Evaluation of etchants for deoxidizing aluminum components

Abstract

In the canning operation, the preparation of aluminum components is important in assuring uniform wetting of the aluminum surfaces with Al-Si. The surfaces of the components should be free of dirt and grease and should also have a minimum retention of non-metallic film, normally aluminum oxide, for the best wetting properties. At present, dirt and grease are removed satisfactorily by a vapor degreaser followed by a Duponol-alkaline wash. The standard specified method of removing the aluminum oxide film from aluminum cans is a four-minute etch in twenty percent phosphoric acid. The oxide is removed from the caps by an eight-minute etch in twenty per cent phosphoric acid or an alternate etch of a one percent solution of hydrofluosilicic acid. If the phosphoric acid etch is not controlled properly, it is possible to form a thin, inert phosphate coating on the surface of the aluminum, which is undesirable for good wetting characteristics. In an effort to find a better etchant for aluminum components, an investigation of various commercial and non-commercial etchants was undertaken. This report details results of this investigation.