The corrosion and tribocorrosion resistance of PEO composite coatings containing α-Al2O3 particles on 7075 Al alloy

Abstract

Plasma electrolytic oxidation (PEO) of 7075 Al alloy was carried out in silicate base electrolyte containing 200 nm diameter α-Al2O3 particles for producing composite coatings. The process was performed under a soft-sparking regime using a pulsed bipolar signal with several concentrations of α-Al2O3 particles. It was found that the incorporation of α-Al2O3 particles into the coating did not significantly alter the thickness and roughness of the coating. However, the α-Al2O3 particles were detected on surface of the composite coatings. Corrosion tests showed significant improvement in corrosion performance of the composite coatings due to the efficient pore blocking provided by α-Al2O3 particles, which enhances the barrier performance of both inner and outer layers of the coatings. However, the long-term EIS measurements showed that the performance of composite coatings becomes close to that of particle-free coating after 56 days of immersion in chloride containing solution. Tribocorrosion tests showed that adding 3 g·l−1 of α-Al2O3 particles to the electrolyte bath decreased the lost wear volume of the resulted coating from 30 to 10 mm3 (×10−3). Higher α-Al2O3 particles concentration (i.e. 7 g·l−1) showed detrimental effect on both corrosion and tribocorrosion performance of the coating.