Microstructure and degradation properties of C-containing composite coatings on magnesium alloy wires treated with micro-arc oxidation

To improve the corrosion resistance of micro-arc oxidation (MAO) treated Mg alloy wires, carbon (C) nanoparticles were added to the silicate electrolyte to fabricate C-containing composite coatings. Comparisons of microstructure, corrosion resistance and mechanical properties of uncoated wires, MAO-C-free coated wires and MAO-C coated wires were performed. The results indicated that the surface layers of Mg alloy wires treated with MAO were primarily composed of MgO and Mg2SiO4 phases and demonstrated porous morphologies. The C nanoparticles in the electrolyte were capable of incorporating into composite coatings, forming a dense composite coating microstructure and resulting in the formation of a small amount of MgAlO4 and Mg3SiO4F2, which improve corrosion resistance in static and dynamic immersion environments. Measurements of mechanical properties demonstrated that the MAO-C coated wires retained a high tensile strength of 180 MPa, even after dynamic immersion for 50 days, and possessed better corrosion resistance than the wires coated with MAO-C-free coating. The effects of shear stress and localized environment changes resulting from the flowing corrosive media were proposed as two mechanisms that accelerated corrosion in a dynamic immersion environment.