The breakdown of passive films on active metal anodes by halide ions in oxidizing molten salts

The various effects of halide ions on Ca, Zn, and Mg anodes in oxidizing molten salts can be related to a corrosion model involving a nonstoichiometric passive film containing an excess of metal ions. Competitive adsorption between passivating and agressive anions attracted to the positively charged surface determines the stability of the film. The addition of chloride ions in molten LiNO3 leads to pitting corrosion of the calcium anode and improvement of the open-circuit potential by as much as 1.3V. In molten NaNO3 and KNO3, the formation of passivating peroxide and superoxide anions greatly diminishes the effects of added chloride ions on the calcium anode. In molten LiClO4 and NaClO3, breakdown of the passive film occurs spontaneously due to the direct reaction of calcium with the molten oxyhalide salt to produce sufficient chloride ions to activate the anode. The best performance of the calcium anode is obtained in the oxyhalide molten salts. 31 ref.--AA