Composition and conduction mechanism of the surface oxide film on Ni-based alloys in molten hydroxide

A study of the anodic oxidation of nickel-based alloys in molten NaOH is presented, investigating the mechanism of growth, the composition and structure of the surface oxide films. The potential ranges of passivation, passivity and transpassivity have been estimated by voltammetry. Impedance measurements were performed in the passive potential region to investigate the conductivity mechanism of the oxide films. The surface and in-depth composition of the films has been estimated by X-ray photoelectron spectroscopy and Auger electron spectroscopy. As a result, the main passivation product on the Ni surface was found to be NiO. Further oxidation of the NiO layer was presumed to lead to the formation of Ni(III) ions and nickel cation vacancies. Transport of nickel cation vacancies was assumed to be the rate-limiting step of the overall process in the passive region. A tentative kinetic model of the nickel alloy/oxide/melt system is proposed to describe the impedance spectra. On the basis of the model, several kinetic and transport parameters are estimated.