Fabrication Of Corrosion Resistance Coating On Magnesium Alloy

As the lightest structural material, magnesium and magnesium alloys has been knowed as "green structural material" of the twenty-first century, which owing to high strength and high rigirity, good electrical conductivity and good thermal conductivity, good dimensional stability and good electromagnetic shielding characteristics, good damping performance and good machining performance, good recycling and so on. So magnesium alloys have a wide application prospect, which is a sunrise industry in the twenty-first century. Unfortunately, magnesium and its alloys have poor corrosion resistance, poor wear resistance and other properties, which seriously restrict its wide application in engineering. Over the years, people make use of many surface engineering technologies to improve the corrosion resistance of the coating, but there are some limitations. Especially, a method of diffusion coating on the surface of magnesium and its alloys which has been developed in recent years provides a good idea for the solution of this problem. The diffusion coating by the use of molten salt which has a lot of advantanges provides an opportunity for the solution of the poor corrosion of magnesium and its alloys. The "Al-Zn-Mg" alloying coating by the use of molten salt has not been reported at home and abroad.This paper uses AICl3, ZnCl2, and NaCl mixed molten salt to obtain alloys on the surface of magnesium. The study includes two major parts:the influence of pretreatment process of magnesium alloy substrate, reaction atmosphere; the mixture of the molten salt and temperature in the forming process of the alloying coating; the study of the coating formation mechanism, the thermal shock resistance, thermal oxidation resistance and strong acid resistance of the coating, which is obtained in the optimum process.In order to simulate industrial corrosion environment, the open circuit pentials and potentiostatic polarization of samples were measured in3.5%NaCl solution by electrochemical workstation, which is in order to study the corrosion resistance of the coatings. Furthermore, the coating of morphologies was observed by using of scanning electron microscopy, and the use of energy spectrum instrument was for coating compositions.The results of the study show that:the coating on the surface of magnesium alloy substrate which had been treated before the diffusion is divided into two layers, the outer layer and the diffusion layer. The thickness of the coating is about l0μm, the coating is continuous and has litter cracks. The quality and the corrosion resistance of this coating are better than the coating without pretreatment of substrate surface; the coating which is obtained with argon protection in the treatment process has fewer cracks, good continuity, and the thickness of the coating is about5μm. The quality and the corrosion resistance of this coating are better than the coating which is obtained without argon protection in the treatment process; when the mass percentage of AICl3and ZnCl2in the molten salt is9:1, also the temperature of the treatment is300℃, the quality and the corrosion resistance of the coating is the best than the coating, which is obtained when the temperature of the treatment is200℃or400℃; When the mass percentage of AICl3and ZnCl2in the molten salt is1:1, also the temperature of the treatment is300℃, the quality and the corrosion resistance of the coating which is obtained with argon protection in the treatment process is the best; The coating formation process is mainly divided into3stages, each stage has the atomic diffusion, and the main reaction is3Mg+2Na[AlCl4]→2Al+3MgCl2+2NaCl.; When the coating is obtained in the optimum process, the strength between the coating and the substrate is high, and the coating has good thermal oxidation resistance, but in acid environment, the protection of the substrate is limited.