Anti-corrosion microarc oxidation coatings on SiCP/AZ31 magnesium matrix composite

The corrosion-resistant ceramic coatings up to 80 μm thick were fabricated on SiCP/AZ31 magnesium matrix composite by microarc oxidation (MAO) technique in Na3PO4 + KOH + NaF solution. The microstructure, composition and phase constituent of ceramic coatings were analyzed by SEM and XRD, and the electrochemical corrosion behaviour of coatings was evaluated by the electrochemical polarization method. The thicker coating is compact and displays a good adhesion to the composite substrate. The ceramic coatings consist of MgO, Mg2SiO4, MgF2, Mg3(PO4)2, furthermore, a few residual SiC phases were also found in the coatings by means of SEM observation and EDX analysis. Most of SiC reinforced particles in the oxidized composite substrate have transformed into the oxides under microarc discharge sintering, but a few residual SiC reinforcements in the MAO coatings have not disrupted the continuity of coatings. So the corrosion resistance of the SiCP/AZ31 composite is greatly improved by MAO surface treatment, however, the corrosion resistance of coated composite also depends on the coating thickness.