Marcus P. Corrosion mechanisms in theory and practice

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of a galvanic cell between crevice and external surfaces with two consequences: (a)

a difference in the corrosion potential inside and outside the crevice due to the

solution resistivity

and (b) an additional source of environment modification caused

by the electrolytic migration between the crevice and the bulk solution. This second

step is self-accelerated if the local environment becomes more severe and tends to

increase the galvanic current and the electrolytic migration.

Crevice Corrosion Due to IR Drop

In some instances, the increase of the corrosion rate in the crevice may be due

only to the IR drop. The more classical case where the IR drop is responsible for

crevice corrosion is the corrosion by differential aeration of unalloyed steels in

near-neutral water.

Unalloyed steels exhibit an active-passive transition in near-neutral water

(Fig. 3). In aerated environments, the availability of a cathodic current from oxygen

reduction allows steel passivity to be stable, whereas active dissolution prevails in

deaerated conditions (Fig. 3a). Inside a crevice, the local change of environment is due

to the rapid consumption of the dissolved oxygen. However, a cathodic current from

oxygen reduction may be still provided by the external surfaces. If the IR drop is low

enough, the surface may remain passive inside the crevice (curve 1 in Fig. 3b). If the IR

drop is too large (more efficient crevice effect), however, the corrosion potential inside

the crevice is too low for the passivity to be stable (curve 2 in Fig. 3b) and the surfaces

inside the crevice gap become active. In this case, the corrosion rate in the crevice is

usually much higher than the corrosion rate of the free surfaces in a deaerated environ-

ment because of the supply of cathodic current by the external surfaces.

Crevice Corrosion Due to Local Changes of Anodic Behavior

of the Material

In most cases, crevice corrosion also requires a local change of the environment that

results in a change of the material anodic behavior. For example, crevice corrosion of

stainless steels in aerated chloride environments requires a local depletion of the oxi-

dizing species, but it is triggered by a second step of environment modification that

causes a pH drop and an increase of chloride concentration in the crevice (see later).

Local pH drop is not the single cause of crevice corrosion. For example, local

depletion of passivating species may be the origin of crevice corrosion as is the case

for steels in cooling water with anodic inhibitors such as nitrite or chromate ions.

†

Initiation and Propagation

Because of the general mechanisms described previously, crevice corrosion always

includes two steps: (a) an initiation or incubation period required for the local

environment to undergo a critical change during which no significant dissolution

damage occurs in the crevice and (b) a propagation period during which the corrosion

Crevice Corrosion of Metallic Materials 351

* This potential difference is usually referred to as IR drop.

†

Note that cathodic inhibitors do not cause such severe crevice corrosion because their local

depletion does not significantly increase the cathodic current available in a deaerated crevice.

damages occur. Depending on the cause of the localized corrosion, the initiation time

may or may not be a significant part of the lifetime of a structure suffering crevice

corrosion.

Parametric Effects

Crevice Geometry

The severity of the crevice regarding the initiation stage strongly depends on its

geometry, i.e., the crevice gap (h) and the crevice depth (L).

The crevice gap h (Fig. 2) controls the volume/surface ratio of the crevice: the

smaller this ratio, the faster the environment changes due to surface reactions.

However, the crevice gap must be wide enough for the environment to enter but it

becomes inefficient if it is too wide. Figure 4 [1] shows the extent of crevice

corrosion of unalloyed steel in nitric acid as a function of the gap. For stainless steels

in chloride environments, short incubation times are obtained for crevice gaps of the

order of a micrometer or less [2,3] and the incubation time increases rapidly with the

crevice width. As a consequence, surface roughness is of major importance, and, for

example, crevice corrosion can be avoided by polishing the surfaces in contact with

seals to a smooth finish.

The crevice depth L (Fig. 2) controls the transport processes: the deeper the

crevice, the slower the transport kinetics between the crevice tip and the bulk

environment. A minimum crevice depth is often required for corrosion to occur [2].

Bernhardsson et al. [4] introduced the parameter L

/h as a geometric criterion

of crevice severity. A similar r

/h ratio is included in one of the nondimensional

parameters introduced by Alkire et al. [5].

