The influence of CaF2 and AlF3 upon mechanism and kinetics of the anodic reaction in cryolite-alumina melts

The anodic reaction on spectral pure graphite (SPG) was studied in the pote ntial range from 1.2 V to 2.1 V Versus an aluminium reference electrode in cryolite-alumina melts containing also CaF2 and AlF3 at 1000 degrees C by m eans of Electrochemical Impedance Spectroscopy (EIS). The experimentally re corded 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 o f the Tafel constants, the role of the two additives: CaF2 and AlF3 is clar ified. The addition of CaF2 and AlF3 to cryolite alumina saturated melt, in creases the overpotential of the anodic reaction by decreasing the value of the effective rate constant of the first step, and this decrease is much g reater in case of AIS. In the industrial type melt, the influence of the Ca F2 seems to be prevailing above the influence of AlF3.