Preparation And Characteristics Of Micro Arc Oxidation Composite Coating On Magnesium Alloy In Dual Electrolyte

In this project, ceramic coatings fabricated on AZ91D magnesium alloy were obtained by MAO in a Silicon-aluminum dual electrolyte using micro-arc oxidation technique. The optimal technological process parameters were obtained by investigating constitutes of the electrolyte, power output parameters and power supply modes.These effects were measured by the corrosion resistance of MAO coatings. Moreover, the MAO composite coating combined with SiC particles were formed by adding nano SiC particles into the dual electrolyte in the optimal process parameters. Characteristics of MAO coatings such as thickness, morphologies, phase analysis, element composition and hardness were respectively investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), film thickness meter and Vickers hardness tester. The wear resistance of Mg alloy and coatings were evaluated by friction. The corrosion resistance of the coatings in a 3.5%NaCl neutral solution was evaluated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization test and immersion test. See complete results below:Corrosion resistance of MAO coatings and thickness increases at first and then declines in condition concentrations of each composition of the electrolyte increases. After optimizing through orthogonal experiment, a composite electrolyte consisted of 9 g/L sodium aluminate,15 g/L sodium silicate 2g/L sodium tetraborate,3 g/L sodium hydroxide, 5 mL/L propanetriol,7 g/L sodium citrate and is defined to be the better component formula of the environmentally friendly electrolytic systems.As is shown, power output parameters have a big impact on the corrosion resistance and thickness. Corrosion resistance of MAO coatings increases first and then decreases gradually with the increasing of duty cycle, oxidation time and current density respectively. What’s more, with the increasing of frequency, the corrosion of MAO coatings decreases gradually. After the test optimized through orthogonal experiment, a serial of optimized power output parameters are obtained. Specifically, parameters contain current density (15 A/dm2), frequency (520 Hz),,duty cycle (38%) and oxidation time (15 min). The characteristics of ceramic coatings formed in bipolar mode are superior to those in unipolar mode.With the increase of the nano-SiC, corrosion resistance of the MAO composite coating increases at first and decreases later. According to the experiments, the coating obtained in condition the concentrations of the nano-SiC in the electrolyte is 4g/L has a better performance.The process of micro-arc oxidation on magnesium alloy can be divided into four zones, which are active dissolution zone, active passivation zone, passivation zone and transpassivation zone.The process of the coating can be divided into three stages at a constant current, which are anodic oxidation stage, microarc oxidation stage and local microarc oxidation stage. According to the EDS results, it can be seen that MAO single coating primarily contains Mg, Al, O and Si elements whose distributions change along with oxidation time, and MAO composite coating contains C element which come from SiC particles besides Mg, Al, O and Si element. The C element slightly higher externally and lower internally, but it’s a sprinkle. In various conditions of oxidation time, source electrical output parameters and power modes, XRD analyses indicate that both two coatings are mainly composed of MgO, Mg2SiO4 and MgAl2O4,but their concentrations change with the various conditions.The corrosion resistance and wear resistance of magnesium alloy can be effectively improved. Compared to the magnesium substrate, the MAO single coating shows a lower current density and at the same time, it decreases by three orders of magnitude and the corrosive potential of the coated samples increases by approximate 150 mV. Simultaneously, wear rate decreases by 20%. The corrosion resistance of composite coatings is further improved because of the compound effect of SiC, and their wear resistance improves more clearly. Characteristics of composite coating are superior to those of single coating.