Aluminum foil for the electrolytic capacitor and etching method for forming etching pits

摘要

In manufacturing an aluminum foil for the electrolytic capacitor, the present invention permits the productivity to be enhanced by omitting the intermediate annealing step. The etching pits can be formed excellently in the aluminum foil through the non-electrolytic process.;The aluminum foil that may be used has the composition that is composed of 5 to 40 ppm of Si, 5 to 40 ppm of Fe, 0.1 to 3 ppm of Pb and 15 to 150 ppm of Ni, and for the rest, Al and inevitable impurities, in which the inevitable impurities include less than 10 ppm of Cu and the total quantity of the inevitable impurities except Cu is equal to or less than 100 ppm. In the aluminum foil having the above composition, the oxide film has the thickness of 20 to 60 Å and Ni has the ionic strength ratio that is equal to or more than 1.5 at the depth of 0.5 µm, equal to or more than 10 at the depth of 0.1µm and equal to or more than 2 at the depth of 0.3 µm, where it is assumed that the Ni ionic strength is equal to 1 at the depth of 1 µm. Otherwise, Ni has the concentration ratio that is equal to 5 to 50 when it is located deep in the central area of the oxide film, and 80 to 150 when it is located at the interface between the oxide film and the aluminum foil blank, where the 10 nm depth in which Ni is located 10 nm toward the blank from the interface between the oxide film and the aluminum foil blank is assumed to be 1. The etching pit formation may be performed by following the step of removing the surface layer on the aluminum alloy foil including the oxide film by the thickness of 20 to 200 Å by having it contacted by a solution that contains no hydrochloric acid or solution that contains less than 1000 ppm of chloride ions, without conducting electric current, the step of forming etching pits and the step of enlarging the diameters of thus formed etching pits.