Influence of zincate treatment on adhesion strength of electroless nickel-phosphorus plated film for commercial pure aluminum

Characterizations of zincate films were conducted formed as a pretreatment for electroless nickel-phosphorus platings on commercial pure aluminum substrate (JIS A1050P-H24, 99.5 mass%Al). By using a basic solution which consists of sodium hydroxide and zinc oxide, a zincate film formed by the single zincate treatment showed coarse zinc grains of 1-2 mu m in size. The double zincate treatment by using the basic solution reduced coarse zinc grains, and the thickness of the uniform zincate film was 20-30 nm. Iron-added zincate solutions decreased the number and the size of coarse grains in the single zincate treatments compared with the case of the basic zincate solution, and the double zincate treatments brought about further thin zincate films of similar to 10 nm in thickness. The single zincate treatment by using the basic zincate solution resulted in poor adhesion strength of the electroless nickel-phosphorus plated film which peeled spontaneously off the substrate due to its residual stress. Quantification of adhesion strength by 90 degrees peeling test showed the double zincate treatment by the basic solution increased the adhesion strength of the plated film up to 30 N/m. In the cases of the double zincate treatments by the iron-added zincate solutions, the adhesion strengths of the plated nickel-phosphorus films showed such remarkable increase that those could not be quantified by the peeling test due to the rupture of the plated nickel-phosphorus films. From the observation by transmission electron microscopy of the interfaces between the nickel-phosphorus plated films and the substrates, thickness of a zincate film ought to be similar to 10 nm for eliminating gaps between the plated film and the substrate.