Insights on the corrosion thermodynamics of chromium in molten LiF-NaF-KF eutectic salts

Abstract

The thermodynamic stability of chromium in the Cr0, Cr2+, and Cr3+ oxidation states considering various F− ion coordination compounds was investigated in molten LiF–NaF-KF (FLiNaK) eutectic salt. Potential fluoride ion activity (F− and CrF3−) diagrams were constructed to predict the most stable Cr oxidation states as a function of anion activity, solvation state of chromium ions and potential at 600 °C. The Gibbs free energies of these compounds were estimated by utilizing a combined cyclic voltammetry – Nernst theory analysis approach in FLiNaK salt. To verify the constructed diagrams, X-ray diffraction was utilized after exposure of Cr at various applied potentials to determine whether compounds detected in solidified FLiNaK salts agreed with thermodynamic calculations. This work aims to identify key thermodynamic factors that are significant to chromium corrosion for molten salt nuclear reactor applications. The F− stability region spans the entire region where spontaneous corrosion of Cr occurs. Cr may be oxidized to Cr2+ and Cr3+ in the presence HF (due to moisture as an impurity) spontaneously except some conditions of p1/2H2/aHF etc. This situation is not changed qualitatively for various states of solvation between oxidized Cr solute in F− solvent and this aspect is substantially similar for the two cases (pair 1: Cr0/CrF3−/ CrF63−; pair 2: Cr0/CrF42−/CrF52−) considered in this work.