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XPS and corrosion studies on zinc phosphate treated surfaces of aluminum Heung, Wai Fan


The work in this thesis deals with an investigation of zinc phosphate (ZPO) treated surfaces of 7075-T6 aluminum alloy. X-ray photoelectron spectroscopy (XPS) and corrosion tests are used to characterize the treated surfaces and to examine their corrosion protection performance. The coating processes are performed by immersing polished alloy surfaces into 10 wt. % of ZPO suspension in water adjusted at various pH conditions. Biased XPS is applied to discriminate the physically trapped and chemically absorbed zinc compounds in the ZPO treated surfaces. Ultrasonic rinsing in distilled water for the treated surfaces is found important in the sample preparation procedure so as to remove any physically trapped compounds from the surfaces. Five different pH conditions (pH3.5, 5.0, 6.6, 10.5, and 13.0) are studied in this work. Acetic acid and sodium hydroxide solution are used to adjust the required pHs for the ZPO solutions. The treated surfaces are studied by angle dependent XPS (ADXPS) to obtain chemical information at different probed depths. ZPO is found to be an effective coating compound for the 7075-T6 aluminum surface. At pH=6.6 and 3.5, the coatings found on the treated surfaces are believed to be respectively a ZnOx-AlOx mixed material and a thin ZPO-like compound. In alkaline and weakly acidic conditions (pH=13.0, 10.5 and 5.0), the coatings formed on the surfaces are enhanced and have stmctures with mixed ZnOx-AlOx-ZPO materials. Weight loss measurements, atomic absorption spectrometry (AAS) and scanning electron microscopy (SEM) are involved in the corrosion studies. 3.5 % NaCl solutions are used as the corrosive environments. Surfaces before and after the corrosion tests are compared to examine the corrosion protection performance of each surface. As judged by XPS, the surface prepared at pH=13.O is likely to provide the best corrosion control among the five treated surfaces. Dissolution of aluminum from the alloy is observed in the corrosive environment. The coating is believed to play a role as a physical barrier to suppress the corrosion attack on aluminum.

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