Steady‐State Anodic Leakage Current in Barrier‐Type Aluminum Oxide Films

Abstract

When aluminum is polarized potentiostatically in a barrier‐forming electrolyte, such as aqueous ammonium tartrate, the current passed diminishes steadily to a low value commonly termed the leakage current. This value is not stable, but increases gradually, attaining a constant value after many hours. The dependence of this steady‐state current on potential has been studied for aluminum electrodes in 1M aqueous ammonium tartrate at pH 7 and test temperatures of 30°, 50°, and 70°C. In the overpotential range of approximately 50–80V, current is independent of overpotential, while exponential dependence of the anodic current on overpotential is seen at lower overpotentials, with a significant reverse contribution below approximately 20V. The values of both forward and reverse exponential coefficients are extremely anomalous in comparison to normal Tafel slopes. Above approximately 80V, current increases rapidly with potential until breakdown occurs.