Abstract
Aluminum foil of 99.99% purity was etched in an aqueous solution of 2mol dm-3 hydrochloric acid, 0.01mol dm-3 sulfuric acid, and 0.2mol dm-3 aluminum chloride at 303K by 5Hz AC wave with controlled anodic potential, Ea, and cathodic potential, Ec. The etched porous layer was evaluated by SEM observation, optical observation, electrostatic capacity measurement and CV behavior. Electrostatic capacity and porous structure were grcatly influenced by Ea and Eo values.
To make etched pits, it was necessary to set potential Ea higher than the breakdown potential Ebd appearing in the CV curves. At Ea potentials only slightly higher than Ebd, the etched porous layer was thin and the thickness of the foil decreased at an anodic electric charge of 50C cm-2, while when Ea was too high, the pits showed local interconnection and the surface of the porous layer dissolved and lacked smoothness. The aluminum hydroxide film formed in the etched pits during the cathodic cycles played an important role in the characteristics of pit formation. The thickness of the aluminum hydroxide film increased at low Ec, when etching took place locally with several large pits on the foil. This might be caused by the non-uniformity of the hydroxide film on the bottoms of the pits. At high Ec, on the other hand, film protection was weak, leading to thinning of the porous layer and dissolution of the foil. In this study, the optimum etching conditions that provided high electrostatic capacity and a deep and uniform pit morphology were Ea=0.1V, Ec=-2.4V vs. SCE at an anodic charge C=50C cm-2
