Formation of zincate films on binary aluminum alloys and adhesion of electroless nickel-phosphorus plated films

Abstract

The formation of zincate films and the adhesion of electroless nickel-phosphorus plated films on binary aluminum alloys of Al-2at% Mn, Al-2at% Fe, Al-2at% Cu, Al-2 at%Zn and high-purity aluminum (99.999 mass%) were studied. The precipitation mode of zinc during the zincate treatments significantly varied according to the alloying elements in the substrates. For the first and second zincate treatments of AlMn, AlFe and high-purity aluminum, the zinc excessively precipitated, then porous films of zinc repeatedly fell off the substrate. The surfaces of the Al-Cu and Al-Zn alloys were immediately coated by uniform zincate films during the first and the second zincate treatments. The precipitation of zinc is considered to be uniform if the oxide film on a substrate uniformly and rapidly dissolves in the zincate solution. When an electroless nickel-phosphorus plating was conducted after the second zincate treatment of the AlMn and AlFe alloys, the plated films easily peeled off. Those on the AlCu and Al-Zn alloys showed excellent adhesion, and dimple patterns of the substrates were observed on the partly peeled areas. The poor adhesion is thought to be caused due to the fact that the excess zinc dissolves at the beginning of the plating and generates hydrogen gas, then gaps are formed between the plated films and the substrates. © 2013 The Japan Institute of Metals.