The Research Of Anodizing Technology Of Environment-friendly And Low-energy Consumption For AZ91D Magnesium Alloys

AZ91D magnesium alloy is one of the most widely used casting magnesium alloy currently, which has good development prospects in the field of transportation, 3C products and military industry. However the poor corrosion resistance limits its extensive application and the most effective and direct way to solve this problem is surface treatment technology. Anodizing, due to its easy operation, low cost and other outstanding characteristics, has become one of the most important surface treatment technologies for magnesium alloys. However this method still has some weaknesses, like the high-energy consuming caused by high voltage and spark discharge, the environmental pollution resulted from the harmful composition of electrolyte, and the structure and properties can not meet the production requirements. The aim of this work is to develop the environmental protection and low-energy consumption anodizing technology, and to acquire the layers with homogeneous structure as well as excellent performance by optimizing technological process.Na₂B₄O₇-NaOH-C₆H₈O₇ is used as the main solution system. The electrolyte components are determined by the experiments of addition agents; the optimal process for preparing anodic oxide film on the surface of magnesium alloy AZ91 D is determined by the orthogonal optimization experiments of electrical parameters and the final comprehensive orthogonal experiments. The environment-friendly electrolyte composition consists of sodium tetraborate which is main film former. The black layer with good corrosion resistance and uniform surface can be formed when the pH value is 9.2¹0, and pH has significant effects on the coating appearance and corrosion resistance. Additives can effectively improve the uniformity of the film and corrosion resistance. The process uses AC power, and the voltage of the black anodic oxide film forming range from 4¹0 V, the energy consumption is 0.214 kw·h/（m² μm） with the optimum process condition, which is very low so that can ensure the low-energy consumption and high security during the process.In this work, a uniform, black film with 3 μm was prepared on AZ91 D magnesium alloy. SEM, EDS, XPS, XRD were used to characterize the morphology, crystalline structure and elements chemical state of the film. The oxide film formed on AZ91 D alloy is compact. Conformal flake structures with dense particles can be observed on the surface, which is mainly composed of Mg, C, O, Al, Na, B, N. XRD and XPS indicates that the film consists of crystalline MgO, amorphous Al₂O₃, Mg₂B₆O₁₁, C₁₀H₁₂N₂O₈Na₂, etc.Potential polarization curve method was used to test the corrosion resistance of anodized samples, and the results shows that the optimal sample’s corrosion current density is 0.160 μA/cm², which is even lower than those of the samples treated with Dow17 and HAE process. The adhesion strength and the micro-hardness of the film are also enhanced.