A bipolar model of the passivity of stainless steels-III. The mechanism of MoO₄^2- formation and incorporation

Variable angle XPS was used to examine the surface species formed on pure Mo following anodic polarization in 0.1 M HCl solution at -180 mV(SCE) for 1 h under the influence of Fe or Ni cations. The source of the metal cations was an anodically polarized electrode of pure Fe or Ni brought into close proximity to a high purity Mo electrode. Normally, the passive film formed on Mo in 0.1 M HCl at -180 mV(SCE) is comprised of only MoO₂ and its hydrated form, MoO(OH)₂. In the presence of Fe and Ni cations, Mo was observed by XPS to produce a similar passive film, but in addition, MoO₄^2- was detected on the exterior surface of the film with Fe. No Ni could be detected. However, when Cr was polarized in the vicinity of Mo, no MoO₄^2- was observed to form on the film of Mo. This was expected due to the low Cr cation flux generated by the passive Cr. However, the surface of the Cr passive film was observed by XPS to contain MoO₄^2- and MoO(OH)₂ species. It was experimentally determined that soluble molybdenum chlorides were not responsible for the transference of Mo to Cr.