Az91d Magnesium Alloy Plating Copper / Nickel Process

Magnesium alloys are the lightest metal structural material,named as the "environmental engineering material in the 21st century".However, the application of magnesium alloys has been limited due to its poor corrosion resistance.A metallic coating to magnesium can enhance corrosion resistance,wear resistance,solder ability,conductivity and improve artistic appearance of magnesium alloys.In other ways, electroplating on magnesium alloys is a good way because of its cost saving and the greatest efficiency.Surface electroplating technology of AZ91D magnesium alloy was studied in this thesis.By using gain in weight,SEM,EDS and electrochemical polarization curves research techniques,the effect of pretreatment,operating conditions of bath on Cu/Ni coatings quality has been investigated.The best technological conditions were confirmed for the formation of Cu/Ni electroplating on AZ91D magnesium alloy.The effect of the additives on the electro-crystal behavior of electroplating copper and the zincate process have also been investigated.The research results of the experiments show that:1.The alkaline cleaning was conducted in a solution containing OP emulsion 1.0 g·L^-1 with ultrasonic time for 5 min at the operating temperature of 65℃.The optimal formulations of acid pickling were as follows:H₂NCSNH₂:0.5 g·L^-1;HNO₃(65%):30 ml·L^-1;pH:2.0;room temperature;acid pickling time:20 s.Activation was conducted in a solution containing NH₄HF₂ 10 g·L^-1 under room temperature and for 80～120 s.The optimal formulations of zinc immersion were: ZnSO₄·7H₂O:35 g·L^-1;K₄P₂O₇·3H₂O:140 g·L^-1;temperature:65℃; immersion time:150 s;and some other compositions.SEM,EDS and electrochemical results show that the quality of complex alloy diping layer is much better than traditional zinc layer.2.The results of the zincate process of magnesium alloy show that, when the multi-metal ions is added to zincate solution,the deposit rate of zinc is reduced,the mixed potential obtained from the superimposition of partial anodic and cathodic polarization curves(E_c) is increased as well as the stable potential obtained from E-t curves in zincate process(E_Mg),a fine zinc layer will be attained.3.The optimal conditions of Cu/Ni electroplating on AZ91D magnesium alloy were studied by using the single factor test and the orthogonal design test.The optimal operation conditions of electroplating copper were:pH of 8.5,current density of 1.5 A·dm^-2,temperature of 40℃,operating time of 30 min.The nickel electroplating was carried out under the current density of 5～7 A·dm^-2.SEM shows that copper plating coating is homogeneous,dense,smooth,and has good adhesion to the substrate;EDS analysis shows that nickel electroplating can enhance the coating's adhesion effectively,and reduce the porosity of the plating coating.Polarization curves show that the corrosion potential ascend 1.4 V and the corrosion current descend 5 orders after Cu/Ni electroplating on AZ91D magnesium alloy,so the film can provide effective protection to the substrate.4.Pyrophosphate copper electroplating with assistant complexion developed by us is selected for preplating on magnesium alloy.It is found that the mechanism of copper electrodeposition is not changed by the addition of the additives to the fundamental solution.It still keeps to instantaneous nucleation of Scharifker-Hill model under the diffusion control.