Marcus P. Corrosion mechanisms in theory and practice

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Activation pH This pH is the pH below which an active peak is present on the

polarization curves, (Fig. 40). This criterion was mainly developed by Crolet et al.

[55]. The lower the activation pH, the more resistant the alloy to crevice corrosion.

pH of Spontaneous Film Breakdown It is measured by continuously lowering

the pH of the solution until a freely corroding test coupon suffers a sudden potential

drop due to passivity breakdown (Fig. 41). This criterion, which leads to a critical pH

somewhat lower than the former, was used by Ogawa [42].

Active Peak Current as a Function of pH Several authors [3,103,108] use the

measurement of the active peak current below the activation pH to compare the

propagation kinetics of crevice corrosion on different materials. Figure 42 shows an

example of this kind of measurement. Notice that the peak current of the duplex

stainless steels (UR 47, 31803, 31603) is much higher at low pH values than that of

austenitic grades (304 and 904L). This is consistent with the beneficial effect of

nickel on the crevice propagation rate mentioned earlier.

All these electrochemical criteria suffer from several drawbacks:

1. It is assumed that passivity breakdown in a crevice is due to pH drop but,

as discussed, the actual mechanisms may be different.

Crevice Corrosion of Metallic Materials 391

Figure 40 Measurement of activation pH on 430 SS [107] (2 M NaCl, 23°C).

Figure 41 Measurement of spontaneous film breakdown pH [42].

2. Conventional solutions that are not representative of crevice environment are

used. In particular, the presence of metal cations in the crevice solution is not

considered.

However, these criteria probably allowed a correct ranking of the stainless alloys

because the various methods are sensitive to the alloying elements that improve

passivity (mainly Cr and Mo)

PREVENTION OF CREVICE CORROSION

Prevention of crevice corrosion must be taken into account at different stages of

the life of an industrial apparatus:

–Material selection: the more resistant the materials to crevice corrosion (and

more generally to localized corrosion) in chloride environments, the more

expensive they are. Thus, the selection of a material exhibiting a very high

resistance to crevice corrosion should not be the only way considered to

prevent crevice corrosion: improved design, construction procedures, surface

finish, maintenance procedures, and process management can be of a major

importance.

–Cathodic protection: because crevice corrosion occurs above a critical potential,

cathodic protection is an efficient way to prevent crevice corrosion by

maintaining the potential of the free surfaces below the protection

potential. This is the case of 17-4-PH stainless steel, which is not resistant to

crevice corrosion in seawater but is commonly used because it performs

392 Combrade

Figure 42 Evolution of peak current as a function of pH on different stainless steels [107].

very well under cathodic protection (this protection is often due to galvanic

coupling to unalloyed steel).

–Design: an improved design should minimize the crevice areas, for example, by

avoiding overlapping welded parts and by polishing seals bearing. However,

crevice zones can be created by the formation of deposits during service

and/or shutdown periods. Deposits can be minimized, for example, by

avoiding low flow rate areas and by allowing complete draining of tanks,

–Fabrication procedure and surface finish: the intrinsic resistance of a material to

crevice corrosion may be dramatically hampered by inappropriate fabrication

procedures. The most sensitive points are welding procedures, which must be

optimized by an appropriate energy input and, if necessary, by the use of more

alloyed filler metal; thermal treatments, particularly if stress relief has to be

performed; and surface conditions, which may suffer from fabrication events

(scratches with iron incrustation, drops of welding filler metal, etc.) and which

should be restored at the end of the fabrication process, for example, by a

passivation treatment in the case of stainless alloys.

–In service, transient, and shutdown conditions—maintenance procedures: care

has to be taken of the possible change of conditions during shutdown and/or

transient Periods, which are not necessarily less prone to the development of

crevice corrosion. An example is the case of desalination plants where

concentrated chloride solutions may become very dangerous during shutdown

if strict deaeration is not maintained. The possibility of formation of deposits

during transient and shutdown periods and the occurrence of surface damage

during maintenance operations are among the risks that have to be considered.

Generally speaking, the best possible cleanliness of the apparatus must be

maintained during all the apparatus lifetime.

CONCLUSION

Crevice corrosion occurs in restricted transport zones where the access of reactants

and the elimination of corrosion products can be very slow. The detrimental effect of

a crevice is related to a very small volume of solution in contact with very large metal

surfaces. The restricted transport path between crevice and bulk solution is

responsible not only for low diffusion exchanges but also for the buildup of a

potential difference between the free surfaces and the crevice that becomes more

anodic (ohmic drop effect).

