Stress corrosion cracking of noble metals and their alloys in solutions containing cations of the noble metal: Review of observations relevant to competing models of SCC

摘要

The stress corrosion cracking of Ag, Au and Cu-base alloys, and of pure Ag and Cu, has been studied by J.R. Galvele and others. These authors used solutions that contained the cation of the more-noble metal, so that the tested specimen was at or close to its equilibrium potential in the given solution. The opportunity is taken to review the history of this far-from-new observation and some of its implications. The role of the exchange current density in such cracking is discussed. Observations of Sieradzki and Torchio are used to suggest that in alloys such as brass, SCC is favoured by low, not high, surface mobility, in line with the film-induced cleavage model, which requires very fine nanoporosity at the crack tip - such a favourable condition can only be achieved if dealloying is either very fast or occurs under conditions of low surface mobility. Observations of very slow intergranular SCC in pure metals under dynamic loading are interesting, but not really suggestive of mechanistic continuity with the dramatic mixed-mode cracking that occurs under static loading in brass or AuAg alloys. Torchio's observations on brass U-bends in CuSO_4 solutions of various pH and Cu~(2+) concentrations are particularly hard to interpret using the surface mobility model.