increasing the activity coefficient of

the hydrogen ions.

Some reactions of hydrolysis and complexation are not as fast as usually assumed

(see earlier).

This may suggest that the nature of the cationic species produced by the

anodic dissolution may depend on the surface conditions of the alloy and on the local

environment. In particular, the analyses of passive films on stainless steels showed

[43] that the external layers are very hydrated and contain chloride. On the other

hand, the number of hydroxyl groups and chloride ions adsorbed on active surfaces

is likely to be dependent on the local pH, chloride content, and surface potential.

Thus, dissolution from a passive surface may produce metallic hydroxy-chlorides

different from those resulting from active dissolution, and active dissolution may

produce cations depending on the local conditions.

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Figure 20 Evolution with time of the pH of synthetic solution made of metallic chloride

salts [40].

Other Environmental Evolutions in the Crevice Gap

In the case of stainless steels, Lott and Alkire [47] consider that the dissolution of

sulfide inclusions (MnS) inside a crevice is the main environment change before

passivity breakdown. They showed by direct analysis of the local environment in an

artificial crevice that manganese sulfides were oxidized to thiosulfates, which are

known to promote pitting in aerated chloride environments [48–50] and, more

generally, to inhibit film repair on stainless alloys. More recent work of Brossia and

Kelly [51,52] showed that the dissolution of manganese sulfides actually produced

sulfide rather than thiosulfate ions. Whatever the nature of the reactive sulfur species

produced by the degradation of MnS inclusions, they can be deleterious for the

passive layer, particularly because they may form on the bare metal surfaces an

adsorbed layer of sulfur that enhances the anodic dissolution and inhibits (or retards)

the formation of the passive film [53]. Indeed Crolet et al. [54] showed that in acidic

environments representing crevice solutions an increase in the sulfur content of the

stainless steels favored the onset of active dissolution.

In the case of dissimilar crevices where one of the crevice walls is made of

a nonmetallic material, such as a gasket, the possibility of dissolution of foreign

elements must also be considered.

Breakdown of Passivity inside the Crevice

The detailed mechanism of passivity breakdown inside a crevice is not clearly

established and different possibilities are still discussed. Indeed, it is quite possible

that different mechanisms may be involved depending on the material, bulk

environment, crevice geometry, and, possibly, surface condition of the metal.

“Classical” Mechanism”: General Breakdown of Passivity in Low

pH, High Chloride Solutions

The more “classical” mechanism involves general film breakdown in a critical

environment characterized by a low pH and a high chloride content. Low pH and high

chloride concentrations are known to be deleterious for the passivity of most alloys.

Thus, the progressive evolution of the crevice environment causes a degradation of

the passive film that may result in the following successive situations (Fig. 21a):

1. An increase of the passive current

2. The onset of an active dissolution domain with a peak current that increases with

decreasing pH and increasing chloride content (and, thus, increasing with time)

3. A complete inhibition of passivity

Passivity breakdown is thought to occur either when the corrosion potential of the

crevice surfaces is located in the active peak due to ohmic drop (Fig. 21b) or when

there is no more active-passive transition on the anodic curve.

This mechanism of initiation gave rise to the notion of critical pH for film

breakdow [55], which was extensively used as a criterion to rank the resistance of

stainless alloys to crevice corrosion. For Crolet et al. [55] the critical pH is the pH

corresponding to the onset of an active peak in the polarization curves, while Ogawa

[42] considered the pH of spontaneous film breakdown under free corrosion

conditions. These differences changed somewhat the critical pH values but usually not

Crevice Corrosion of Metallic Materials 367

the alloy ranking. Table 2 gives some results of critical pH evaluated on stainless

alloys. Okayama et al. [56] performed measurements of the depassivation pH of over

50 stainless steels and nickel-base alloys and found that the effect of alloying

elements of austenitic materials was given by

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Figure 21 (a) Evolution of the anodic characteristic of passivated alloy when decreasing

the pH and increasing the chloride content. (b) Activation inside a crevice when the

corrosion potential is located in the activity peak due to ohmic drop in the crevice.

