Microstructure of surface area and etching behavior of high-purity aluminum foils for electrolytic capacitors

Microstructure of high purity Al foils as high temperature rolled and annealed for electrolytic capacitors was analyzed mainly by transmission electron microscopy. Emphasis was on the dislocation microstructure near the foil surface covered with oxide films and also on the microstructure change along the depth from the surface of the foils. The results were discussed in reference to the etching behavior of the foils. As it goes from the surface to the foil thickness of about 10 μm, dislocation structure changes from relatively random to cellular. The dislocation density and the cell-forming tendency increased with cold rolling to 50% reduction. Surface of the foils was examined by AFM to be wavy, with ridges extending parallel to the rolling marks at intervals of 2 to 4 μm. It was also revealed by TEM that band areas with high densities of dislocations and also of oxide particles extended, underneath the oxide layer, along the rolling marks. A larger number of initial-pits were formed by etching along the band areas, suggesting that the areas had acted as preferential nucleation sites for pits. By a prolonged etching, deep tunnel-pits were formed at the sites corresponding to a single dislocation or the aggregates of dislocations. Rolling the foils to 20% reduction prior to etching resulted in increased density and in decreased average size of the pits. This suggests that density of etch pits in the foils could be controlled by controlling the processing conditions before etching.