External Surfaces

The free surfaces outside the crevice gap constitute the cathode, which provides

cathodic current to balance the anodic dissolution in the crevice gap. Thus, the larger

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Figure 3 Crevice corrosion of unalloyed steel in near-neutral water. (a) Behavior of

“uncreviced” steel surface in aerated and deaertated water. (b) Behavior in presence of

crevice—role of IR drop.

the free surfaces and the higher the bulk solution conductivity, the higher the

available cathodic current and the higher the corrosion potential in the crevice. Except

in the cases where the IR drop is the driving factor for crevice corrosion (see earlier),

this usually shortens the incubation period and increases the corrosion rate in the

propagation stage.

IR drop

The IR drop depends on both the crevice geometry and the solution conductivity.

Most of the IR drop usually occurs inside the crevice gap, but the IR drop in the bulk

solution may be of major importance in dilute solutions and limit the effective area

of the cathode.

It is worth noticing that the IR drop and the diffusion fluxes between the crevice

and the bulk solution are controlled by the same restricted transport path. Thus, the

more limited the diffusion transport, the larger the IR drop.

The IR drop is responsible not only for the buildup of a potential difference

between the crevice and the free surfaces but also for potential gradients inside the

crevice. If large enough, the IR drop could make possible water reduction near the

crevice tip and this may create a more complex situation, the crevice tip becoming an

additional source of cathodic current and reversing the environmental evolution [6].

PHENOMENOLOGY OF CREVICE CORROSION OF PASSIVE ALLOYS

IN AERATED CHLORIDE ENVIRONMENTS

The first point to emphasize is that crevice corrosion of passive alloys does not occur

in environments deprived of oxidizing species other than water. Generally, it does not

occur in deaerated solutions. On the contrary, oxidizing agents such as many biocides

(hypochlorite, chlorine, chlorine dioxide, etc.) may cause crevice corrosion.

Crevice Corrosion of Metallic Materials 353

Figure 4 Effect of crevice gap on the amount of corrosion of unalloyed steel in nitric acid [1].

As the other crevice corrosion phenomena, the development of crevice

corrosion on passive alloys in aerated (and more generally in oxidizing) chloride

environments involves two steps:

First, there is an incubation period, which may be very long for the more

resistant materials. During this period, short corrosion potential transients can

possibly be observed (Fig. 5), but, when examining the surface inside the crevice,

no significant traces of corrosion are observed.

Then, initiation of rapid corrosion occurs inside the crevice and the

propagation period begins during which the corrosion proceeds quite rapidly. The

formation of deposits outside the crevice allows visual detection of the corrosion.

Initiation of corrosion in the crevice is also marked by a significant drop in the

corrosion potential of the material [3] (Fig. 5). Thus, monitoring of the corrosion

potential can be an efficient way to detect crevice corrosion (as well as other types of

localized corrosion) in its early stages or even during the incubation period.

Figure 6 shows an example of severe crevice corrosion of stainless steel

bolts in seawater.

Effect of Potential: Critical Potentials for Initiation and

Repassivation

For a given bulk environment, crevice initiation appears to be dependent on the

corrosion potential of the free surfaces: initiation occurs when this potential exceeds

a critical value E

crev

. The propagation may occur at potentials significantly lower than

the potential that caused the initiation to occur, and indeed, the potential of a freely

corroding material is generally lower during the propagation compared with the

incubation period (Fig. 5). However, propagation stops below a critical potential for

the crevice repassivation E

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Figure 5 Potential drop due to initiation and propagation of crevice corrosion (crevice

area ratio 1:1) in two different tests on a 316 stainless steel in 1 M NaCl, pH 6 solution at

room temperature. (From Ref. 3.)

As a consequence, the polarization curves drawn on a specimen equipped with

a crevice former device exhibit (Fig. 7) features very similar to those typical of

pitting corrosion. During the upward potential scan, crevice corrosion initiates quite

abruptly at a critical potential E′

crev

that is generally lower than the pitting potential.

Then, during a subsequent backward potential scan, corrosion stops at a critical

potential E′

significantly lower than E′

crev

However, both crevice and repassivation potentials are dependent on the

experimental conditions and particularly on the scan rate when determined by using

potentiokinetic techniques. Nevertheless, long-term experiments tend to show that

there is a minimum value of E

crev

above which the incubation time decreases as the

Crevice Corrosion of Metallic Materials 355

Figure 6 Severe crevice corrosion of stainless steel bolts in seawater (after A. Désestret).

