This work focused on the stability of the protective Cr 2 O 3 layer formed on alloys in molten fluorides with different fluoroacidities and ZrF 4 contents. Results showed that fluoroacidity only slightly affected Cr 2 O 3 due to its stability in basic FLiNaK and neutral FLiBe melts. When more than 1.1 mol% ZrF 4 was added to FLiNaK, the new structure [ZrF 7 ] 3− formed by the interaction of ZrF 4 and F − could be detected by Raman Spectroscopy and X-ray diffraction, and Cr 2 O 3 solubility in FLiNaK-ZrF 4 increased 19 times. In the case of FLiBe salt, the structure of [ZrF 6 ] 2− could be detected when 5 mol% ZrF 4 was added, and Cr 2 O 3 dissolution increased from 183 ppm to 320 ppm. Further structural changes from [ZrF 6 ] 2− to [ZrF 8 ] 4− occurred after adding free F − (LiF) to FLiBe-ZrF 4 (5 mol%), and solubility of Cr 2 O 3 doubled. Hence, the molar ratio of ZrF 4 additive to free F − greatly affected Cr 2 O 3 dissolution, whose mechanism can be ascribed to Cr dissolving in the form of soluble CrF 3 and the oxide ions O 2− combining with ZrF 4 and free F − to form ZrO x F y 4-2x−y complex.