The Influence of CaF_2 and AIF_3 upon Mechanism and Kinetics of the Anodic Reaction in Cryolite-Alumina Melts

摘要

The anodic reaction on spectral pure graphite (SPG) was studied in the potential range from 1.2 V to 2.1 V versus an aluminium reference electrode in cryolite-alumina melts containing also CaF_2 and AlF_3 at 1000 癈 by means of Electrochemical Impedance Spectroscopy (EIS). The experimentally recorded impedance spectra were interpreted in terms of an equivalent circuit approach and the kinetic theory of Bai and Conway. Using the values of the effective rate constants of the two charge transfer steps and the values of the Tafel constants, the role of the two additives: CaF_2 and AlF_3 is clarified. The addition of CaF_2 and AlF_3 to cryolite -alumina saturated melt, increases the overpotential of the anodic reaction by decreasing the value of the effective rate constant of the first step, and this decrease is much greater in case of AlF_3. In the industrial type melt, the influence of the CaF_2 seems to be prevailing above the influence of AlF_3.