The dissolution behaviour of barrier layer of porous anodic oxide film on Al-1wt.% Si-0.5wt.% Cu alloy has been investigated as a function of applied potential and pH. Porous film was formed on the Al alloy at various applied potentials from 1 to 8 $V_{SCE}$ in 0.1 and 1 M $H_2SO_4$ solutions. Potential drop in the passivation range was applied to the porous film in aqueous $H_2SO_4$ solution. From the response of current density with time to the potential drop, we obtained an induction time after which a sudden increase in current density was encountered, and the induction time increased with decreasing ratio of potential drop and with increasing pH of electrolyte. From capacitance measurement, it was found that the thinning of the barrier layer occurred during the induction time. The experimental result was discussed in terms of the combined effects of the applied potential and pH on the electrochemical dissolution of barrier layer of the porous anodic oxide film on the Al alloy. From the calculation of the amount of charge corresponding to the growth of the oxide film, it is found that the oxide growth occurs simultaneously with the oxide dissolution during the thinning of the barrier layer and the rate of the formation of the porous anodic oxide film on the Al alloy is determined by the dissolution of the barrier layer. From all the experimental results, it is suggested that the dissolution of the barrier layer of the porous oxide film on the Al alloy is crucially determined by the electric field distributed over the oxide film and chemical reaction occurring at the oxide/electrolyte interface.
