THE CATHODIC POLARIZATION OF ALUMINUM COVERED WITH ANODIC OXIDE-FILMS IN A NEUTRAL BORATE SOLUTION .1. THE MECHANISM OF RECTIFICATION

Aluminum covered with barrier type anodic oxide films was polarized anodically and cathodically in a neutral borate solution in a vacuum to investigate rectification phenomena. Rectification took place only in the solution, suggesting that it is due to a high rate of proton reduction during cathodic polarization. Potentiostatic cathodic polarization in solution showed a threshold potential of -2.0 V(Ag/AgCl). At more positive potentials, a minute cathodic current flows, while the current increases with time below -2 V(Ag/AgCl). The increase in current was accompanied by hydrogen evolution at weak spots in the oxide film, causing the formation of a small number of pits. The mechanism of pit formation is discussed in terms of alkalization at the bottom of pits.