The corrosion resistance of SiO2-hexadecyltrimethoxysilane hydrophobic coating on AZ91 alloy pretreated by plasma electrolytic oxidation

Although plasma electrolytic oxidation (PEO) can be used to fabricate hard ceramic protective layers on the exterior of magnesium alloys, it cannot achieve ideal corrosion resistance because of the formation of micropores. In this study, AZ91 alloy pretreated using PEO was modified in two steps involving SiO2 gel for sealing and hexadecyltrimethoxysilane (HDTMS) for hydrophobisation. The experimental results demonstrated that the microholes of the PEO coating were well sealed by the SiO2 gel and the final coating exhibited a uniform and dense morphology with improved hydrophobicity, as indicated by the increase of the contact angle to 135 +/- 4 degrees. The gel layer exhibited excellent adhesion to the PEO coating and the corrosion resistance of the substrate alloy was also significantly improved after the SiO2 and hexadecyltrimethoxysilane modifications, which resulted in a lower corrosion current density of 1.05 x 10(-7) A.cm(-2). Additionally, no significant changes were observed on the surfaces of the treated samples after 30 days of immersion in NaCl solution. These results indicated that the two-step treatment is a promising approach for the post-treatment of PEO coatings to enhance the protection of the matrix.