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

Effect of alloying elements and generation of hydrogen gas during zincate treatment and electroless nickel-phosphorus plating were studied by using 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 continuing oxidation of the substrate and reduction of proton and zincate ions. On the other hand, the surfaces of ternary Al-2 at%Zn-0.2 at%Mg and Al-2 at%Zn-4.5 at%Mg alloys were immediately covered with thin and uniform film of zinc. Magnesium in aluminum alloys and the excess precipitation of zinc were found to lower adhesion of 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 poorly adhered to the substrates. Despite the thin and uniform zincate film, the plated film on Al-2Zn-4.5Mg alloy was easily peeled due to the voids formed between the plated film and the substrate. Magnesium in the alloy may suppress precipitation of nickel, and accelerate generation of hydrogen gas at the beginning of the plating.