The effects of applied potential and pH on the electrochemical dissolution of barrier layer in porous anodic oxide film on pure aluminium

The electrochemical dissolution of the barrier layer in porous anodic oxide film on pure aluminium film has been investigated as functions of applied potential and pH by using the potential drop method and AC impedance technique. The porous film was formed on pure aluminium film at various potentials from 5 to 8 V (SCE) in 1 M aqueous H2SO4 solution and a potential drop in the passivation range was applied to the porous film in aqueous H2SO4 solutions. An induction time, after which a sudden increase in current density occurs, was observed. The change in reciprocal capacitance of the barrier layer with time revealed that the thinning of the barrier layer occurs mainly during the induction time. It was found that the dissolution of oxide from the barrier layer/solution interface occurs simultaneously with the formation of oxide at the aluminium/barrier layer interface during the induction time, indicating that the real dissolution rate of the barrier layer is higher than its thinning rate. The real dissolution rare increases with increasing relative change of potential drop to the initial applied potential, implying that the real dissolution of the barrier layer proceeds via an electrochemical dissolution (field-assisted dissolution) at the barrier layer/solution interface. The electrochemical dissolution rate does not change with pH, suggesting that the electrochemical dissolution of the barrier layer is crucially determined by aluminium ion-removal rate at the barrier layer/solution interface.