二元アルミニウム合金上の亜鉛置換皮膜形成と無電解Ni–Pめっき皮膜の密着性

Abstract

Formation of zincate films and adhesion of electroless nickel-phosphorus plated films were studied for binary aluminum alloys of Al–2at%Mn, Al–2at%Fe, Al–2at%Cu, Al–2at%Zn and high-purity aluminum (99.999 mass%). Precipitation mode of zinc in the zincate treatmens greatly varied according to the alloying elements in the substrates. In the cases of the first and the second zincate treatments for Al–Mn, Al–Fe and high-purity aluminum, zinc precipitated excessively, then, porous films of zinc repeatedly fell off the substrate. The surfaces of Al–Cu and Al–Zn alloys were immediately coated by uniform zincate films in the first and the second zincate treatments. Precip-itation of zinc is considered to be uniform if the oxide film on a substrate dissolves uniformly and rapidly in the zincate solution. When electroless nickel-phosphorus plating was conducted after the second zincate treatment of Al–Mn and Al–Fe alloys, the plated films easily peeled off. Those on Al–Cu 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 because 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.