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. Surface morphologies of zincated surfaces, electrochemical properties during zincate treatment and depth profiles on activated and zincated surfaces were investigated by microscopy and measurement of temporal changes of electrode potential.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 except for the case of A7075. Temporal changes of electrode potential during zincate treatments and morphologies of the zincated surfaces showed that copper and zinc promoted uniform precipitation of zinc and resulting rapid termination of conversion reaction. Excess zinc was shown to dissolve into the plating solution and cause the formation of lowdensity area or gaps between plated films and substrates.Activation for A1100 by various conditions showed that formation of oxide film had a remarkable effect on precipitation of zinc during zincate treatment. 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.

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.

It is well known that the adhesion of the electroless Ni-P coating on aluminum alloy substrate can be remarkably improved by introducing the double zincate treatment as a pretreatment process. This study investigated the effects of the alloying element and zincate treatment on adhesion of electroless Ni-P coating on various aluminum alloy substrates using peeling test, FE-SEM and XPS. Surface morphology of zinc deposit from the 1st zincate treatment and its adhesive strength were changed, depending on the alloying element. The zinc deposit from the 2nd zincate treatment became uniformly thin, and the adhesive strength was improved irrespective of the alloying element. By dipping the zinc deposit obtained at the 1st zincate in 5% HNO 3 solution, most of the zinc deposit on aluminum substrate was dissolved, but XPS analysis revealed that the existence of zinc, whose state was metal, at the surface of aluminum alloy substrate after this dipping. This zinc at the surface should be an important factor influencing morphology of zinc deposit at the 2nd zincate treatment, so that zinc deposit became highly uniform and thin. This zinc at the surface can be attributed to the diffusion of zinc and aluminum between the zinc deposit from the 1st zincate and the aluminum alloy substrate.

It is well known that adhesion of electroless NiP coating onto aluminum alloy substrate can be remarkably improved by introducing the double zincate treatment as a pretreatment process. This study investigated the effects of alloying elements and zincate treatment on adhesion of electroless NiP coating onto various aluminum alloy substrates by using peeling test, FE-SEM and XPS. Surface morphology of zinc deposit from the 1st zincate treatment and its adhesive strength were changed, depending on the alloying element. The zinc deposit from the 2nd zincate treatment became thinly uniform, and the adhesive strength was improved irrespective of the alloying element. By dipping the zinc deposit obtained at the 1st zincate in 5% HNO 3 solution, most of the zinc deposit on aluminum substrate was dissolved, but XPS analysis revealed that the existence of zinc, whose condition was metal, at the surface of aluminum alloy substrate after this dipping. This zinc at the surface should be an important factor influencing morphology of zinc deposit at the 2nd zincate treatment, so that zinc deposit became highly uniform and thin. This zinc at the surface can be attributed to the diffusion of zinc and aluminum between the zinc deposit from the 1st zincate and the aluminum alloy substrate.