Solubility of Cr2O3 in Molten Fluorides with Different ZrF4 Contents and Fluoroacidities

Abstract

This work focused on the stability of the protective Cr2O3 layer formed on alloys in molten fluorides with different fluoroacidities and ZrF4 contents. Results showed that fluoroacidity only slightly affected Cr2O3 due to its stability in basic FLiNaK and neutral FLiBe melts. When more than 1.1 mol% ZrF4 was added to FLiNaK, the new structure [ZrF7]3− formed by the interaction of ZrF4 and F− could be detected by Raman Spectroscopy and X-ray diffraction, and Cr2O3 solubility in FLiNaK–ZrF4 increased 19 times. In the case of FLiBe salt, the structure of [ZrF6]2− could be detected when 5 mol% ZrF4 was added, and Cr2O3 dissolution increased from 183 ppm to 320 ppm. Further structural changes from [ZrF6]2− to [ZrF8]4− occurred after adding free F− (LiF) to FLiBe–ZrF4 (5 mol%), and solubility of Cr2O3 doubled. Hence, the molar ratio of ZrF4 additive to free F− greatly affected Cr2O3 dissolution, whose mechanism can be ascribed to Cr dissolving in the form of soluble CrF3 and the oxide ions O2− combining with ZrF4 and free F− to form ZrOxFy4–2x−y complex.