X-RAY PHOTOELECTRON-SPECTROSCOPY AND CORROSION STUDIES OF ZINC PHOSPHATE COATED ON 7075-T6 ALUMINUM-ALLOY .2. EFFECT OF COATING PH

Coated layers which formed on 7075-T6 aluminium alloy surfaces after t reatment in zinc phosphate (ZPO) suspensions adjusted to different pH values (3.5, 5.0, 6.6, 10.5 and 13.0) were characterized by X-ray phot oelectron spectroscopy (XPS). Comparative XPS tests were also made aft er the treated surfaces were exposed to a 3.5% NaCl solution for 5 h. The processes of coating from solution, oxidation, etching and bulk di ffusion were all involved, although in different proportions, in the d ifferent treatments. The amounts of zinc and phosphorus in the uppermo st layers were enhanced when the coating was at pH values of 5.0, 10.5 and 13.0, and in each case a mixed ZnO(x)-AlO(x)-ZPO structure was ap parently formed. The coating prepared at pH = 13.0 appeared to provide the best corrosion protection.