UBC Theses and Dissertations
Zinc phosphating on 6061-T6 aluminum alloy Shi, Lei
There is an urgent need to develop new non-chromating coating methods for the corrosion protection of aluminum alloys, and this thesis reports studies to establish optimal working conditions for forming zinc phosphate coatings on 6061-T6 aluminum alloy. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and corrosion tests were used to characterize the treated 6061-A1 surface, and to assess the effects of different parameters in the coating bath, as well as different pre-treatments and post-treatments. The initial study investigated different working conditions for the zinc phosphating, and this built on other studies from this laboratory. An initially considered coating bath had 16.0 ml H3P04 (85%), 5.36 g ZnO and 0.5 g NaF per liter, and optimal conditions for coating involved dipping the 6061-A1 samples into this bath at 60 °C for 6 min. But the addition of more fluoride was shown to be effective for increasing the zinc phosphate coverage on the aluminum alloy; the optimal range of F" being from 400 ppm to 600 ppm for these conditions. Based on the working condition study, the effects of pre-treatments, posttreatments, and associated procedures involved with the zinc phosphating of 6061-A1 alloy on the quality of the final coating were investigated. Comparisons were made between acid etching and mechanical polishing in the pre-treatment stages, and the mechanical polishing involved was done by either machine polishing or hand polishing. New post-treatment procedures were investigated using methyltriethoxysilane. It results in a considerable increase in the zinc phosphate coverage and coating thickness on the aluminum surface. This gave a greatly improved corrosion resistance by the coating compared with samples that did not have the silane post-treatment. A related silane treatment can also be used at an intermediate stage in the zinc phosphating immersion. In order to improve the final coating, Cu²⁺ and Ni²⁺ ions were studied as accelerators. Cu(N0₃)₂ (0.002 wt%) and Ni(N0₃)₂ (0.0004 wt%) were separately introduced to the basic phosphating baths containing H₃PO₄; ZnO and NaF, and each was shown to not only increase the phosphating speed, but also to improve the corrosion resistance and adhesion, insofar as the coating contained smaller crystalline grains and increased coverage of zinc phosphate on the surface. Of these two accelerators, Ni²⁺ appeared especially effective at increasing the corrosion protection ability of the final coating. Although NO³⁻ has some capabilities as an accelerator it was shown that its effect was negligible in this context at the concentrations used.
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