| This paper developed some new methods to improve corrosion resistance of magnesium alloy and solve its poor corrosion resistance the problem in application. After analysing the mechanisms of pretreatment and deposited process of electroless Ni plating, a new electroless Ni plating process was developed. After the tranditional electroless plating method was broken, a new idea-clad electroless plating process was proposed by using organic coating and electroless plating, the surfaces of magnesium alloys were deposited respectively compact Ag, Cu and Ni films so that the corrosion resistacne of the substrates increased greatly.The process of traditional electroless Ni plating on magnesium alloy was investigated first. The Chromic acid etching could remove metal oxides and rusts. However it resulted in the surface corrosed severely, which needed careful control of the etching time. After aicd eching, the surface was losse and porous, which would pollute the plating bath and decrease the using period. When the sample was immerged in the solution, Nickel was firstly deposited on the bare substrate, and then the nickel coating rapidly covered the whole surface, which resulted in closing early the loose layer on the surface and there were lots of holes between the coating and substrate.In order to avoiding these disavantages, this paper provided a new electroless plating Ni process. The optimum bath of the new electroless Ni plating was proposed by the orthogonal experiment,14g/L C6H8O7·H2O, 10g/L NH4HF2, 10g/L KF, pH=6. And the effects of the bath components on deposition rate and corrosion resisitance were discussed. Compared with traditional process, new process uses no poisonous materials, such as hydrofluoric and chromic acid and the process of pretreatment was simplifies. Moreover, the current density of coated sample decreased one order of magnitude than that of AZ31 magnesium alloy.However, the potentiometry of plating coating is higher than that of magnesium alloy, thus the coating and magnesium form easily corrosive couple. To avoid this disadvantage, new process-clad electroless plating was developed by electroless plating after organic coating (organicsilicon heat-resisting varnish), in which organic coating acts as interlayer between the substrate and coating. Compared with epoxide resin, the organic-sillicon varnish coated magnesium alloy more perfectly. The result of the cross-cut test indicated that the adhesion between the substrate and the interlayer reached level 2. In order to increase the adhesion between the interlayer and coating, the surface of interlayer was roughed, and then was activated for subsequent plating.Firstly, Ag was coated on the AZ31 magnesium alloy by clad electroless plating. X-ray diffraction and SEM analysis showed that growth of the silver film was like "island"model. The potentiodynamic polarization showed that, the potential of the sample clad electroless plating increased by 300mV compared with AZ31 magnesium alloy.Secondly, Cu was coated on AZ31 magnesium alloy by clad electroless plating. After plating, a compact Cu film was deposited the surface of the substrate. The tests showed that the optimum bath contains 8～10g/L CuSO4·5H2O,30～35g/L NaH2PO2,10～20g/LNa3C6H5O7, lg/L NiSO4·6H2O,30～40g/L H3BO3, pH=9, T=70℃The polarization curves showed that the sample clad electroless plating increased by 200mV.Lastly, Ni was coated on AZ31 magnesium alloy by clad electroless plating. A Ni-P film with fine and dense structure was obtained on the substrate. The electrochemical measurements showed that the sample clad electroless plating exhibited lower corrosion current density and more positive corrosion potential than that of AZ31 magnesium alloy. Furthermore, the Ni-P coating on the AZ31 magnesium alloy exhibited high corrosion resistance in the accelerated corrosion test.Clad electroless plating process developed in this paper was environmentally friendly, no fluoride or hexacalent chromium compounds. In addition, it provided a new concept for plating the metals, such as Al alloys and Ti alloys which were considered difficult to plate due to high reactivity.
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