Effect of zincate treatment on adhesion of electroless Ni–P coating onto various aluminum alloys

It is well known that adhesion of electroless Ni–P 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 Ni–P 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% HNO3 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.