Figure 7 Polarization curves of alloy 825 in chloride solution: comparison of curves for

pitting and crevice corrosion. (From Ref. 8.)

potential increases (Fig. 8 [8] and 9 [7]). Thus, adding an oxidizing agent (for

example, biocides) in a chloride environment strongly enhances the crevice initiation

(as well as the pitting probability).

Conversely, there is some agreement that a critical value E

(usually referred to

as protection potential) exists that is the maximum possible value of E

below which

no crevice corrosion may propagate significantly. This is consistent with the fact

that crevice corrosion of passive alloys in chloride environments does not occur in

deaerated environments.

Figure 8 suggests that long-term crevice and pitting potentials tend to the same

value [8], but this still needs confirmation. In particular, it remains to be established

whether this common value is intrinsic to the alloy-environment system or whether it

depends on the crevice geometry.

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Figure 8 Effect of time on the crevice and repassivation potentials of alloy 825 in chloride

solutions. (From Ref. 8.)

Figure 9 Effect of potential on the initiation time of crevice corrosion on 316 stainless

steel (SS) in 3% NaCl solution. (From Ref. 7.)

Effect of the Environment

For a given crevice geometry, the critical potentials for crevice initiation and

repassivation decrease with increasing chloride content (Fig. 10) and increasing

temperature (Fig. 11) of the bulk solution. This means that the susceptibility to

crevice corrosion of passivated alloys increases with the chloride content and the

temperature. For example, titanium alloys become sensitive to crevice corrosion only

in hot concentrated chloride solutions around 100/150°C [9,10]. Propagation rates

also increase with temperature.

The presence of sulfate ions or buffering species may retard crevice initiation,

at least in dilute chloride solutions [13].

Crevice Corrosion of Metallic Materials 357

Figure 10 Effect of chloride concentration on the crevice and repassivation potential of

alloy 825 at 95°C [11].

Figure 11 Effect of the temperature on the critical potentials for crevice and pitting

corrosion on 304 SS in 0.5 M NaCl solution [12].

Geometric Factors

The incubation time is strongly dependent on the crevice geometry (see also

Parametric Effects earlier). The initiation time decreases with decreasing crevice gap.

By changing the crevice gap from about 0.2 to 5 μm, Oldfield [3] raised the initiation

time from 6.5 to 350 h on AISI 430 stainless steel.

The crevice gaps that exhibit the greater efficiency in crevice initiation are of

the order of tenth of micrometers to a few micrometers. At this scale, surface

roughness is of major importance.

The area of the free surface surrounding the crevice zone is also of importance: a

large free surface area causes shorter crevice initiation time, faster propagation rate,

and smaller potential drop at the crevice initiation.

Alloy Composition

Chromium, molybdenum, and nitrogen are the most efficient alloying elements for

increasing the resistance to crevice (and to pitting) corrosion initiation of stainless

alloys (Fe-Ni-Cr-Mo-N- … alloys). A pitting index has been derived that characterizes

the resistance of stainless alloys to these forms of corrosion [14] (Fig. 12 [15]):

PREN = (wt %Cr) + 3.3(wt %Mo) + 16 to 30(wt %N)

As an example, PREN values of about 40 to 45 are requested for a stainless alloy to

resist crevice corrosion in natural seawater. This leads to the use of sophisticated

alloys containing usually about 23 to 25% Cr, up to 7% Mo, up to 0.45% N, and very

often Cu and W additions.

The harmful effect of sulfur [16] as an alloy impurity has been mainly attributed

to the presence of reactive sulfide inclusions (usually MnS), which constitute

very efficient sites for initiation of localized corrosion (see later). Low sulfur or Ti

alloying* improves the alloy resistance to crevice corrosion [16].

358 Combrade

Figure 12 Effect of composition on the resistance to crevice and pitting corrosion of

stainless alloys in chloride environments—the PREN index. (From Ref. 15.)

* The beneficial effect of Ti alloying is probably due to the formation of sulfide compounds

more stable than mangnese sulfide.

