Oxidation of stoichiometric nickel ferrites used as inert anodes for aluminium electrolysis in molten cryolite mixtures

Abstract

Thanks to their good electronic conductivity and their low solubility in cryolite melts, nickel ferrites are considered to be among the most suitable ceramic materials that could be used as inert anodes in the electrowinning of aluminium. In this work, electrodes composed of single phased and stoichiometric nickel ferrite NixFe3-xO4 (0≤x<1) have been studied by electrochemical techniques (linear voltammetry and potentiostatic electrolysis) in a molten cryolite mixture at 960°C. The aim was to understand the oxidation reactions susceptible to take place inside the material under anodic polarization and oxygen evolution. Ex situ characterization of the electrodes by SEM-EDX and microprobe analysis allows proposing a degradation mechanism of the pure nickel ferrites: the formation of haematite Fe2O3 at the grain boundaries was evidenced and it resulted in a slow degradation. The influence of the Ni content in the ceramic phase was investigated, and it was shown that rich-Ni compositions exhibit a better resistance to corrosion.