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

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 was studied. 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 has been estimated by X-ray photoelectron spectroscopy and their in-depth composition by Auger electron spectroscopy. The main passivation product on the Ni surface was 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.