Effects of alloying elements in aluminum alloys and activations on zincate treatment and electroless nickel-phosphorus plating

Zincate treatment and electroless nickel-phosphorus plating for aluminum alloys of A1100, A2017, A5052 and A7075 were researched from the viewpoints of alloying elements and activation conducted prior to zincate treatment. Adhesion strengths of the plated films were measured by 90° peeling test, which showed that double zincate treatment improved adhesion strength especially when ferric ion was added into the zincate solution. Temporal changes of electrode potential during zincate treatments and morphologies of the zincated surfaces showed that copper and zinc in the substrates promoted uniform precipitation of zinc. Excess zinc was shown to dissolve into the plating solution and cause formation of low-density areas or gaps between plated films and substrates. Activation for A1100 by various conditions showed that formation of oxide film remarkably affected the precipitation of zinc. When a mixture of nitric and hydrofluoric acid was used for activation, finer particles of zinc were formed near etch pits compared with the cases of hydrochloric or nitric acid. On the other hand, the surface was uniformly covered with fine zinc particles in the case of the activation with a solution of sodium hydroxide. This indicates that uniformity of oxide film on substrate before zincate treatment is thought to bring about uniform precipitation of zinc.