Corrosion Behavior of AM60 Magnesium Alloy Based Composites with and without Plasma Electrolytic Oxidation Coating

7 vol.% Fibre/ AM60, and (7 vol.% Fibre + 3 vol.% nano-Particle)/AM60 composites, named 7FC and MHNC-7F3NP, respectively, and the unreinforced matrix alloy AM60 were prepared by using the preform-squeeze casting technique. The corrosion behaviors of the 7FC and MHNC-7F3NP composites and the matrix alloy were investigated by using the potential-dynamic polarization test in 3.5 wt.% NaCl aqueous solution. Compared with the matrix alloy, the introduction of micron-sized alumina fibres decreased the corrosion resistance of the matrix alloy AM60, however the addition of the nano-sized particles led to almost no further reduction in the corrosion resistance of the MHNC-7F3NP. To enhance the corrosion resistance of as-cast magnesium alloy AM60-based composite, the plasma electrolytic oxidation (PEO) coatings with NaAlO2 and KOH electrolytes were applied on the surfaces of as-cast AM60 (PEO-AM60), 7 vol.% Al2O3 fibres/AM60 (PEO-7FC) and 7 vol.% Al2O3 fibres +3 vol.% Al2O3 nano-sized particles/AM60 (PEO-MHNC-7F3NP). By comparing the corrosion test results of the coated AM60 and the composites with those of the uncoated counterparts, it was found that the PEO coating significantly improved the corrosion resistances by up to 154 times. The corrosion rates of the coated composites were higher than the coated AM60, although the PEO coating reduced the corrosion rates of the composites by two orders of magnitudes. The addition of nano-sized particles barely increased the corrosion rate of the composite. The microstructure of each uncoated and coated material was observed and analysed by scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS).