Electroless Nickel Plating On Mg-Li Alloy By Two-step Process

Mg-Li alloy has low density, high specific strength and specific modulus and so on. So it can be applied widely in many fields, for example, the aerospace, aviation industry and the 3C(computer, communication, consumer electronics products) products. However, its Poor corrosion and high chemical reactivity have hindered wide use of magnesium alloys in many applications. Studies on Mg-Li alloy were mainly foused on the synthesis technology of the Mg-Li alloys.The researeh on corrosion resistance method on Mg-Li alloy is seldom reported.Therefore, surface treatment is necessary to improve the corrosion resistance of Mg-Li alloy.In view of limitation of other surface modified technologies, this article introduces electroless plating technology for enhancing corrosion resistance of the Mg-Li alloy nickel by two-step process. Firstly, the samples pre-treated pre-plating in NiCO3·2Ni(OH)2·4H2O, and then the samples having been plated a thin film of Ni-P alloy plate in the solution which NiSO4·6H2O is main salt. The purpose of doing that is to avoid both the SO42- magnesium lithium of corrosion and reduce the cost of the basic carbonate-based nickel salt. Based on the above ideas, two-step technical processes of electroless nickel plating are carried out on the sufurce of Mg-Li alloy.This paper analyzes the surface morphology and structure of the coatings, were evaluated by metallographic observation, scanning electron microscopy (SEM), energy dispersive X-ray Detector(EDX), Corrosion resistance, binding force and hardness of the coatings were tested by micro-hardness polarization, curve measurements, traditional tests of bending test, file lest, scratching test. The results showed that we get excellent nickel coating plating on Mg-Li alloy by two-step process. The research shows the results as follows:(1)By means of experiment, two step method preservative treatment technology on Mg-Li alloy was obtained, including pretreatment, pre-plating and electroless plating three processes. The Ni-P coating was dense and uniform by the technology.(2)The Ni-P coating and substrate combined closely, and the coating thickness was about 20μm. The EDX test showed that the P contents in the coating were 13.564%, belonging to high phosphorus deposit.(3)Coating performance test results showed that:coating and the substrate were well bonded, there were no stripping, laps and so on; the Ni-P coating registered a free corrosion potential of about-0.249 V, (vs SCE), shifting positively more than 1400 mv. Corrosion resistance of Mg-Li alloy surface coating significantly improved. The average Vickers hardness reached 549 HV, increased more than six times comparing with the substrate hardness.