Slip-oxidation mechanism

Quantitative predictions of the crack propagation rate via the slip-oxidation mechanism are based on the Faradaic relationship between the oxidation charge density on a surface and the amount of metal transform from the metallic state to the oxidized state (e.g., MO or M jq). The change in oxidation charge density with time following the rupture of... 

Schematic of the crack enclave and the relevant phenomena associated with the slip-oxidation mechanism of crack advance. (Adapted from Ford, F.P., The crack tip system it s relevance to the prediction of cracking in aqueous environments, in Proceedings cf First International Conferemx on Environmentally Assisted Cracking cf Metals, Kohler, Wl, October 2-7, 1988, Eds. R. Gangloff and B. Ives, Published by NACE, pp. 139-165 Ford, F.P. et al.. Corrosion-assisted cracking of stainless steel low-alloy steels in LWR environments. Report NP5064S, February 1987, EPRI, Palo Alto, CA.)...

Based on the arguments made above for unirradiated stainless steels, it was hypothesized that the slip-oxidation mechanism of crack propagation was a relevant cracking model for irradiated stainless steels and that the basic Equations 18.6 and 18.10 apply, but with modifications to account for the effect of irradiation on the inputs to that model. Thus, the relevant system parameters that might be affected by irradiation and which are important to the slip-oxidation mechanism of crack propagation are... 

Experimentally validated elements of these earlier proposals have been incorporated in the current slip dissolution model, which relates the crack propagation to the oxidation that occurs w hen the protective film at the crack tip is ruptured [39-44], Different types of protective films have been proposed, including oxides, mixed oxides, salts, or noble metals left on the surface after selective dissolution of a more active component in the alloy. As discussed below, quantitative predictions of the crack propagation rate via the slip dissolution mechanism are based on the faradaic relationship between the oxidation charge density on a surface and the amount of metal transformed from the metallic state to the oxidized state (e g., MO or M ). 

With the advent of extremely sensitive analytical and crack monitoring capabilihes, many of the earlier cracking hypotheses have been shown to be untenable, and the candidate mechanisms for environmentally assisted crack propagation for alloys in water-cooled reactors have narrowed down to slip-oxidation, film-induced cleavage, and hydrogen embrittlement. 

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... 

In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102... 

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