Growth of etch pits formed during sonoelectrochemical etching of aluminum

Anodes of aluminum electrolytic capacitor, which requires large surface area for a high capacitance, are prepared with electrochemical etching. Ultrasound contributes to the increase of etch pit density by prohibiting anodic oxide film formation and induces uniform tunnel length distribution by enhancing the transport of corrosion product, AlCl3, present inside etch tunnels. Compared to low 28 kHz, smaller and higher density of cavitation bubbles at 68 kHz frequency induces higher pit and tunnel density and increases the survival rate of tunnels to 55.8% in the process of tunnel growth from 3 to 12 μm. It results in the increase of capacitance by 110% at anodization voltage of 100 V.