The mechanism of the anodic reaction in cryolite melt is studied in presence and in absence of a layer of a l u m i n u m metal in the cell. These two experimental conditions give rise to large differences which are interpreted on the basis of changes in the interracial 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 a l u m i n u m 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,The industrial preparation of a l u m i n u m by electrolysis of alumina dissolved in fused cryolite involves complex electrochemical reactions, especially on the anodic side. In order to improve the working conditions of the electrolytic cells, much research is devoted to the study of these reactions (1); the nature of the ionic associations present in the bath has been studied (2), the diffusion coefficients (3), and the transport numbers of the constituent elements are k n o w n (4). F r o m these data different hypotheses have been built up to obtain a better insight into the mechanism of the electrolysis.However, it has been recently (5, 6) shown that an important part of the observed overvoltage arises from the gas formation on the graphite anode. The kinetic parameters of the electrochemical reaction depend on the pressure in the cell, and the properties of the electrolyte-graphite interface (wetting angle ~ and interfacial tension 7) have a considerable influence on the electrode process.We plan to give here the results of the chronopotentiometric study of these phenomena. Special attention will be paid to the presence of dissolved a l u m i n u m in the bath (7) for it has been observed that, when molten cryolite is in contact with liquid a l u m i n u m metal, a very large change of the surface tension occurs (8). As a consequence, the results of chronopotentiometry are quite different to those obtained in pure ionic melts.In industrial cells the electrolyte is in contact with a layer of liquid a l u m i n u m ; a part of the faradaic loss can be attributed to the oxidation of the dissolved metal (9). Our research indicates also that this presence induces a large change in the interracial tension at the electrode and influences the gas coverage of the electrode surface.Experimental The experiments with liquid a l u m i n u m in the bath ar...

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