| Magnesium alloys are called as the green engineering material with the most developing and applied potential and good prospects in the 21 st century. They are widely used in automotive, aerospace, medical, electronics and other industries. However, magnesium alloys have poor corrosion resistance and carry on serious corrosion reactions in the humid atmosphere, soil and water. The engineering applications of magnesium alloys can bring about the major security risks. To enhance the intrinsically safe performance of magnesium alloy materials, the surface treatment technologies of magnesium alloy are usually used to deposite the anti-corrosion coatings on magnesium alloys. Because magnesium alloy surface is easily covered by the oxide film that reduces the cohesive strength between coatings and substrate materials, so the research on the plating and coating pretreatment process of magnesium alloy has important practical significance. In this paper, the research object is the pretreatment of electroplating and coating and the main results obtain are presented as follows:(1) A process of pulse pre-plating nickel on magnesium alloy was developed. Through the electrolytic degreasing, acid pickling, electrical activation, the surface state of AZ91 D magnesium alloy was helpful for direct plating nickel. By orthogonal test and single factor analysis methods, the pulse formula and technology of the plating nickel on magnesium alloys surface were optimized. The formula is Nickelous Sulfate 40g/L, Sodium Citrate 10g/L, Ammonium Bifluoride 5g/L, Sodium Borate 1.8g/L, pH 8.5. The form of pulse current is double-pulse current which is composed of pulse current(+) and direct current(-). The mean density of forward current is 0.63 A/dm2, its duty cycle is 20% and it lasts 100 ms. The mean density of reverse current is 0.125 A/dm2 and it lasts 10 ms. The structure and properties of the coating were investigated by Scanning Electron Microscopy(SEM), X-ray diffraction(XRD), Electrochemical Polarization Curve and other analytical methods, respectively. The results show that when the forward of double pulse current is pulse and the reverse is direct current, and the reasonable parameters are set, the pre-plating nickel film with excellent properties on magnesium alloy can be obtained. This film can be used as the pre-plating coating of magnesium alloy and helpful for plating metals on magnesium alloys.(2) A process of phosphating treatment on magnesium alloys was developed. The formula and technology of the phosphating treatment on magnesium alloys surface were optimized by the orthogonal test and single factor analysis methods. The formula is Sodium Dihydrogen Phosphate 40g/L, Calcium salt 40g/L, Hexamethylene Tetramine 1g/L, Organic Ligands D 15g/L, pH 3.0. The phosphating temperature 30℃ and the phosphating time is 30 min. The uniform density and corrosion resistance of phosphating film can be deposited on magnesium alloys in the above environmental phosphating liquid. The effect of Hexamethylene Tetramine, phosphating time and phosphating temperature on the properties of phosphating film was investigated by Scanning Electron Microscopy(SEM), X-ray diffraction(XRD), Electrochemical Polarization Curve. The results show that the Hexamethylene Tetramine can help the growth of grain, the phosphating time has an influence on the stability of the coating and thickness, the phosphating temperature has an effect on the speed of grain growth. The structure and performance of film obtained is good when magnesium alloys are phosphated for 30 minnutes at the temperature of 30℃, which can be used as the pre-coating of magnesium alloy. |
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