Etching Anode Foil with Branch Tunnels for Aluminum Electrolytic Capacitors

Al foil for high-voltage aluminum electrolytic capacitor was first D.C. etched in HCl-H₂SO₄ mixed acidic solution to form main tunnels and then D.C. etched in natural NaCl solution containing 0.1% H2C2O4 and different trace amounts of Zn(NO₃)₂. Between the two etching processes, Zn nuclei were deposited on the interior surface of the main tunnels by the natural occluded corrosion cell effect to form micro Zn-Al galvanic local cells. The effects of Zn nuclei on the cross-section etching and electrochemical behavior of Al foil were investigated using scanning electron microscopy, polarization curve measurement, and electrochemical impedance spectroscopy. The sub-branch tunnels can form along the main tunnels owing to the formation of Zn-Al micro-batteries, in which Zn is the cathode and Al is the anode. Increasing Zn(NO₃)₂ concentration increases the number of Zn nuclei that can serve as sites for branch tunnel initiation along the main tunnels, thereby enhancing the specific capacitance of etched Al foil.