Electrochemical Corrosion Studies in Molten Chloride Salts

Abstract

This study investigates corrosion of Fe–(Cr)–Ni model alloys in eutectic molten chloride salt mixtures that broadly simulate the heat transfer circuits of molten salt cooled nuclear reactors. The primary focus is impurity-driven corrosion and selective dissolution of electrochemically reactive element(s) (dealloying), which are known degradation modes for Fe– and Ni-based alloys in molten salts. This study demonstrates that a Mg rod performs close to a true Mg∣Mg2+ reference electrode system in a MgCl2 containing mixture. Mg dynamic reference electrodes validated the performance of the ordinary Mg reference electrode. The residual moisture content in molten salts is measured by cathodic polarization of Pt, which showed that convection of molten salts, after step-wise heating, through a bundle of Mg ribbons dropped the moisture content by 30-fold. The effect of impurities in the molten salt mixture, moisture and NiCl2, on dealloying was also investigated at low homologous temperature. It was found that the universal parting limit for dealloying is decreased to a value of approximately 32 at% Fe when Ni from the parent alloy is close to equilibrium with Ni2+ ions added to the eutectic molten salts. Results support the percolation/surface diffusion concept for dealloying in molten salts at low homologous temperature.