Effect of Sealing in Alkali Earth Metal Ion Solutions on the Resistance to Pitting of Al Anodized in a Sulfuric Acid Solution

Abstract

The sealing in alkali earth metal ion solutions was effective to increase the resistance to pitting of the aluminum anodized in a sulfuric acid solution. After sealing, formation of crystalline BaSO₄ in the anodic oxide film was confirmed by XRD and EPMA. The formation of sulfate and/or carbonate of Ca (II) and Sr (II) was suggested, though no direct evidence was obtained except for Ca 2p XPS spectrum. The resistance depended on the composition of sealing solution and increased in the order of doubly distilled water<Ba (II) solution<Sr (II) solution<Ca (II) solution. This is in reverse order of the insolubility of each sulfate, so that the effect cannot be explained simply by the formation of insoluble sulfate by sealing. Less increment of resistance with Ba (II) was due to the formation of crystalline BaSO₄ in the anodic oxide film during sealing, which eventually comes off the film, leading to the formation of large defects in the film. The best results were obtained with Ca (II) but the reason is not clearly explained so far.