Chronopotentiometric Investigation of the Anodic Reaction in Cryolite Melts: Influence of Dissolved Metal Traces

Abstract

The mechanism of the anodic reaction in cryolite melt is studied in presence and in absence of a layer of aluminum metal in the cell. These two experimental conditions give rise to large differences which are interpreted on the basis of changes in the interfacial properties at the electrode. The chronopotentiometric technique is used to obtain a better insight into these surface variations and on the gas evolution on the graphite anode. The dissolved metal reacts with the graphite to form at its surface aluminum or sodium carbides and gives a better contact between the electrolyte and the electrode. The analysis presented here is illustrated by the generation of three types of anode effects: By depletion of dissolved metal near the electrode which very much changes the wetting properties; by decrease of the pressure in the cell which, in absence of liquid metal, increases the dimension of retained gas bubbles; and by rapid repetition of the galvanostatic pulses which indicates that the gas evolution is a slow process. These observations have the same origin, i.e., the gas covering of the electrode which markedly depends on the presence of dissolved metal.