Effect of Zincate Treatment on Adhesion of Electroless Ni–P Plated Film for 2017 Aluminum Alloy

The present authors have investigated the changes in adhesion of electroless Ni–P plated films on an aluminum alloy substrate (JIS A2017P-T3, Al–4 mass%Cu) from the viewpoint of the preceding zincate treatment and the subsequent heat treatment. The precipitation state of zinc and adhesion of Ni–P plated film differed with the number of zincate treatments. Without zincate treatment, some of the Ni–P plated film peeled off during the plating process, and the film obtained after the single zincate treatment also showed poor adhesion to the substrate. The highest adhesion was obtained by the double zincate treatment, and the triple zincate treatment resulted in a poorer adhesion. Existence of aluminum at the surface of the zincate film was shown to be necessary to obtain higher adhesion of the Ni–P plated film, on the other hand, excess zinc obtained by the triple zincate caused lower adhesion. Interdiffusion of aluminum and nickel between the Ni–P plated film and the substrate through the medium of zincate film with an appropriate thickness is thought to promote strong bond between the Ni–P plated film and the substrate.