Synergistically strengthened corrosion protection for MAO-coated magnesium alloy by micropore-sealing and enhanced hydrophobicity

A synergistic sealing protocol involving modifying micro-arc oxidation (MAO) coatings with a mixture of stearic acid (SA) and yttrium nitrate was developed to intensify the long-lasting corrosion protection. The micropore-sealing effects of two yttrium-containing solutions at different times and the corresponding morphologies and compositions of the composite coatings were investigated. The organic-inorganic SY-MAO-20 composite coating treated in the mixture for 20 minutes exhibited mostly enhanced corrosion protection, ascribed to the decreased surface porosity resulting from the sealing effect of (C 17 H 35 COO) 3 Y (Yttrium stearate) compared to the unmodified coating. The SY-MAO-20 coating possesses an extremely low corrosion current down to 10.9 nA·cm -2 and a high impedance modulus up to 6.52 MΩ·cm -2 at low frequency, which are decreased by two orders of magnitude and increased by 50 times compared to the MAO film without modification. The enhancement of the as-prepared coating in corrosion resistance is also caused by the transition of surface wettability from hydrophilicity (water contact angle: 20.4° ± 4.2°) to hydrophobicity (141.2° ± 1.2°) due to the SA modification. These results demonstrate an effective strategy to address the micropore issue in the MAO coating and thus significantly intensify the physical barrier action for achieving long-term corrosion protection for the magnesium alloy.