Table 2 Examples of Depassivation pH Measured according to Crolet et al. [47] and

Oldfield and Sutton [54] Criteria

This relation showed the well-known beneficial effect of Cr and Mo and confirmed

the very deleterious effect of sulfur (see previous section).

On very resistant stainless alloys, the criterion of depassivation pH tends to

be replaced by a criterion of critical crevice solution (CCS) [19,57] and Gartland

assumes that crevice corrosion occurs when the solution is so aggressive that

passivity is no longer possible over the whole potential range.

However, this classical mechanism appears not to be consistent with different

observations, at least on less resistant materials:

Several experiments [58–60] with in situ local measurement of pH and/or chloride

concentration with microelectrodes showed that the large changes in crevice

pH and chloride content mentioned in the former paragraph occurred not

before but after passivity breakdown. In these experiments [58,61], the

potential difference between crevice and free surfaces (i.e., the ohmic drop) is

often very low before initiation (few mV) and becomes larger only after

crevice initiation.

This mechanism does not explain clearly why the initiation time is so dependent on

the potential of the external surfaces [60,62], particularly if the passive current

is not significantly dependent on the potential as is assumed in most crevice

models. This strong dependence on the applied potential would support the

idea that the pH drop rate in the crevice is controlled by the migration process

and/or by the available cathodic current. This is hardly understandable at

the very low current densities involved during the initiation stage, when

considering that dissolution rates several order of magnitude higher can be

sustained during the propagation stage.

The following points have been raised to account for these observations:

The corrosion initiates by local passivity breakdown in the crevice as a result of (a)

microcontacts due to wall asperity or (b) pitting inside the crevice.

The critical environment involves the buildup of deleterious species such as (c)

sulfur or (d) chloride species rather than low pH.

“Microcrevices” inside the “Macrocrevice” Gap

Several authors [1,14,60] raised the point that at the scale of the crevice gap, the

surface roughness may play a major role in the environment evolution. Local contact

between the opposite surfaces may constitute very tight local “microcrevices”

inside the “macrocrevice” (Fig. 22).

Due to their very narrow gap, these microcrevices could be very efficient in

promoting a very local pH change (see Modeling Crevice Corrosion later) and a local

Crevice Corrosion of Metallic Materials 369

Figure 22 Crevice formed by asperity contact [14].

passivity breakdown by the mechanism presented in the former paragraph. As

soon as active dissolution occurs somewhere inside the macrocrevice, the breakdown

of passivity spreads out progressively to the whole crevice provided the potential

inside the crevice is high enough.

Thus, according to this view, the classical mechanism could operate at the scale

of the microcrevices because of asperity contacts rather than at the scale of the whole

crevice. Most of the “apparent” incubation period would be a period of very local

corrosion extending progressively into the whole crevice zone, and thus, it would be

sensitive to the potential because of the locally active dissolution.

This interpretation is also consistent with the observation that pH and chloride

microelectrodes detect only a significant environment evolution after a time tag

following the current increase because the size of the sensitive area of these electrode is

much larger than the size of the crevice nuclei inside the crevice. Thus, these microelec-

trodes detect only the spreading of the corrosion through the whole crevice (Fig. 23).

Pitting in the Crevice Gap

The initiation of crevice corrosion by pitting inside the crevice gap is mainly

invoked for stainless steels. Indeed, several investigators [23, 46, 63, 64] have

observed pitting and further coalescence of the pits inside stainless steel

crevices. Eklund [64] showed that pitting on MnS inclusions near the mouth of

the crevice may be the initial stage of crevice corrosion of stainless steels.

Oldfield and Sutton [46] have given a detailed description of the sequence of

micropitting, pit coalescence, and crystallographic etching that results in a

rapid dissolution stage in a stainless steel crevice in 1 M NaCl solution (Fig. 24).

In this case, early corrosion damages progressively spread through the crevice,

suggesting an effect of the environment evolution in the crevice. By using

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Figure 23 Development of localized corrosion inside a crevice gap formed by asperities

and corresponding response of current [60].