(Invited) The Complementarity of Cr and Mo in Reinforcing Oxide Protectiveness on Ni-Cr-Mo Alloys.

Ni-Cr-Mo alloys are extremely corrosion resistant as a result of the passive oxide film that spontaneously develops on the alloy surface upon exposure to the environment. We report on investigations of the influence of the oxide film composition on the susceptibility of the alloy to crevice corrosion, the rate, morphology and extent of crevice corrosion damage, and the tendency to stifle or repassivate the crevice. Chromium oxides are well known to provide a strong barrier to corrosion in solutions of moderate pH, over a wide range of oxidizing conditions. The corrosion susceptibility, however, can be decreased by alloying additions of Mo, which permit the deposition of Mo-rich corrosion products in response to the failure or partial failure of the Cr oxide barrier in acidic solution or crevice corrosion -a kind of “back-up plan” for the workhorse Cr. Mo can also help to defend against transpassive dissolution of Ni-based alloys, although this influence seems to be a transient effect that is dependent on the local pH. Here we report the results of galvanostatically driven crevice corrosion experiments, online analysis of dissolved corrosion products by atomic emission spectroelectrochemistry (AESEC) during transpassive polarization, and immersion testing in strong acid solution.