Study on preparation and corrosion resistance of micro-ARC oxidation coating on Mg-Zn-Ca

To improve the corrosion resistance of Mg-Zn-Ca, Micro-arc Oxidation (MAO) technology was used to prepare corrosion-resistant coating on Mg-Zn-Ca by adjusting the positive duty cycle (20%, 30% and 40%) in Na2HPO4, NaOH and C3H8O3 solutions. The phase composition, surface morphology, coating thickness, bonding force and corrosion resistance were characterized by XRD, SEM, optical microscope, scratch tester and electrochemical workstation. XRD results show that the coating phases are mainly MgO, Mg3(PO4)2, ZnO and Zn3(PO4)2; the intensity of the diffraction peak of the same object is getting lower and lower with the increasing of positive duty cycle when the 2θ angle is 32.4°, 37.2°, 43.1° and 62.8°; SEM results show that the coating pore size and the diameter of granular coating become larger with the increase of the positive duty cycle, the coating is dense when the positive duty cycle is 20%. The scratch tester results show that the maximum bonding strength is 61.70 MPa when the positive duty cycle is 20%. The coating thickness test results show that the maximum thickness of the coating is 15.89 μm when the positive duty cycle is 40%. Electrochemical test results show that the maximum resistance of the coating is 490.41 Ω, the largest corrosion potential is –1.16 V and the smaller corrosion current is 4. 89×10–5 A/cm2 when the positive duty cycle is 30%. The polarization form of the Mg-Zn-Ca in 3.5%NaCl solution is mainly electrochemical polarization. In summary, when the positive duty cycle is increased from 20% to 30%, it can alleviate corrosion resistance, but when the positive duty cycle continues to be increased to 40%, it will lead to excessive coating pore size and porosity, while corrosion resistance of the material is reduced. © 2021, Chongqing Wujiu Periodicals Press. All rights reserved.