The propagation rate also depends on the composition of the alloy, but Oldfield

[3] observed that the ranking of alloys may be different because the effects of

elements are different for initiation and propagation. For example, he claimed that Cr,

which is very efficient in retarding the initiation, favors fast propagation rates Mo

definitely has a positive effect on both stages [17,18], but Ni seems to have a positive

effect only on the propagation rate [16,18].

Propagation of Crevice Corrosion

Crevice corrosion occurs at very high rates, which make this form of corrosion very

difficult to manage as the lifetime of a corroding structure may be determined by the

initiation period.

Laboratory studies showed that for most alloys, the corrosion rate is higher

near the mouth of the crevice [19,20]. However, alloys such as ferritic stainless

steels may corrode faster near the dead end of the crevice [20].

An interesting observation on long-term crevice tests of Fe-Ni-Cr-Mo-N alloys

[21] is that well-developed crevice corrosion may stop spontaneously even though

the environmental conditions do not change. If confirmed, such behavior could be of

major importance because it would mean that crevice corrosion may not necessarily

cause the failure of structures, particularly in applications involving thick materials

such as the containers for long-term storage and disposal of radioactive wastes.

PROCESSES INVOLVED IN CREVICE CORROSION OF PASSIVE

ALLOYS IN AERATED CHLORIDE ENVIRONMNENTS

The generally accepted scenario is that the environment in the crevice suffers a

progressive evolution that leads to the breakdown of passivity and to a propagation

stage during which the metal inside the crevice undergoes active dissolution. The

following paragraphs briefly describe the current understanding of the different

processes, i.e., environment evolution, passivity breakdown, and active dissolution in

the crevice gap.

Environment Evolution in the Crevice Gap

The following mechanism is generally accepted to describe the evolution of the

crevice environment on passivated alloys exposed to aerated chloride environments.

Step 1: Deaeration of the Crevice Environment

On the free surfaces, the cathodic reaction is the reduction of oxygen. However, the

environment in the crevices becomes deaerated after periods of time that can be very

short, at least compared with the lifetime of an industrial apparatus. As an example,

a passive current of 10 nA/cm

will cause the deaeration of a crevice with a

surface/volume ratio of 10

–1

(i.e., a crevice gap of 20 μm between two metal

surfaces) in about 3 h.

The lack of oxygen causes the inhibition of the cathodic reaction inside the

crevice. Thus, the local anodic reactions must be balanced by cathodic reactions

occurring on the surfaces exposed to the bulk solution. This builds up a galvanic cell

Crevice Corrosion of Metallic Materials 359

between the inside and the outside of the crevice. As a consequence, the IR drop

between the crevice and the bulk is responsible for the buildup of a corrosion

potential difference between the inside and outside surfaces, the former becoming

more anodic. However, the contrary to what happens on unalloyed steels (see

earlier), the stainless alloys exposed to neutral chloride solutions are passive in both

aerated and deaerated solutions and the local oxygen consumption has no significant

effect of the dissolution kinetics. Thus, this step does not induce any corrosion

damage but it is essential to trigger the following process.

Step 2: pH Evolution and Concentration of Dissolved Species

The anodic dissolution of the metal inside the crevice produces metallic cations

(and/or oxides that contribute to thickening the passive layer, at least before initiation

of the propagation stage).

Production of solid oxides or oxyhydroxides

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Production of solvated metallic cations

These metallic cations cannot concentrate indefinitely and they undergo

hydrolysis.

The earlier theories assumed the formation of simple hydroxides:

It is now recognized that metallic cations may also form metallic chlorides and/or

complex oxy-hydroxy-chlorides according to reactions such as

The formation of metallic polycations should also be considered [4].

These anodic processes result in a local excess of cations inside the crevice.

Because there is no cathodic reaction inside the crevice, the restoration of electrical

neutrality requires the onset of a galvanic current in the base metal and a migration

flux in the liquid. At least in the early stages of crevice evolution, most of the ions

conveyed by this migration are chloride ions that are transported from the bulk

solution into the crevice [22].

All these processes are summarized in Figure 13, where hydrolysis reaction (3)

is assumed. As a consequence, the crevice environment becomes more and more

acidic and more and more concentrated in metallic ions (including metal chloride

complexes) and free chloride ions. We will see later that, in some instances, this

pH evolution, combined with an IR drop, can make possible the reduction of water in

the crevice.