Effects of copper additive on micro-arc oxidation coating of LZ91 magnesium-lithium alloy

•The corrosion resistance of the LZ91 alloy significantly improved after being treated with the micro-arc oxidation process.•The color, structure and corrosion behavior of the coatings are all affected by the additives in the electrolyte.•The coating thickness increased and became more porous with increasing concentrations of copper ions in the base electrolyte.

The improvement in corrosion resistance for magnesium-lithium alloys in an aggressive environment is an important issue for their widespread application. In this research, the different colors of ceramic coatings are developed by micro-arc oxidation technology in order to protect the surfaces of LZ91 magnesium-lithium alloy using electrolytes containing different concentrations of copper compound. The MAO coatings are compared based on their thicknesses, porosities, surface compositions and corrosion performances. The surface morphology, cross-section and chemical compositions of phosphate coatings were investigated via scanning electron microscopy (SEM) accompanied by energy-dispersive X-rays. The corrosion performance and porosity of the coatings were evaluated by electrochemical characterization which was performed in a 3.5% NaCl solution at 25 °C. The findings showed that the color of the coating fabricated by the MAO process, with the addition of 3% and 5% Cu in the base electrolyte, changed from brown to dark brown. The coating thickness increased and became more porous with increasing concentrations of copper ions in the base electrolyte. The corrosion resistance of the LZ91 alloy was significantly improved after being treated with the micro-arc oxidation process.