Electroless Plating Ni-P On The Surface Of Magnesium Alloy

Magnesium alloys are widely used in aerospace,automobile manufacturing and electronics industry because its small density,high specific strength,high stiffness,electromagnetic shielding performance,good damping,shock absorption performance and other advantages.However,the chemical properties of magnesium is very active.It is very prone to severe corrosion.In addition,magnesium has a low surface hardness,scratching the surface easily.These properties restricted the application of magnesium alloys.Therefore,magnesium alloys material must be treated by the surface treatment.In this work,The phosphating treatment was prepared between the Ni-P coating and on AZ63 magnesium alloy.The best formula of zinc and manganese phosphating is obtained by orthogonal test.The electroless plating on the best zinc and manganese phosphating film is carried out.The effect of pH and temperature on the performance of the coating was studied by single factor method.Scanning electron microscope(SEM),Energy Disperse Spectroscopy(EDS),X-ray Diffraction(XRD),Hardness.Electrochemical testing and other test methods have been used to characterize and analyze the morphology,the composition.The result indicated that:(1)The phosphating on the surface of magnesium alloy can be modified.It can be obtained by orthogonal experiment based on the thickness of the film and the corrosion resistance time,and the optimum Zn series and Mn series chemical formula and process parameters are as follows:Na2HPO46.6×10-2 mol/L,4.2×10-2 mol/L Zn(NO3)2,4.3×10-2 mol/L NaNO2,3.8×10-2 mol/L NaF,t=25 min,T=50?,pH=4.5,t=1h;The technological parameters of the optimum chemical formula for Mn series are 0.520 mol/L Na3PO4,0.190 mol/L KMn04,0.022 mol/L NaWO4,pH=6,T=60?,t-20min.(2)The chemical Ni-P plating can be made of the phosphating layer surface containing Zn or Mn.Based on the corrosion resistance and deposition rate of the coating by the orthogonal test and single factor method can be obtained the optimum proportion and process parameters of the chemical plating solution that is NiSO4·6H2O 7.6×10-2 mol/L,0.17 mol/L NaH2PO2·H2O,3.4×10-2 mol/L Na3C6H5O7·2H2O,0.18 mol/L NH4HF2,1.32×10-2 mol/L thiourea,pH=8,T=85 ?,t=1 h.(3)The amorphous structure of coating of magnesium alloy in different phosphating treatment can get into the cellular morphology,The deposition rate of coating was above 25?m/h and the hardness of coating on magnesium alloy has relatively large degree of surface hardness of Zn phosphating layer on the surface of electroless Ni-P coating of 747 HV,hardness Mn phosphating layer the surface chemical coating is up to 712 HV.(4)The electroless plating of Ni-P in the best Zn and Mn phosphating film,Zn and Mn provide electroless Ni-P plating with a self-catalytic activity.Mn phosphating film is more suitable for electroless plating on magnesium alloy,which can got a coating in 3.5%NaCl solution the corrosion current density only 4.09×10-6 A/cm2 and the corrosion potential of-0.62V.and the corrosion current density(3.04×10-4 A/cm2)of sample was lower two order of magnitude than the uncoated,which play a good role in the protection to magnesium alloy substrate.