Effect of halides on the corrosion behaviour of nickel in molten (Na, K)NO3

Abstract

The effect of alkali halides on the corrosion behaviour of nickel in (Na, K)NO3 eutectic has been investigated at a temperature of 400° C. Initial addition of the halide (Cl−, Br− or I−) increases the steady-state potential to more noble values than for the pure melt. Further addition decreases the potential according to the equationE=a-blog C. Gravimetric measurements show that the corrosion of nickel increases in the presence of Cl− and Br− and decreases in the case of I−. The anodic polarization increases in the presence of Cl− and Br− and decreases when I− is added to the melt. These results are discussed, in the light of previous work, on the basis of the interaction of these halides with the nitrate melt to give the corresponding halogen or their catalytic effect on the dissociation of nitrate to give excess amounts of O2− ions. The liberated iodine may form a physically adsorbed layer on the nickel surface, offering some protection to the metal. In this respect Cl− and Br− behave as acidic additives and I− behaves as a base.