ESCA studies of Ni‐25 At% Fe alloy. 2—dissolution and passivation

Abstract

AbstractThe surface compositions of a Ni‐25 at% Fe single crystal (100) and (110) were analysed by ESCA with respect to the electrochemical behaviour in 0.05 M H2SO4 (anodic dissolution and passivation). When the alloy surface is exposed to the electrolyte solution, or anodically polarized in the active region (anodic dissolution), iron is selectively dissolved and the surface is enriched with nickel. A stationary surface composition is attained which does not depend on the polarization potential or dissolution current. Once the stable surface composition is attained, the anodic dissolution of nickel and iron takes place with the same ratio as in the alloy. The extent of the surface enrichment with nickel, calculated from a layer by layer model for XPS signal intensities is found to be almost one monolayer of nickel at the alloy surface, each following plane in depth having the same composition as the bulk (Ni3Fe). The passive film is enriched with nickel. It consists or an inner layer of nickel oxide (NiO) and an outer layer of nickel and iron hydroxides. The thickness of the passive film is 8–10 Å. The alloy/oxide interface is also enriched with Ni in its metallic state.