Effect of sulfuric acid on pit propagation behaviour of aluminium under AC etch process

Pit propagation on high purity aluminium electrode in 2 M HCl solutions with and without H2SO4 under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H2SO4 concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H2SO4 concentration the size of the pit clusters increased. There is an optimal H2SO4 concentration, which is 0.01 M H2SO4 in this study, to form a deep etched layer of uniform thickness with high surface area. At H2SO4 concentrations higher than 0.01 M, the pit propagation proceeded on limited foil surface sites and deep etched regions were formed locally, since sulfate ions assisted passivation and reduced the number of pit nucleation sites on foil surface. Analysis of potential transients during the anodic half-cycle supports the hypothesis that sulfate ions retarded the pit nucleation.