KINETICS AND MECHANISM OF THE AL(III) AL ELECTRODE-REACTION IN CRYOLITE-ALUMINA MELTS/

Using the relaxation method (RM) with galvanostatic perturbation and e lectrochemical impedance spectroscopy (EIS), exchange current densitie s and activation parameters were determined for the electrode reaction on the aluminium electrode in pure cryolite melt and in cryolite-alum ina melts with the addition of 2-12 wt % Al2O3. In all these melts a t hree step electrode process was observed, comprising a preceding chemi cal reaction followed by two charge transfer steps. The exchange curre nt densities for two charge transfer steps were determined as a functi on of temperature, together with the equilibrium constant of the prece ding chemical reaction and its kinetic and diffusion impedance. The th ird step was found to be independent of diffusion and of the concentra tion of alumina, whereas the second step showed mixed characteristics. The exchange current densities were of the order of 5-15 A cm(-2).