Slip-step dissolution

Fig. 8.2 Strain-generated active path mechanisms, (a) Often referred to as the film rupture model and (b) the slip step dissolution model. In both cases growth is by dissolution film rupture is the rate controlling step, not the mechanism of crack growth...

Figure 4.43 Strain generated active path mechanisms (A) Film rupture model (B) slip step dissolution model. (From Sedriks, A.J., Slattery, P.W. and Pugh, E.M. (1969). Trans. Am. Soc. Met, 62, 1238. Reproduced by kind permission of ASM, Metals Park, Ohio, USA)...

Once the crack is initiated, the metal surface inside the crack may be quite different from the normal surface of the metal. Thus, in the course of plastic deformation, the metal could have developed slip steps [see Fig. 12.77(c)] which contain crystallographic planes of high Miller index at which the specific dissolution rate (or exchange current density) may be larger than that at the normal metal surface. Anodic current densities of some 104 times those at a passive surface have been shown to appear at a metal surface that is yielding under stress (Despic and Raicheff, 1978). 

For some material-environment combinations it has been shown that accelerated anodic dissolution of yielding metal is the significant mechanism. This is the case for austenitic stainless steels in acidic chloride solutions. In these steels, plastic deformation is characterized by a dislocation pattern giving wide slip steps on the surface. For such systems, Scully [7.50] has proposed a model for initiation and development of stress corrosion cracks, which has been supported by other scientists [7.51]. The model in its simplest form is illustrated in Figure 7.52. A necessary condition is that flie surface from the beginning is covered by a passivating film (A). 

It has been suggested that vacancies produced by the anodic dissolution at slip steps (Staehle, 1987) are injected in the material and increase the dislocation motion near the specimen surface, as discussed elsewhere... 

Significant phenomena occur when plastic strain is applied to materials protected by a surface film which strongly reduces anodic dissolution and, in many circumstances, cathodic reaction kinetics. In the Sec. 5.2.4.2, the emergence of slip steps on the surface of a strained material was shown to cause localized damage to the protective film. The following example shows the evolution of dissolution transients during cy-... 

When the applied stress causes some plastic strain and dislocation creep, the periodical emergence of slip steps promotes local excess dissolution. Crack advance is attributed to this metal removal, with no mechanical rupture events even at a microscopic level. Thus the crack propagation rate do/dr may be estimated by calculating the metal loss by dissolution at the crack tip. In general, two different situations are considered ... 

As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve diffusion of water molecules or hydrogen ions to the crack tip reduction to hydrogen atoms at the crack tip surface diffusion of adsorbed atoms to preferential surface locations absorption and diffusion to critical locations in the... 

The mechanism of the slip dissolution process [86] takes place in the following steps ... 

Note that, in this model, the role of dissolution is essential, but indirect. First, vacancies can enhance the plasticity at the crack tip and, second, hydrogen absorption requires the existence of a critical defect in passivated metals. This defect is created by dissolution at the slip band emergence, which is often experimentally observed as the crack initiation step. Vacancies may also act as hydrogen trapping sites, thus increasing its crack tip concentration. 

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