These conditions trigger an evolution of the local environment in the crevice.

Crevice corrosion may occur when the environment becomes more aggressive in

the crevice than in the bulk environment.

On passivated alloys, and particularly on stainless alloys (Fe-Ni-Cr-Mo-

…alloys), crevice corrosion occurs when the local conditions in the crevice cause

passivity breakdown and the onset of active dissolution. As a consequence, crevice

corrosion exhibits two different stages: an incubation period during which the

local environment in the crevice evolves toward critical conditions and

Crevice Corrosion of Metallic Materials 393

a propagation period that starts at the initiation time corresponding to the passivity

breakdown.

The most widespread cases of crevice corrosion of passivated alloys are caused

by aerated (or more generally oxidizing) chloride solutions such as sea or brackish

water. In these chloride solutions, the environment in the crevice becomes

progressively more acidic and more concentrated in chloride anions and metal

cations. There are several possible causes of passivity breakdown, including low pH,

high chloride content, presence of metallic chloride complexes, and pitting inside the

crevice gap. Passivity breakdown occurs only if the corrosion potential of the free

surface exceeds a critical value, but the relationship between the potential of the

external surfaces and the evolution of the environment in the crevice is not

completely understood.

Whatever the initiation mechanism, propagation causes further evolution of the

local environment, which eventually becomes saturated (with concentrations

estimated to be several mol/L of chloride and metal cations). At this point, a salt layer

may form on the corroding surfaces. Thus, active corrosion in the crevice is rarely

controlled by an activation process but rather by the ohmic drop and, for well-

developed crevices, by cation transport through the salt layer, the thickness of which

determines the corrosion rate.

An interesting feature that can be used to avoid crevice corrosion is that

propagation stops if the potential of the free surfaces becomes lower than a critical

value, referred to as repassivation potential. At least for alloys exhibiting a

significant resistance to crevice corrosion, it is believed that this corrosion arrest is

due to a repassivation of the metal surface inside the crevice. It is quite possible that,

after long exposure periods, the critical potential for crevice initiation and the

protection potential have a common value, but this point must be confirmed. The

“protection” or “repassivation” potential can be a safe design criterion to avoid

localized corrosion in long-term service. In particular, it provides a criterion

independent of the surface condition because it guarantees against propagation of

corrosion of the bulk material.

As the basic mechanisms of crevice corrosion have been understood to some

extent, many models have been developed either to describe the steady state of an

actively corroding crevice or to calculate the environment evolution and predict the

passivity breakdown. These models are able to reproduce some major features of

crevice corrosion, particularly geometric effects, but they all suffer from the lack of

knowledge of the complex concentrated solutions that are formed in the crevice.

Crevice corrosion is a typical example of a corrosion problem in which the

solution involves more than just the selection of fully resistant alloys. Design,

fabrication procedure, and surface finish must be optimized and attention paid to the

management of service conditions, as well as transient and shutdown periods.

Optimization of maintenance procedures to maintain the best cleanliness of the

surfaces may also contribute to prevent crevice corrosion.

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Crevice Corrosion of Metallic Materials 397

Stress-Corrosion Cracking Mechanisms

Roger C. Newman

University of Manchester Institute of Science and Technology, Manchester, England

Several testable models for stress-corrosion cracking (SCC) of metals are discussed

in terms of the main experimental variables: stress, metallurgy, and environment.

Slip-dissolution, film-induced cleavage, and hydrogen embrittlement models

are all shown to be consistent with experimental data in particular systems.

Other models that cite effects of corrosion (without a film) or adsorption on

crack tip deformation, leading to microcleavage or plastic microfracture, are

less easy to test. No model can be “universal” in view of the demonstrable

multiplicity of mechanisms. In many cases the atomistic mechanism is

unknown, yet cracking can be controlled or predicted via the localized corrosion

process that precedes SCC.

DEFINITION AND SCOPE OF STRESS-CORROSION CRACKING

SCC is defined as the growth of cracks due to the simultaneous action of a stress

(nominally static and tensile) and a reactive environment. For metals, “reactive”

excludes gaseous hydrogen, cathodic polarization, and liquid metals but includes

aqueous and nonaqueous electrolytes and reactive atmospheres (H

O, I

, Cl

Related phenomena occur in inorganic glasses and ceramics, especially in water,

and are thought to be involved in major geological processes including earthquakes

and midocean vulcanism. Static stress does not exclude slow monotonic straining

or low-amplitude cycling (“ripple loading”), which accelerate SCC in many

metallic systems—for example, by promoting oxide film rupture at the crack tip.

