This invention relates to the removal of nodular copper from aluminum foil surfaces, particularly etched aluminum capacitor foil, by contacting the aluminum with a solution of ammonium persulfate.
When aluminum foil is etched for use in capacitors, a thin uniform film of copper appears on the foil surface. Scanning electron microscopy (SEM) and electron microprobe analysis have revealed that for the most part the copper is present as a monolayer, but sometimes copper nodules are also present.
Since aluminum foil contains some copper, copper as well as aluminum will dissolve during the etching of aluminum foil. The copper that is present in small amounts in the foil is dissolved in the treating solutions and then redeposited on the foil surfaces. These deposits take the form of extremely thin films, even as thin as a monolayer, and spherical or irregular nodules. It was believed that the occurence of copper nodules on the foil surface might be related to copper ion concentration in the etch solution.
It was found that copper originating from the foil itself redeposited as an atomic monolayer on the foil surface. However, deposition of the monolayer was not always complete, as the amount of copper redeposited is dependent on the copper content of the foil; a large fraction of such dissolved copper is redeposited at the foil-solution interface. When the copper ion concentration in solution reached a high level, copper deposited as nodules on the foil surface.
Nitric acid washes have been used in the prior art to remove copper from the surface of aluminum capacitor foil. W. J. Bernard in Ser. No. 155,728 filed June 2, 1980 teaches removal of copper from the surface of aluminum foil by low voltage anodization in sodium potassium tartrate or ammonium phosphate.