The contribution of Cr and Mo to the passivation of Ni22Cr and Ni22Cr10Mo alloys in sulfuric acid

The beneficial effects of Mo on passive film formation were investigated for Ni22Cr10Mo with AESEC and XPS by comparison with Ni22Cr and pure Ni and Mo. Both spontaneous (E approximate to -0.2 V-SCE) and anodic passivation (E = 0.3 V-SCE) showed Cr surface accumulation, while significant Mo accumulation (as Mo(IV)) only occurred during spontaneous passivation with a markedly reduced open circuit corrosion rate as a consequence. Mo facilitated the active to passive transition in the low potential domain probably by favoring the oxidation of Cr to Cr2O3 over Cr(OH)(3). At higher potential, Mo dissolved as Mo(VI) and did not accumulate in the film.