Pit Growth during Tunnel Etching of Aluminum Foil for Electrolytic Capacitors

Abstract

The dissolution behavior on the tip and on the wall of tunnel pits formed on aluminum during DC etching in hot HCl solutions was studied. Initiated pits grow into a tunnel by crystallographic and auto-catalytic dissolution on the tip, independent of both the pH and the chloride ion concentration of a bulk solution. The tip dissolution proceeds under diffusion control at a rate of several μm/s, and then the tunnel growth abruptly stops at a limiting length when the tunnel tip may be repassivated. Although the tunnel walls must be passive during etching, they are subjected to general dissolution because of a high etching temperature. The tunnel widths increase continuously at a rate of several nm/s and the square cross section of the tunnels changes to circular shape.