Electrochemistry of Cr(III) in the Molten System NaF–AlF3–(Al2O3) at 900°C

Abstract

The diffusion coefficient of Cr(III) determined by chronopotentiometry in the molten system NaF–AlF3–CrF3 was found to be D(Cr(III)) = (5.28 ± 0.14) × 10−5 cm2 s−1. The molar ratio CR = n(NaF)/n(AlF3) was 1.4, which allowed us to perform measurements at 900°C. When oxygen anions were present in the melt, chromium was deposited simultaneously with aluminium at the cathode, and the apparent diffusion coefficient related to Cr(III) was too high by two orders of magnitude. This may be explained by the formation of oxofluoro complexes of chromium and aluminium. The solubility of Cr2O3 was studied in the above-mentioned melt at 900°C by two methods: chemical analysis of solidified samples and by chronopotentiometry. It was found that saturation was achieved after several hours, and the concentration of chromium was (177 ± 6) ppm, corresponding to (258 ± 9) ppm Cr2O3.