| Magnesium alloys are considered as green engineering materials in the 21st century and have been gradually used in automobile, aerospace, electronic industry and communication equipments. Unfortunately, magnesium alloys have high chemical reactivity and poor corrosion resistance, which have limited their widespread applications. Many coatings on magnesium alloys have been developed to improve their corrosion resistance. Electrodeposition is considered as one of the most effective ways. Traditional electrodeposition processes have the limitations of poor adhesion, poor corrosion resistance and the usage of toxic cyanides. In order to overcome the limitations mentioned above, a new process of Cu/Ni/Cr assembled plating on nickel pre-plating coating was investigated to protect and decorate the products of magnesium alloys in this dissertation.The effects of the key zinc immersion step on the performances of nickel coatings were mainly investigated; the mechanism of initial nickel pre-plating on magnesium alloys was suggested. In order to establish the optimal parameters of nickel pre-plating process, the effects of concentration of main salt and F- in nickel bath, the cathodic current density and temperature of nickel plating on qualities of nickel coatings were studied. Manufactory tests of plating on magnesium alloy parts of motorcycles were carried out by the developed process. The research results are as follows:1. After 30-90 s zinc immersion treatment, magnesium alloy substrate is covered by a zinc film containing small amount of Mg(OH)2 and MgF2.The zinc film can improve the self corrosion potential, depress the self corrosion current of magnesium alloy substrate in nickel bath and boost the conductivity of the surface so that nickel pre-plating coatings with good ahesion and low porosity are obtained.2. When pre-plating nickel is conducted on zinc immersion film of magsneium alloy substrate, electrodeposition of nickel occur as accompanied with chemical reduction of nickel synchronously until the substrate is completely covered by nickel layer. In order to obtain nickel coating of good quality, it is very important to control the low speed of chemical reduction of nickel.3. The optimum composition and operating conditions of nickel pre-plating on magnesium alloy are:NiSO4·6H2O 100-120 g·dm-3,(NH4)3C6H5O7 5 g·dm-3,F-1.2-1.5mol·dm(-2),C7H4NNaO3S·2H2O 3 g·dm-3, pH = 4.8-5.4,current density 2.0 -4.0 A·dm-2, temperature 45-55℃, time 25-30 min.4. Cu/Ni/Cr assembled plating is carried out after the pre-plating nickel coating iscompleted on magnesium alloy parts of simple shape. The coatings obtained havehigh coverage and good corrosion resistance. For the workpieces of complex shape,elecroless nickel plating is adopted as the rendering layer, then Cu/Ni/Cr assembledplating is carried out and the good plating workpiece is obtained. However, theproblems of poor stability and a short use period of elecroless nickel bath need to be further researched.
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