Abstract
The corrosion cell is exploited to enhance the effective surface area of high-purity aluminum foils for aluminum electrolytic capacitors by forming tunnels. The morphology of tunnels is characterized, and the growth mechanism is discussed. Detailed investigations indicate that tunnels could grow only when nitric acid was added into the mixed acid electrolyte in the corrosion cell. Also, the tunnel morphology in the aluminum foil is relevant to the concentration of nitric acid and hydrochloric acid. In the mixed electrolyte solution that consists of 3.0 mol L^–1 H₂SO₄, 1.0 mol L^–1 HCl solution and 0.2 mol L^–1 HNO₃, the current density of the corrosion cell on the aluminum foil surface can reach 0.2 A cm^–2, which leads to square tunnels of high density on the surface of aluminum foils. The length and width of the tunnels are about 25–30 and 2 μm, respectively. The morphology of the tunnels is similar to that obtained by DC etching. The current results indicate that the generation of tunnels has a close correlation with the hydrogen evolution corrosion and oxygen absorption corrosion in the corrosion cell.

Key words
aluminum foil, aluminum electrolytic capacitor, tunnels, corrosion cell

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