Effects of Crystalline Oxide on Pit Nucleation of Alminum Foil for Electrolytic Capacitors during D. C. Etching

The electrode potential of high-purity aluminum foils for electrolytic capacitors moves from a high potential period to a stability period during early DC etching in hot hydrochloric acid. We observed changes in pit structures in transmission electron micrographs of films stripped from etched foils, and measured a potential change using a digital oscilloscope. Pitting attacks occurred around MgAl2O4 spinel or γ-Al2O3. Pits varied from hemispherical to half-cubic, and the etch tunnel growth was promoted by crystallographic dissolutions of the (100) faces. Pit wall surfaces of a half-cubic pit and part of the tips of etch tunnels were passivated. Both MgAl2O4 spinel and γ-Al2O3 crystals were observed at the metal ridges on the aluminum substrate by transmission electron micrographs of ultramicrotomed sections. Pits are considered to be initiated at cracks associated with oxide film crystallizations. MgAl2O4 spinel crystals were observed in a foil containing 123 wt-ppm magnesium after annealing at 445°C for 5h in an Ar gas atmosphere. At 575°C, γ-Al2O3 crystals were only observed in a foil containing 1 wt-ppm magnesium. The growth of MgAl2O4 crystals was accelerated above 1 wt-ppm magnesium.