Study On The Process And Corrosion Resistance Of Electroless Nickel Plating On AZ31 Magnesium Alloy

Magnesium is the lightest metal structural material in engineering application. It will be widely used in aerospace, automobile and electronics industry for its outstanding characteristics, such as high strength-to-weight ratio, excellent cast ability, electromagnetic shielding property, machinability, damping property, and recycle characteristics. However, poor corrosion resistance has often prevented it from further exploitation. Electroless nickel plating has being one of the effectively surface protection methods for magnesium alloys due to its good corrosion and wear resistance, deposit uniformity, and good surface quality. The pretreatment for electroless nickel plating and technology of electroless nickel plating of AZ31 magnesium alloy, which has good plastic deformation ability, has been studied; The corrosion resistance, initial deposited and corrosion failure of coating have also been studied systematically by using metallographic observation, SEM observation, EDS analysis, XRD analysis, salt spray test, electrokinetic potential polarization curve research techniques.By using orthogonal test method and the adhesion of coating as the evaluating indicator, one step pickling and activation techniques were ascertained as pretreatment techniques for AZ31 magnesium alloy, this is because it can predigest the pretreatment techniques, improve the bond strength and quality of electroless nickel coating, bring no deleterious matters such as Cr⁶⁺,HF etc to environment. The results show that there is MgF₂ film on magnesium surface after pretreated, which may protect the magnesium alloy from corrupting in the acidic electroless nickel plating solution. Furthermore, the surface state, thickness, bond strength with matrix of MgF₂ film also influence the quality and bond strength of electroless nickel coating.The solution and technology of electroless nickel plating on magnesium alloy were studied by using orthogonal test. Period use of electroless nickel plating solution was studied. The determination method of Ni²⁺ in solution was improved. The results show that the Ni-P coating obtained through the optimized techniques has relatively good quality, excellent bound strength and good corrosion resistance proved by SEM observation, EDS analysis, bond strength test, total immersion test.The experiments of 96h neutral salt spray show that the Ni-P coating of 20μm could properly protect the matrix and achieved ninth protection level. The main reason for protect invalidation of coating is the disfigurements existing in the surface and interior of coating, which can be decreased by improvement of electroless nickel plating solution and strict control over technological parameters.Electrokinetic potential polarization curve show that the corrosion potential ascend 1185mV and the corrosion current descend three orders after plating Ni-P coating on AZ31 magnesium alloy. The polarization curve exist evident passivated region, and the interval of potential has reached to 398mV, which shows that the passivative treatment could effectively improve corrosion resistance of Ni-P coating.