The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles

The enrichment and dissolution of Cr and Mo were monitored as a function of time during potentiostatic active passive cycles and during the spontaneous passivation of a Ni-Cr-Mo alloy (Hastelloy C-22) in 2 M H 2 SO 4 . Atomic emission spectroelectrochemistry was used to directly measure the elemental dissolution rates of the alloy components in real time, and combined with a mass balance, the Cr and Mo enrichments associated with the passive film were determined. Cr enrichment occurred during the passivation treatment either at an applied potential or spontaneously at open circuit. Mo enrichment only occurred during spontaneous passivation and partially redissolved during polarization in the passive domain. The dissolution behavior of Mo corresponded well with the elemental polarization curves and thermodynamic calculations.