Investigation On Process, Microstructure And Performances Of Microarc Oxidation Coating In Dual Electrolyte Fabricated On ZK60 Magnesium Alloy

Micro-arc oxidation (MAO), as a novel and unique surface treatment technique based on anodic oxidation, has been used for surface modification of many metals. At present, MAO researching of magnesium alloys mainly focus on AZ and AM alloys, but few about Mg-Zn-Zr alloys such as ZK60. In this paper, a new dual electrolyte system was developed under constant current micro-arc oxidation mode and the microstructure and composition of the coating were analyzed by modern surface analysis methods such as SEM, EDS, XRD. Some valuable researches about ZK60 have been carried out and some innovative results have been obtained.Firstly, Micro-arc oxidation (MAO) process was carried out in a dual electrolyte system of NaAlO₂-Na₃PO₄ and NaAlO₂-Na₂SiO₃ and Na₂SiO₃-Na₃PO₄ respectively. Test of weight loss was conducted at a 3.5%NaCl solution to assess the resistance to corrosion. The results reveal that the best dual electrolyte system is NaAlO₂-Na₃PO₄.Secondly, by means of single variable experiments, the effect of every element of the dual electrolyte system on voltage-time responses during MAO process and the coating characteristic was analyzed and discussed systematically. Combined with the orthogonal experiment an optimized dual electrolyte system was developed, which composes of 12.5g/L NaAlO2, 5g/L Na₃PO₄, 3g/L NaOH, 2g/L NaB4O7 and 3.6g/L C₆H₅Na₃O₇.The optimized electronic parameters are current density:20 A/dm²; power frequency: 500Hz; duty cycle ratio: 38%; oxidation time: 15min.The microstructural characteristics of coating were investigated by XRD and SEM coupled with EDS. Studies have shown that the optimized micro-arc oxidation coating growth process is mainly composed of the following stages: anodic oxidation stage, micro-arc oxidation stage, voltage droping stage and tiny spark discharge stage, The coating prepared with the optimized process parameters was characterized with compact, smooth, glossy, colorful and exquisite surface. The main phase of surface of Coating is MgAl₂O₄ and MgO phase.The optimized coating in the electrochemical experiments exhibited higher impedance value. In addition, The microhardness test results show that the coating's microhardness is 726HV, the wear resistance test results show that friction coefficient of optimized MAO coating decreases with increasing of loads under the condition of dry sliding wear. The adhesion test results showed that the bonding between coating and substrate achieve the top 1 level of corresponding standard.