Cellular Structure of Aluminum Capacitor Grade Foil and Pit Initiation during Electrolytic Etching

Abstract

The initiation behavior of etch pits on the microstructure of aluminum capacitor grade foil was investigated. The microstructure, which consists of a fine network of cells, approximately less than 1μm in diameter, was revealed by the chemical polishing process of high-purity aluminum. Two series of 99.99wt.% pure aluminum specimens in the form of 100μm thick foils were prepared. One series of foils contained copper in the range of 1 to 300wt.ppm, and the other contained lead from 0.2 to 1.5wt.ppm. Observation and analysis for the reaction morphology after treatments were employed using SEM, AFM and analytical TEM. The cellular size changed on the basis of the amount of copper and lead and the chemical polishing time. Concerning the pit initiation during electrolytic etching, the cellular structure of the aluminum foil was examined in order to clarify the role of the copper and lead. The etching was carried out in a hydrochloric acid solution using a square anodic pulse current. Several examples of the electrochemical attack on different amounts of the impurities and at various orientations are presented. The first step of the preferential sites for pit initiation during the electrochemical action predominately occurred on the wall leading to the interior of the cellular structure.