Electrochemical Etching of Aluminum Foil for Electrolytic Capacitors
The increase of capacitance on dielectric layer is associated with enlargement of etched surface area of electrodes for aluminum electrolytic capacitors. Etched tunnels extending along  directions are formed by crystallographic dissolution of aluminum in a chloride solution. This study has been focused primarily on the effects of addition of inhibitors to hydrochloric acid solution on the increase of the surface area by Electrochemical Impedance Spectroscopy (EIS). The addition of 3 % ethylene glycol was more effective for the enlargement of the surface area comparing with those of 1M sulfuric acid and no additive. And it was observed by TEM that the tunnels were grown straight and parallel to  directions.
Z.Y. Zhong, H. Saka, T.H. Kim, E.A. Holm, Y.F. Han and X.S. Xie
C.-S. Chi et al., "Electrochemical Etching of Aluminum Foil for Electrolytic Capacitors", Materials Science Forum, Vols. 475-479, pp. 385-388, 2005