Effect of voltage on microstructure and corrosion resistance of micro-arc oxidation coating on Mg-Zn-Ca biodegradable alloy

This work discussed the influences of voltage on the microstructure and corrosion resistance of micro-arc oxidation (MAO) coating developed on Mg-2Zn-0.3Ca (ZX20) alloys. The MAO coatings on ZX20 alloy surface consist of MgO and Mg2SiO4 compounds. They feature two layers, namely a porous outer layer and a dense inner layer. Higher voltage forms a thicker MAO coating on ZX20 alloy, meanwhile, larger diameter micropores and more thermal micro-cracks are generated in the coating. The MAO coating on ZX20 alloy obtained at 400V has the largest thickness (5.98 +/- 0.23 mu m), the largest diameter of the micropores (1.02 +/- 0.64 mu m), the most thermal cracks and the most amount of Mg2SiO4. The MAO coating on ZX20 alloy obtained at 300V has the largest contact angle value and the highest bonding strength. During the immersion corrosion, the corrosive ions such as Cl- can enter the outer layer of the coating and react with MgO to generate Mg(OH)2, resulting in the dissolution of the coating. With the prolong of the immersion time, local corrosion and cracks form at the weak site of the coating, speeding up the corrosion. While, the dense inner layer containing chemically stable Mg2SiO4 has less defect, providing an effective physical barrier for the substrate and enhancing the corrosion resistance of ZX20 alloy. The ZX20 alloy coated at 300V exhibits the lowest degradation rate (PH = 0.13 mm y-1) due to its dense inner layer, as well as having the largest contact angle value and the highest bonding strength.