Effects of Electrolyte Immersion Time on Pit Formation of High-purity Aluminum Foil for Electrolytic Capacitors during DC Etching

The effects of surface dissolution of high-purity aluminum foils for electrolytic capacitors during immersion in hot hydrochloric acid on distributions of pits initiated by direct current (DC) etching are discussed. Pit structures were observed with TEM of oxide films stripped from etched foils in I2/methanol solution. Anodized oxide film replicas were prepared for SEM to observe pit distribution. γ-Al2O3 crystals with sizes of 0.2-0.3μm were observed on rolling lines of oxide films. With increasing electrolyte immersion time before DC etching, areas of surface dissolution of aluminum foils around γ-Al2O3 crystals expanded. When immersion time was prolonged from 0s to 60s, pits were initiated uniformly because of the increase in active sites. Excess active sites on surfaces formed above 60s delayed tunnel pit growth.