Fabrication of environmentally friendly anti-corrosive composite coatings on AZ31B Mg alloy by plasma electrolytic oxidation and phytic acid/3-aminopropyltrimethoxysilane post treatment

In order to improve the corrosion resistance of magnesium alloys, environmentally friendly anti-corrosive composite coatings were fabricated on AZ31B Mg alloy. Mg alloys were first treated by plasma electrolytic oxidation (PEO) technique and then modified by phytic acid/3-aminopropyltrimethoxysilane (PA/APTMS) hybrid. Besides, the technique parameters were optimized. The composition and structure of the composite coatings were characterized by SEM, EDS, IR and XPS, the corrosion resistance was evaluated by electrochemical tests in 3.5% NaCl solution and the possible corrosion resistance mechanism was proposed as well. The results showed that the composite coating's corrosion resistance was remarkably improved by two or three orders of magnitude, compared with Mg alloy substrate and the single PEO coating. The PA/APTMS hybrid in network structure and the stable covalent bonds within the composite coatings effectively restricted the penetration of a corrosive medium into the Mg alloy surface, representing an excellent corrosion barrier effect.