Low etch alkaline zincate composition and process for zincating aluminum

Abstract

A method is provided for zincating aluminum substrates for metal plating thereon wherein the plated aluminum product has smoothness, dimensional integrity and increased production yield of the plated products. The substrates also have enhanced paramagnetic thermal stability of ENP coatings used on memory disk products. A zincate bath contains as additives Fe.sup.+3 and NaNO.sub.3, and a chelator to chelate the iron, with a preferred iron chelator being Rochelle Salt and with the amount of Fe.sup.+3 being controlled at a preferred concentration of 0.2 to 0.3 g/l. A preferred zincating method employs an etchant composition comprising HNO.sub.3, H.sub.2 S0.sub.4 and H.sub.3 PO.sub.4 to etch the aluminum substrate prior to zincating. Use of this etchant composition, either alone or with the zincate bath of the invention, is particularly effective for aluminum substrates which have been ground to a smoothness of less than 100 .ANG.. The etchant is non-aggressive and removes metal oxides formed by the grinding and annealing process to form the aluminum substrates used to fabricate the memory disks. The etchant also preserves the dimensional integrity of the substrate and prepares the surface for zincate deposition. It is highly preferred to use the etchant and zincate bath of the invention in the same metal plating process to provide an enhanced process and metal plated product. The etchant or zincating bath may also be used alone in other plating processes requiring these type substrate treatments.