Effect of Zincate Treatment on Adhesion of Electroless Nickel-Phosphorus Coating for Commercial Pure Aluminum

The present authors have conducted characterization of zincate films formed as a pretreatment for electroless nickel-phosphorus coatings on commercial pure aluminum substrate (JIS A1050P-H24, 99.5 mass%Al). By using a basic solution which consisted of sodium hydroxide and zinc oxide, a zincate film formed by single zincate treatment showed coarse zinc grains of 1–2 μm in size whose (0001) planes were mainly parallel to the surface of the substrate. A double zincate treatment by using the basic solution reduced the coarse grains, and the thickness of the uniform zincate film was 30–40 nm. A commercial zincate solution which contained iron decreased the number and the size of coarse zinc grains for the single zincate treatment compared with the case of the basic solution, and the double zincate treatment formed a further thin zincate film of 10–20 nm in thickness.The single zincate treatment by using the basic zincate solution resulted in such poor adhesive strength of the electroless nickel-phosphorus plated film as to peel off the substrate due to its residual stress. Quantification of adhesive strength by 90° peeling test showed the double zincate treatment increased the adhesive strength of the plated film up to 30 N/m. In the case of the commercial zincate solution, the adhesive strength obtained by the single zincate treatment was 125 N/m. Furthermore, the adhesive strength of the plated nickel-phosphorus film was too high to conduct the peeling test in the case of the double zincate treatment by the commercial solution, where dimple patterns were observed due to ductile fracture of the substrate.Observation by transmission electron microscopy of the interface between the nickel-phosphorus plated film and the substrate showed gaps in the case of the double zincate treatment by the basic zincate solution. On the other hand, the double zincate treatment by the commercial solution eliminated gaps and a strong interface was obtained.