Research Of Electroless Nickel Plating On AZ91D Magnesium Alloy

Magnesium alloys have many excellent properties which make them have broad application prospects.But their low corrosion resistance limit further development and applications.It is important to develop a simple,green and low cost corrosion protection technology in current magnesium alloy application engineering.In this thesis the process of electroless Ni-P plating on AZ91D magnesium alloy have been studied.The coating morphology was observed by optical microscope and scanning electron microscope(SEM).The chemical composition of the coating was determined by Energy Dispersed X-ray Spectrometer(EDS).The corrosion resistance of the coating was determined by weight loss method in which samples were diped in 3.5%NaCl solution.The adhesion strength between the coating and the substrate was measured by cross cut test.The results showed that both the concentrations of NiSO4,NaH2PO2,NH4HF2 and the plating solution pH and temperature can influence the coating deposition rate,coating structure and coating properties.Among them,the NH4HF2 concentration has a significant impact on the coating deposition process.If there is no NH4HF2 in plating solution,a complete coating can't be formed on magnesium alloy.When the NH4HF2 content was between 5 and 20 g/L,the plating speed decreased while the coating performance and the pH stability of the plating solution became better with the increase of NH4HF2 concentration.The best plating conditions are as follow:the molar ratio of NiSO4 and NaH2PO2 was 0.34,NH4HF2 concentrations was 20 g/L,solution pH was about 6.5,and plating temperature is 80?.With the optimized plating solution,the pickling and activation pretreatment process of the electroless nickel plating on the AD91D magnesium alloys was also studied.A chromium-free pretreatment process which is relatively environmentally friendly was developed.In this process sodium molybdate is used to replace chromic acid in pickling process and NH4HF2 is used to replace the volatile hydrofluoric acid in activation process,respectively.