Study On Mg-12Li-2Al-2Zn Alloy Of Surface Protection Coatings

The magnesium-lithium alloy is the lightest structure material, has been widely applied on the 3C and many other fields because of its excellent ductility, it is useful for its promoted application with carring out its superficial anti-corrosion research.First studied the effect of three kinds of pretreatment process on the surface parkerising coating and the chemical plating nickel layer quality of the Mg-12Li-2Al-2Zn alloy,then the optimal pretreatment process is determinded by using OM, SEM/EDS, XRD, micro-hardness tester and potentiostat.Take the optimal pretreatment process as the foundation, studied the effect of various process parameters on phosphate conversion coating, permanganate-phosphate conversion coatings and nickel plating by thickness, the appearance and the anti-corrosive performance.The optimal process of each testing plan was determined by the orthogonal experiments.The research shows that the optimal pretreatment process of Mg-12Li-2Al-2Zn alloy is formula 2; The preparation of phosphate coating surface and the coating quality affected by the process parameters, the phosphate coating greatly affected by the process temperature, but the permanganate-phosphate coating affected by the process time to be more prominent.The optimal process parameter of phosphate conversion (pH=1.5) is T=80℃, t=20min and the optimal process of permanganate-phosphate conversion(pH=3) is T=70℃,t=30min under this experimental conditions;The coating of the phosphate and the permanganate-phosphate conversion coating are characterized by microscopic morphology of the crack network, the former assumes the iron grey and the latter assumes the reddish brown.The phosphate conversion coating is mainly composed by Mg3(PO4)2 and BaHPO4 and the permanganate-phosphate conversion coating is mainly composed by BaHPO4 and MnO2. The corrosion resistance of the permanganate-phosphate conversion coating are better than the phosphate conversion coating by surface coating appearance, compactness, thickness and polarization curve.The effects of PH value, temperature, time on the electroless nickel-plating on Mg-12Li-2Al-2Zn alloy were investigated. The results show that the optimal process parameter of electroless nickel-plating is pH=7±0.2,T=80±2℃and t=50min.In the technical parameter,the hardness of the coating was about 435Hv and the thickness was about 13-15μm;The coating is Ni-P amorphous state, composed by many compact alveolar;Under the optimal process condition, the magnesium-lithium alloy surfacing can obtain compact chemistry conversion coating or the nickel plating, thus improves corrosion resistance through phosphate, permanganate-phosphate or chemistry nickel plating. The corrosion resistance of the magnesium-lithium alloy surface nickel plating surpasses the permanganate-phosphate conversion coating or the phosphate conversion coating.through analysis comparison.