Etched Layer Structure Produced by AC Etching of Aluminum in Hydrochloric Acid Solution

Alternating current etching of aluminum in chloride solution produces an etched layer composed of an ant-like porous structure that is used as electrodes in electrolytic capacitors for a large specific surface area. The effect of frequency and etchant on the layer formation was examined by means of capacitance measurement of etched foil, scanning electron microscopy and Auger electron spectroscopy. The structure of the layer can be characterized by etched depth, etch products amount and its distribution and cubic etch cell size. The amount of etch products was greater at lower etchant temperature, at higher frequency of current, and at the subsurface than at the bottom of a layer. Changes in the typical etch cell size with frequency to the -0.63th power was discussed in terms of current efficiency.