XPS Studies of the Al Alloy Surface with Zincate Treatment

It is well known that the adhesion strength of metal plating film on Al substrate should be remarkably improved by introducing double zincate treatment as a pretreatment process for metal plating on Al. At each step of alkaline etching, desmutting, the first zincate treatment, HNO3 dipping, and the second zincate treatment in the double zincate process, depth profiles and the chemical state of the surface elements were extensively measured by XPS. A set of surface structure models in the process was proposed to explain the improvement of the adhesion strength by double zincate treatment.By dipping the first zincate film in HNO3 solution, a granular zinc deposit of about 1 μm thickness, roughly deposited on Al passivation film at the first zincate treatment, was dissolved, and zinc nano-particles were believed to be concurrently formed in the Al passivation film of about 3nm thickness. In the second zincate treatment the nano-particles should be exposed to the second zincate solution to act as nuclei for formation of a highly uniform and thin zinc film. The zinc particles were so close to the metallic Al substrate, that Dipolar coupling such as Alδ+…Oδ&minus…Znδ+ would act to improve the adhesion strength of double zincate film on Al substrate.