Stresses arise in practice from applied loads or from residual stress due to welding

or inhomogeneous plastic deformation.

This review deals mainly with metals. The literature on their SCC is immense,

and there are many useful texts and conference proceedings [1–12]. A brief history

was published by Newman and Procter [13] and should be consulted for further

historical details, especially the metal-environment combinations known to exhibit

SCC. Plasticity plays a key role in SCC of metals, in contrast to inorganic glasses and

ceramics, which are brittle solids and crack via reaction of the corrodent with highly

stressed bonds at an atomically sharp crack tip [14]. Metal-induced fracture [15],

formerly called liquid-metal embrittlement, remains a tantalizing phenomenon and

399

there is no agreement on the mechanism or the relationship to SCC. Several

authors have proposed universal models of SCC and corrosion fatigue [16–18] or

even universal models of environment-assisted fracture, encompassing aqueous,

gaseous, and liquid-metal environments [19,20]. We shall conclude that although

these models may apply to particular systems, they are not universal, and there

remain several valid mechanisms of SCC. Finally, hydrogen effects play an

important part in SCC but are also the subject of a large separate literature [21,22].

The atomistic action of hydrogen will be covered quite briefly here but can be

investigated further by referring to Refs. 1–12, 21, and 22.

CANDIDATE MODELS FOR SCC

We have selected three testable cracking models that may account for most known

cases of SCC in metals: slip-dissolution, film-induced cleavage, and hydrogen

embrittlement. Other important models involve microcleavage [16,17] or plastic

microfracture [19] induced by dissolution or adsorption, respectively. A sixth model,

surface mobility [20], will be discussed only briefly, as there appear to be serious

problems with the basic physics of this model [23], although not necessarily with the

general concept. A model based on vacancy injection during dissolution, even of pure

metals, was proposed by Jones [18]; this deserves attention but has not yet made any

successful new predictions, and it relies on processes that might be discounted in the

future using modern techniques such as scanning tunneling microscopy.

Slip-Dissolution Model

According to this model [24–33], crack growth occurs by extremely localized

anodic dissolution. The sides of the crack are protected by a film, ususally an

oxide, which is fractured as a result of plastic strain in the metal at the crack tip.

Crack growth proceeds by a cyclic process of film rupture, dissolution, and film

repair (Fig. 1). This model was once favored for both intergranular and transgranular

SCC in a wide variety of systems [25] but is now applied mainly to the intergranular

cracking of ferritic steels in passivating environments such as carbonate-bicarbonate

solutions [26,27] and to sensitized stainless steels [28–32].

Film-Induced Cleavage Model

Film-induced cleavage originated with the suggestion by Edeleanu and Forty [34]

that superficial dezincification of α-brass in ammonia solution could trigger a brittle

fracture through several micrometers of the unattacked substrate; they believed that

the material through which this crack was propagating (between slip bands) was

embrittled by short-range ordering. Their optical microscopy was elegant and came

near to proving the reality of the crack-jumping phenomenon, which was later confirmed

by Pugh and others using the scanning electron microsope (SEM) [35–39]. In the

modern theory of film-induced cleavage [40–45] there is no special property of the

face-centered cubic (fcc) substrate through which the crack jumps; the special

properties lie in the film itself, which must be able to trigger a crack with a velocity

of hundreds of meters per second within less than 100 nm (Fig. 2). Nanoporous

400 Newman

metallic layers produced by dealloying seem to be very effective, as they can be

both brittle and epitaxial, or at least strongly bonded to the substrate. Tests of the

film-induced cleavage model [33,46–51] have relied on producing single-shot

brittle events through thin alloy foils and have shown that intergranular fracture

can also be triggered by dealloyed layers in gold alloys. It may turn out that

film-induced fracture involves a slower and more plastic process than originally

envisaged by Sieradzki and Newman [40], but the reality of the phenomenon is

now well established.

Stress-Corrosion Cracking Mechanisms 401

Figure 1 Schematic drawing of crack initiation and growth by slip-dissolution. (Adapted

from Ref. 25.)

Figure 2 Schematic drawing of crack growth by film-induced cleavage in an fcc metal

or alloy. (Adapted from Ref. 36.)