Effect of temperature and melt composition on the passivity of a Ni-10%Cr alloy in a molten electrolyte

Abstract

This chapter presents a study on the effect of temperature and carbonate addition on the mechanism of growth and the composition of anodic oxide films on a Ni-10 wt.% Cr alloy in molten NaOH. Impedance measurements were performed in the passive potential region to investigate the conductivity mechanism of the oxide films as depending on temperature and melt composition. The surface composition of the oxides is estimated by X-ray photoelectron spectroscopy and their in-depth composition by Auger electron spectroscopy. The main passivation product on the Ni surface is found to be a non-stoichiometric NiO doped with Cr close to the metal/film, and Na or Li at the film/melt interface. The transport of nickel cation vacancies through the barrier sublayer was demonstrated to be the rate-limiting step of the overall process in the passive region. The picture of the anodic film that emerges from the present study is that of a non-stoichiometric layer with appreciable ionic and electronic conductivity, the main current carriers being identified as cation vacancies and electron holes. It can be tentatively concluded that the surface film is composed of two layers, an inner barrier-like layer which grows by a solid-state mechanism and an outer layer growing by a dissolution-precipitation mechanism.