Etching Behavior and Dielectric Film Formation on Aluminum Foil Stocks for Electrolytic Capacitors: A Review.

This review delineates the mechanisms of pit nucleation and growth, establishes the foundational principles of etching technology, and presents the findings from investigations on the behavior of anodic dissolution and anodic film formation on high-purity aluminum foils for electrolytic capacitors based on electrochemical analyses and surface electron microscopic observations of etched surfaces. To elucidate pit nucleation and growth mechanisms, the effects of crystalline oxide and small amounts of lead on etching behavior were investigated. Pits initiate at cracks surrounding MgAl₂O₄ spinel or γ-Al₂O₃, resulting from the crystallization of the oxide film at metal ridges on the aluminum substrate. Using ultra-high-resolution field-emission scanning electron microscope (FE-SEM), high-angle backscattered electron (BSE) images revealed the presence of lead as the bright nanoparticles, approximately 10 nm in size, at the surface oxidation layer along rolling lines attributable to pick-up inclusions during hot rolling. Pitting attacks predominantly occur in the oxidation layer owing to the less noble potential for tunnel dissolution in the initial DC etching phase. Increased titanium content within the aluminum foil accelerated hydrogen evolution in the pit and hydrous oxide formation during the cathodic half-cycle of alternating current etching. The crystallization of anodic oxide films around MgAl₂O₄ spinel crystals, formed in a boric acid solution, was observed using transmission electron microscopy (TEM). Round-shaped γA-Al₂O₃ formed around the MgAl2O4 crystals and expanded across the surface as the formation voltage increased. [ABSTRACT FROM AUTHOR]