Effect of Alloying Elements and Generation of Hydrogen Gas on Zincate Treatment and Electroless Nickel-Phosphorus Plating of Aluminum Alloys

The effect of alloying elements and the generation of hydrogen gas during zincate treatment and electroless nickel-phosphorus plating were studied by electron microscopy. Hydrogen gas was vigorously evolved during the zincate treatment for binary Al-2 at%Mg and Al-2 at%Si alloys, and the zincated surfaces were nonuniformly covered with excess coarse zinc particles due to the continuing oxidation of the substrate and reduction of proton and zincate ions. On the other hand, the surfaces of the ternary Al-2 at%Zn-0.2 at%Mg and Al-2 at%Zn-4.5 at%Mg alloys were immediately covered with a thin and uniform film of zinc. Magnesium in aluminum alloys and the excess precipitation of zinc were found to lower the adhesion of the electroless nickel-phosphorus plated films. When the zincated Al-2Mg and Al-2Si alloys were immersed in the plating solution, dissolution of the excess zinc generated hydrogen gas, and the plated films were poorly adhered to the substrates. Despite the thin and uniform zincate film, the plated film on the Al2-Zn-4.5Mg alloy was easily peeled off due to the voids formed between the plated film and the substrate. Magnesium in the alloy may suppress precipitation of the nickel and accelerate generation of hydrogen gas at the beginning of the plating.