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

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 trans passive 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.