STUDIES ON THE CORROSION AND THE BEHAVIOR OF INERT ANODES IN ALUMINUMELECTROLYSIS

The corrosion rates of inert anodes based on tin oxide and nickel ferr ite cermet materials were studied as a function of some operating para meters. To reach a better understanding of the corrosion mechanism, th e behavior of the anodes was observed under some specific conditions, such as in pure cryolite, at high current densities, at different pote ntials, and at varying cathode surface areas. It was confirmed that lo w alumina concentrations led to catastrophic corrosion of the anodes a nd that high current densities and high as well as low NaF/AlF3 molar ratios were also detrimental. The corrosion rate of tin oxide based an odes showed a minimum (so-called ''normal corrosion'') at anodic poten tials of 2.2 to 2.4 V with respect to aluminum. The normal corrosion i s due to chemical dissolution of the anode material and reduction of t he corrosion products into the cathode metal. The corrosion rate incre ased with increasing cathode surface area. At potentials higher than s imilar to 2.5 V, the anodes showed catastrophic corrosion. Catastrophi c corrosion can be ascribed to decomposition of the anode material by depletion of alumina at the anode surface provoked by low bulk concent ration of alumina and/or high current density.