Corrosion resistance and mechanisms of smart micro-arc oxidation/epoxy resin coatings on AZ31 Mg alloy: strategic positioning of nanocontainers

Smart micro-arc oxidation (MAO)/epoxy resin (EP) composite coatings were formed on AZ31 magnesium (Mg) alloy. Mesoporous silica nanocontainers (MSN) encapsulated with sodium benzoate (SB) corrosion inhibitors were strategically incorporated in the MAO micropores and in the top EP layer. The influence of the strategic positioning of the nanocontainers on the corrosion protective performance of coating was investigated. The experimental results and analysis indicated that the superior corrosion resistance of the hybrid coating is ascribed to the protection mechanisms of the nanocontainers. This involves two phenomena: (1) the presence of the nanocontainers in the MAO micropores decreased the distance between MSN@SB and the substrate, demonstrating a low admittance value (∼ 5.18 × 10−8 Ω−1), and thus exhibiting significant corrosion inhibition and self-healing function; and (2) the addition of nanocontainers in the top EP layer densified the coating via sealing of the inherent defects, and hence the coating maintained higher resistance even after 90 days of immersion (1.13 × 1010 Ω cm2). However, the possibility of corrosion inhibitors located away from the substrate transport to the substrate is reduced, reducing its effective utilization rate. This work demonstrates the importance of the positioning of nanocontainers in the coating for enhanced corrosion resistance, and thereby providing a novel perspective for the design of smart protective coatings through regulating the distribution of nanocontainers in the coatings.