Summary: | 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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