Two kinds of metal oxide coatings were fabricated on 7050 Al alloy by hard anodization (HA) and micro-arc oxidation (MAO) techniques. The microstructure, phase composition, and corrosion behavior of the two coatings were studied by scanning electron microscopy, X-ray diffraction, and corrosion tests, respectively. When compared with the HA coating, the MAO one was more effective to isolate the substrate from the corrosive environment. In addition, as confirmed by electrochemical tests, the MAO coating was of better corrosion resistance than the HA coating. Furthermore, it was revealed by neutral salt spray test that the MAO coating could protect substrate alloy over 1140 h, while the HA coating can only protect substrate alloy for 46 h due to the amorphous composition and through thickness defects (micro-pores and micro-cracks).

Micro-arc oxidation (MAO) was performed on AZ31 and AZ91 Mg alloys to explore the relationship between Mg substrates and corrosion behaviors of resultant coatings. The microstructure and long-term corrosion performance of the two coatings were investigated. Results showed that MAO coating on AZ91 alloy provided a longer-term corrosion protection to substrate than that on AZ31 alloy, despite having similar microstructure and phase composition. However, once corrosion occurred, the corrosion area of MAO coating on AZ91 alloy enlarged at a faster rate compared to that on AZ31 alloy during immersion tests, and their corrosion morphologies were different. Based on the corrosion features of Mg substrates and MAO coatings, roles of substrates on the corrosion processes of coatings were analyzed.

Micro arc oxidation (MAO) coatings doped with graphene oxide (GO) were prepared on the 6061 Al alloy substrates in the electrolyte composed of an aqueous solution of sodium silicate (Na₂SiO₃·9H₂O, 20 g L⁻¹) and 100 mg L⁻¹ GO. The microstructure, mechanical property and tribological behaviour of the MAO + GO composite coatings were comparatively investigated. Results indicated that GO was easily dispersed into the solution for MAO coatings preparation. Little of GO in the composite coating was observed by field-emission scanning electron microscopy and energy dispersive spectrometer. This composite coating with increased proportion of α-Al₂O₃ phase still exhibited a porous microstructure like MAO coating but with an improved binding force, mechanical property and tribological behaviour. Furthermore, the tribological behaviour of this composite coating sliding against with hard alloy steel (YG6), silicon nitride (Si₃N₄) and High-carbon chromium bearing steel (GCr15) had been analysed.