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Formation of zinc phosphate coatings on aluminum alloys Kok, Wai-Hoong


This thesis describes research to determine some of the factors that affect zinc phosphate coatings of aluminum alloys. The basic parameters for a coating bath to be used for dipping AA6061-T6 alloy are P-to-Zn atomic ratio, pH and F~ ion concentration, but the effect of acid etching in the pre-treatment was studied for both the AA6061-T6 and AA2024-T3 alloys. The surface characterization methods used are X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy, and the corrosion protection provided by the coatings formed are assessed by electrochemical polarization measurements. Three main zinc phosphate coating recipes used in this study are referred to as the highratio normal-zinc (h-Zn), normal-zinc (n-Zn) and low-zinc (/-Zn) formulations. It is shown that the h-Zn formulation (modified from n-Zn), with a P-to-Zn atomic ratio of 15, provides the best coating condition at the original pH as compared with the n-Zn and 1-Zn formulations. The coating baths have optimal pH values of 4 for the main types of coating recipe, and this is judged in terms of the quality of the coatings formed and by the observed corrosion resistance. The best coating recipes determined here are for the /-Zn process at pH 4 with 600-1000 ppm of added F⁻ and for the n-Zn process at pH 4 with 200-400 ppm F~. The inclusion of acid etching in the pre-treatment of AA6061-T6 aluminum alloy yields better coatings than when mechanical polishing alone is used; it appears that the acid etching yields a rougher surface on which the coating nucleates more favorably. However, acid etching of the AA2024-T3 alloy results in the presence of enriched copper at the alloy/film interface, and the amount of copper detected by XPS increases with the time of the acid etching. A study was made of the effect of this copper enrichment on the zinc phosphate coating formed on the AA2024-T3 alloy. For the conditions studied, a 5 min acid etch leads to zinc phosphate coatings that have a uniform distribution of small crystallites and improved corrosion resistance. However, when the copper is in excess, for example from a 10 min acid etch, the coating formation is strongly inhibited

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