New Method for the Corrosion Resistance of AZ31 Mg Alloy with a Porous Micro-Arc Oxidation Membrane as an Ionic Corrosion Inhibitor Container

This work introduces a new composite anticorrosion coating for the AZ31 magnesium alloy, based on the synergistic effect of an organic/inorganic composite coating with a micro- and nanoporous micro-arc oxidation (MAO) membrane as the container of ionic corrosion inhibitor (M-16). The surface morphologies and size of the micro/nanocontainers in the porous MAO membrane before and after filling with M-16 corrosion inhibitor are examined by scanning electron microscopy (SEM). The effectiveness of M-16 for corrosion suppression on AZ31 Mg alloy with and without epoxy coating as the top sealing layer is demonstrated by electrochemical impedance spectroscopy (EIS) and salt spray tests. The potentiodynamic polarization and electrochemical impedance spectroscopy measurements show that, compared with the bare AZ31 Mg alloys, the composite coating has superior corrosion resistance with the a lower corrosion current (9.7 × 10^-9 A/cm²) and a higher protection efficiency (99.3%) after immersion in 3.5 wt % NaCl solution and, meanwhile, has stronger salt spray resistance within 30 days. The results demonstrate the synergistic effect of the isolation protection of the micro-arc oxidation layer and the inhibition of M-16 and that the epoxy coating contributed to the protection for AZ31 Mg substrate to some extent. Therefore, it is anticipated that the composite coating has a potential application in the protection of metals and their alloys.