Insights on the Corrosion Thermodynamics and Kinetics of Chromium in Molten Fluoride Salts for Gen-IV Reactor Applications

Chromium, a ubiquitous and powerful alloying element possible structural material in advanced nuclear reactor alloys, is susceptible to selective dissolution (when present as an alloying element) in molten fluoride salts in the presence of hydrofluoric acid, which is formed by the adsorption and reaction of moisture impurity in molten fluorides. In this project, the thermodynamic stability of Cr in the Cr⁰, Cr²⁺ and Cr³⁺ oxidation states considering various F^- ion coordination was investigated in molten LiF-NaF-KF (FLiNaK) eutectic salt. Potential-fluoride ion activity (F^- and CrF₃-) diagrams were constructed to predict the F- stability region where spontaneous corrosion of Cr occurs. Electrochemical methods were utilized to relate the interface corrosion kinetics to the predicted stability region. An In-situ electrochemical method was also developed to understand the corrosion behavior of Cr and its corrosion products in real-time, simultaneously. The present work aims to identify key thermodynamic and kinetic factors that are significant to the chromium corrosion for molten salt nuclear reactor applications.