The ac etching process on high purity aluminum foil used in electrodes for electrolytic capacitors was investigated in 3.6wt% hydrochloric acid solution at 303K and 323K by applying an asymmetrical triangular current of 0.3A/cm
2 with 5Hz. The time at the top of the current form of the anodic half cycle, t
p, was varied in the range of 0 to 0.1s. Etching products developed on the etching cell during the process were characterized by SEM, XPS and gravimetric analysis. The etched layer contained the products of a duplex structure, consisting of an inner anodic film of Al
2O
3 and an outer hydrous oxide layer of Al
2O
3·1.6H
2O, which was formed at all t
p at 303K and the t
p range of 0.075~0.1s at 323K, while no etched layer was observed at t
p ≤0.05s at 323K, except for only a thin film of Al
2O
3. It seems to be that the variation of t
p affects the concentration gradient of chloride ions in the vicinity of the aluminum electrode and results in the change of the specific surface area of aluminum, and t
p also affects the concentration of the aluminum hydroxide deposited on the electrode during the cathodic half cycle.
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