| Magnesium alloys have low density,high specific strength,high specific stiffness,good electrical conductivity and electromagnetic shielding properties,which show broad application prospects in the fields of aerospace,automobile and electronics industry.However,the poor corrosion resistance of magnesium alloy limits their application in practice.Electroplating method is one of the effective ways to improve the corrosion resistance of magnesium alloys.However,magnesium alloys exhibit unstable in the conventional electroplating solutions.Thus,looking for a proper surface pretreat process,and electronplating solution and process of magnesium alloys becomes a hot topic in the surface treatment of magnesium.Therefore,in this thesis,the AZ91 D magnesium alloy was taken as the research object,and the surface pretreatment of magnesium alloy and nickel plating process were investigated.The corrosion resistance of the magnesium alloy after plating was evaluated.The main results are as follows:(1)The surface pretreatment processes,including pickling,activating and zinc dipping,of AZ91 D magnesium alloy were studied.The results showed that,by the acid washing for 40 s in the solution(phosphoric acid 242mL/L+ concentrated nitric acid 12mL/L),and then activating for 4 min,and finally immersing in zinc melt for 15 min,a high quality zinc layer can be obtained on the surface of the magnesium alloy.It is found that acid cleaning and activation treatment can remove the residual oxides on the surface of the sample,and a slight pitting corrosion surface is favorable for the deposition of zinc coating.Moreover,the composition of the electroplated nickel layer on the surface obtained is even with a high interface bond strength,in comparison with that of the primary zinc dipping treatment.(2)The process of nickel plating on AZ91 D magnesium alloy was studied by comparing analysis of the differences between pulse plating nickel process and direct current plating process.It is found that the DC plating method exhibits a higher efficiency than the pulse electroplating,while,by using the pulse plating(pulse frequency 1000Hz+ current is 1.25A/dm2)with the duty cycle of 20% and 80%,and the plating yield a relative high plating efficiency.By using the DC plating(current 1.25A/dm2)and pulse plating(1000Hz+ frequency and duty ratio is 60% 80%+ and current 1.25A/dm2)methods,we can obtain the homogeneous nickel electroplating.It is found that the AZ91 D magnesium alloy with a DC nickel coat exhibits the superior corrosion resistance.The bond strength of the interface between the nickel coating and the substrate meets the standard requirements of the shock test according to the American Society of Materials Standard(ASTM B571-97).(3)The processes of electroplating Cu/Ni composite coating on AZ91 D magnesium alloy were investigated.By using the plating solution developed in the present work,the copper coating with compact interface and good covering was prepared on the AZ91 D magnesium alloy.Furthermore,By using pulse electroplating(pulse frequency 1000Hz+ current density 2A/dm2+ 80% duty cycle)and DC plating(current density 2A/dm2),we can obtain the homogeneous nickel electroplating.It is found the coating obtained by DC plating method exhibits a high corrosion resistance than that by pulse plating method.The nano hardness of nickel coating was much higher than that of magnesium alloy matrix.Moreover,the bonding strength of the interface between the copper nickel coating and the substrate the standard requirements of the shock test according to the American Society of Materials Standard(ASTM B571-97). |
PDF Full Text Request