Preparation And Performance Research Of Micro-Arc Oxidation Coatings On AZ31 Magnesium Alloy

The preparation of micro-arc oxidation ceramic coatings in phosphate and silicate electrolyte was investigated respectively in this thesis. The thickness and the rate of weight growth of ceramic coatings in various process conditions were tested. The corrosion resistance of ceramic coatings was evaluated by polarization curves and electrochemical impedance spectroscopy in 3.5% NaCl solutions with different pH value. The surface morphology and phase composition of micro-arc ceramic coatings were analyzed using SEM and XRD.The optimal technologcial conditions of micro-arc oxidation in phosphate electrolyte are that: 15 g·L^-1 sodium phosphate, 4 g·L^-1 sodium hydroxide, 3 g·L^-1 sodium fluoride, 15 g·L^-1 sodium tetraborate, the current density 5 A·dm^-2, the oxidation time 30 minutes. The optimal technologcial conditions of micro-arc oxidation in silicate electrolyte were: 8 g·L^-1 sodium silicate, 2 g·L^-1 sodium hydroxide, 3 g·L^-1 sodium fluoride, 6 mL·L^-1 glycerol, the current density is 5 A·dm^-2and the oxidation time acts as 20 minutes.The even and compact micro-arc ceramic coatings could be fabricated in phosphate electrolyte and silicate electrolyte, and the coatings fabricated in silicate electrolyte in the aspect of rigidity and corrosion resistance are better than those fabricated in phosphate electrolyte. Technology parameters such as electrolyte constitutes, the current density and the oxidation time could effect the performances of micro-arc oxidation coatings to some extent, which can be used to control the surface morphology and the thickness of micro-arc oxidation coatings. The results of electrochemical measurement technology indicated that the micro-arc oxidation coatings have perfect corrosion resistances in acidic, neutral and alkaline aqueous solution, and could protect magnesium alloy substrate effectively. The surface of micro-arc oxidation ceramic coatings was composed of numerous fused particles with different sizes, which top accompanied with micro-holes, and the section is composed of an inner compact layer and an outer loosed layer. The coatings fabricated in silicate electrolyte have smaller pore size and smoother surface compared with those fabricated in phosphate electrolyte. During the process of micro-arc oxidation, the electrolyte ions could be involved in the reaction to form coatings. The micro-arc oxidation ceramic coatings prepared in phosphate electrolyte is composed of MgO, while the coatings prepared in silicate electrolyte is mainly composed of MgO and Mg₂SiO₄.