Molybdenum surface enrichment and release during transpassive dissolution of Ni-based alloys

[Display omitted] •Ni, Cr, Mo and Fe dissolution were monitored as a function of time in the passive and transpassive domain.•Mo surface enrichment, probably as H2MoO4, was observed during potentiostatic experiments in the transpassive domain.•The excess Mo was released into the electrolyte when the potential returned to the passive domain.•It is proposed that Mo precipitation and redissolution are driven by changes in the local pH.•This phenomenon, reported here for the first time, may contribute to the positive effect of Mo on repassivation kinetics. The role of alloyed Mo during transpassive dissolution of four commercially available Ni-based alloys in neutral chloride solution was investigated by atomic emission spectroelectrochemistry. Time-resolved dissolution rates of Ni, Cr, Mo, and Fe were obtained as a function of applied potential. Mo enrichment occurred at the transpassive potentials and redissolved when the potential returned to the passive domain. These results suggest a mechanism of Mo enrichment and release that could play a significant role in repassivation in initially neutral electrolytes such as occurs during crevice corrosion. It is proposed that Mo precipitation and redissolution are driven by local pH changes.