Nickel ferrite as inert anodes in aluminium electrolysis: Part II - Material performance and long-term testing

The behaviour of three different compositions based on nickel ferrite-nickel oxide-copper cermets was investigated as anode materials in laboratory electrolysis tests for 50 h in a conventional cryolite-based electrolyte. The corrosion of the anodes was assumed to be mass transfer controlled and the transfer of impurities into the electrolyte and subsequently into the cathodically deposited metal was studied. The results indicate that the materials corroded in a controlled manner. Mass transfer coefficients of species from the anode to the electrolyte were of the order of 10(-4) m s(-1) while the mass transfer coefficients for transfer of the species from the electrolyte into the deposited metal were of the order of 10(-6) m s(-1). Nickel exhibited significantly lower mass transfer coefficients than those of iron and copper. The extrapolated corrosion rates of the anode ranged 1.2-2.0 cm year(-1), which is acceptable from an industrial perspective. The contamination of the deposited aluminium with respect to Ni and Cu was, however, too high to meet current specifications for commercial grade metal. Post-electrolysis examination of the anodes showed that a reaction layer of approximately 50 mu m thickness was formed on the anodes. This layer did not contain any metal grains and seemed to prevent preferential corrosion of the metal phase in the underlying cermet.