Preparation of white ZrO2 coating with low solar absorptance on aluminum alloy by plasma electrolytic oxidation

Abstract

The plasma electrolytic oxidation (PEO) coatings with the color of black or white have good prospects in thermal control application. However, it still remains a challenge for obtaining the PEO coatings on Al alloys to achieve high whiteness that in turn could give adequate low absorption thermal control characteristic. Herein, a bright white oxide coating mainly composed of ZrO2 was successfully prepared on 6061 aluminium alloy by PEO process from a fluorozirconate electrolytic solution. The microstructure and composition of the coating were studied by SEM, EDS, XRD and XPS. The thermal control properties were measured by UV-VIS-NIR spectrophotometer and infrared emissivity spectrometer, respectively. The stability of thermal control properties of the coating was also evaluated by simulated space environmental tests. The corrosion protection performances of the coating were investigated by potentiodynamic polarization and salt spray tests. Results show that the oxide coating is composed of t-ZrO2 as well as a little of m-ZrO2 and γ-Al2O3 with a porous microstructure. The coating exhibits low solar absorptance (αs) of 0.205 and high emissivity (ε) of 0.84, which is comparable for the typical space white paints. In addition, the coating owns excellent stability in thermal control properties under simulated space environments. The coating has a good durability in corrosive environment though it provides only slight improvement in corrosion resistance of 6061 aluminium alloy due to the presence of many larger micropores in the coating.