| Although the density of magnesium alloy was only twenty-five percent of the steel, but the strength of the magnesium alloy is obviously higher than steel. As one of the lightest metal materials in modern industry, magnesium alloy has good casting properties and machinability, welding strength is larger, lower elastic modulus and has the advantages of good electromagnetic shielding performance, therefore widely used in aerospace, military industry, transportation and other fields. Due to the low magnesium electrode potential, therefore the chemical stability is very poor, poor corrosion resistance becomes an important restricting its application difficulty. Electrochemical activity of magnesium and other metal materials, mainly displays in the magnesium in the air can quickly form have ability to protect the surface oxide film layer. Due to the formation of porous membrane layer, poor adhesion, and is not uniform, so we cannot provide a satisfactory corrosion protection ability. Therefore, the development of magnesium alloy corrosion protection technology is extremely important.One of effective ways to improve the corrosion resistance of magnesium alloys is a layer of isolation from forming on the surface of the external environment of protective coating. Chemical conversion treatment is a highly efficient, economic and easy to implement and can realize factory important processing method. In this paper using phosphate conversion process of AZ91D magnesium alloy surface treatment, through the orthogonal test with different proportion of conversion processing fluid experiment, through electrochemical testing, evaluation through conversion processing under various combinations of magnesium alloy corrosion resistance of phosphate coating, so as to select the best conversion processing formula. In the process of magnesium alloy surface phosphate treatment, a rotating magnetic field was applied. Study of different magnetic field conditions on the phosphate coating on the corrosion resistance of magnesium alloys and the influences of magnetic field on the surface morphology structure, analyzes the corrosion resistance of phosphate coating and the characteristics of the surface topography have been studied. Results show that the magnetic field on the surface morphology and structure of magnesium alloys have a positive impact, within a certain range, the auxiliary magnetic speed is higher, the phosphate coating on the surface of the more uniform, dense, and phosphorus content is higher, and its corrosion resistance is higher, more than the best speed, the quality of phosphate coating on the surface of a magnesium alloy has a negative effect.For traditional magnesium alloy chemical conversion coating issue such as difference of crack size, corrosion resistance, this paper studied the phosphate coating two kinds of preparation methods of composite membrane, which is magnesium alloy phosphate/silicon nitride composite membrane and the magnesium alloy phosphate/super hydrophobic composite coating. Preparation process of composite coating is the first use of magnesium alloy surface phosphate conversion treatment, and then use PECVD process and electro-deposition process technique for composite coating structure. Analyzed through physical and electrochemical testing, the results show that composite coating can make up for the magnesium alloy surface physical defects of phosphate coating, and exhibits a uniform, structure compact coating have the excellent the corrosion resistance. |
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