Kinetic studies of dissolution behaviour of bismuth and anodic oxide films in sulphuric acid

Abstract

Preliminary investigations of the open circuit behaviour of bismuth by the impedance measurement technique indicates the dependence of the oxide growth and dissolution. processes on the medium. Slow oxide formation was observed in NaOH and at a higher rate in Na₂SO₄ in contrast to H₂SO₄ in which the dissolution process was observed to increase with increase of acid concentration. The oxide dissolves according to a zero order mechanism, which in practice produces a constant rate of change of reciprocal series capacitance with time. Results indicate that the anodically formed oxide film may be represented by a two layer model. The rates of dissolution of both layers increase with increase of formation voltage. The outer layer is more defective, partly due to contamination by anions during anodisation. Change in acid concentration affects the dissolution of both layers. Increase in temperature enhances the dissolution of both layers to the extent of apparently dissolving the anodicfilm completely at temperatures ≥ 40°C.