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Optimization of zinc phosphate coating on 7075-T6 aluminum alloy |
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| Authors: | J F Ying B J Flinn M Y Zhou P C Wong K A R Mitchell T Fostert |
| Institution: | a Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 b Chemistry Group, Defence Research Establishment Pacific FMO, Victoria, British Columbia, Canada VOS 1B0 |
| Abstract: | A zinc phosphate coating process on 7075-T6 aluminum alloy has been significantly improved through a combined approach of titanium colloid pretreatment, pH level adjustment, fluoride additive and ZnO/H3PO4 ratio adjustment. In particular, optimum phosphate coatings were formed in coating baths with a ZnO/H3PO4 ratio of 0.205 and 0.256. Coating morphology studied by SEM reveals that the adjustment in the coating procedure produces an improved crystalline coating layer with high coverage and small grain size. For coatings formed in a bath with a ratio of 0.205, the coverage of the crystalline patches on top of the amorphous coating base is almost complete. The chemical composition of the coating layers has been characterized by XPS and SIMS. The crystalline and amorphous coating phases formed in the bath with a ratio of 0.205 were found to have compositions given schematically as Znx(PO4) and ZnxAly(PO4), respectively. In contrast, neither coating phase formed in the bath with a ratio of 0.256 shows the presence of Al. |
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