Electrochemical behaviour and structure of anodic oxide films formed on aluminium in a neutral borate solution

Abstract

99.99% Al specimens were anodically oxidized in a boric acid-borate solution (pH = 7.4) at temperatures of 20, 40 and 60, by applying a constant potential of 50V ( vs sce). The anodic current and the amount of dissolved Al ions were measured as a function of the anodizing time. Observations of sections of the oxide films were made by electron microscopy using an ultra-microtome for sample preparation. The current was found to decrease exponentially with time to a steady value which increases with increasing temperature. The current efficiency for the formation of oxide decreases gradually with decreasing current and is less than 70% in the period with steady current. The low efficiency of the reaction is exclusively due to the field-assisted dissolution of the oxide. In general, the oxide film is composed of two layers. The inner layer next to the metal is compact and is responsible for the polarization behaviour, its thickness being almost constant during anodizing but increasing with decreasing solution temperature. The outer layer, on the other hand, has many vertical pores and continues to grow even in the period with steady current. The porosity of the outer layer is estimated to be 22% for 40 and 60° films and the 60° film contains 4.5% bound water. The current-potential relationships measured for the anodized specimens in a 20° solution indicate that the protective ability of the inner layer increases with increasing anodizing time and temperature.