One-step in situ growth of a simple and efficient pore-sealing coating on micro-arc oxidized AZ31B magnesium alloy

Micro-arc oxidation (MAO) is an environmentally friendly and effective way to generate a dense and homogeneous coating with high corrosion resistance on magnesium alloys. However, the inherent porous structure of MAO coating can lead to the invasion of corrosive media to the substrate and thus reduce its corrosion resistance. Herein, we reported a novel one-step in situ growth of a pore-sealing coating on MAO-coated AZ31B alloy by hydrothermal treatment in Na3PO4 solution. The surface morphology, chemical composition and growth process of the pore-sealing coating were thoroughly studied by FESEM, XRD, FTIR and XPS. The micro-pores and micro-cracks of the MAO coating can be fully covered by the pore-sealing coating with spherical structure, which is mainly composed of Mg(OH)2 and Mg3(PO4)2. The corrosion resistance of the coatings prepared at different hydrothermal time was evaluated by electrochemical test and immersion test. Compared with the bare AZ31B substrate, the current density of the composite coating with hydrothermal treated for 18 h decreased by three orders of magnitude from 8.40 × 10−5 A/cm2 to 1.59 × 10−8 A/cm2, indicating a remarkable enhancement of the corrosion resistance. The MAO/Mg(OH)2/Mg3(PO4)2 composite coating shows high adhesive strength with the substrate, especially the outermost Mg3(PO4)2 layer with the inner Mg(OH)2 layer. In addition, the in situ growth and corrosion protection mechanisms of the pore-sealing composite coating are also proposed.