Pitting mechanism of aluminum in hydrochloric acid under alternating current

The etching of high-purity Al foils in HCl under a.c. was investigated. Pit development and characteristics of etched foils were studied taking into account the a.c. amplitude, frequency, etching time and electrolyte temp., using light micrography and scanning electron micrography. A max. of electrical capacitance C sub M of etched and anodized foils is observed for certain frequency value v sub M C sub M ; and v sub M depend on the experimental conditions. The aluminum oxide formed on the metal etched surface exhibits min. thickness for bath temp. approx 60 deg C. Galvanodynamic voltammetry allows the determination of a pitting current and an induction time which depends on frequency. The pit density and calculated anodic current efficiency for metal dissolution are found to decrease markedly with increasing frequency. 17 ref.--AA