The effect of sulfuric acid on tunnel etching of aluminum in hydrochloric acid

Aluminum foil for high voltage aluminum electrolytic capacitor is etched by D. C. current in the hot chloric solution. The several studies of tunnel growth have ever studied using the hot hydrochloric acid. Effects of sulfuric acid in the hot hydrocloric acid on the tunnel etching of aluminum were studied. The concentration of sulfuric acid has no effect on the initial tunnel growth rate, but has definite effect on the limit of tunnel length. As increase in the concentration of sulfuric acid causes a decrease in the tunnel length. Initial etching pit sites scatter partially on the surface by adding sulfuric acid. The multiplication of etch tunnels occurs in sequence after previous tunnels have reached their limits of length.