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 H(2)SO(4) under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H(2)SO(4) concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H(2)SO(4) concentration the size of the pit clusters increased. There is an optimal H(2)SO(4) concentration, which is 0.01 M H(2)SO(4) in this study, to form a deep etched layer of uniform thickness with high surface area. At H(2)SO(4) 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. (C) 2009 Elsevier Ltd. All rights reserved.