Electroless Plating Ni-P(W) And The Coatings With A Chromium-Free Pretreatment On AZ91D Magnesium Alloy

Magnesium alloys have been recognized as green engineering materials in 21st century, and thus have various industrial applications so far in the fields of automobiles,3C products, bicycles, and etc. However, magnesium has high chemical reactivity and its alloys usually have poor corrosion resistance, which is actually the most serious obstacle that impedes the application of magnesium alloys in practical environments. Electroless plating on magnesium is an available way to improve their corrosion resistance but with a limited effect up to date. The traditional pretreatment, involving in the chromium acid pickling and fluoridization, causes serious environmental pollutions and thus does not conform to green chemistry principles. So it is impotant to develop a novel technology of chromium-free pretreatment.The purpose of this study is to improve the corrosion resistance of the magnesium alloys, AZ91D magnesium as one model alloy in this work, through modification of the traditional electroless plating Ni-P coatings. Direct electroless Ni-W-P platings on the AZ91D magnesium alloys and one novel technology of chromium-free pretreatment were studied in this work. The surface morphologies were examined by the laser scanning confocal microscope (LSCM) and scanning electron microscope (SEM); the structure of the coatings were analyzed by the X-ray diffraction technique (XRD) with Cu Kαirradiation; the hardness of different coatings were checked by microscopic hardness tester and their corrosion resistance of the coatings were examined by the polarization curve test. Also the surface morphologies of the displaced nickel were observed by LSCM and SEM. The optimized time of Ni displacement was decided by the curve of time-potential. The main conclusions are as follow:(1) Electroless plating Ni-P and Ni-W-P coatings on AZ91D magnesium substrate were succeded with a traditional pretreatment. Compared with the Ni-P coatings of amorphous structure, the Ni-W-P coatings are of crystalline and with finer and denser micro structure as well as higher hardness. The corrosion resistance of the Ni-W-P coatings is a litter poorer than that of the Ni-P coatings due to its crystalline structure rather than amorphous one. (2) The microstructure of the Ni-W-P coatings became smaller when annealed at 250℃for 50 min. Furthermore, the corrosion potential of the Ni-W-P coatings after annealing markedly shifts to the positive by 150 mV, and shifts by 166mV as compared to that of the Ni-P coatings, implying the corrosion resistance of the annealed Ni-W-P coatings has been much improved.(3) One novel pretreatment processing, called the nickel-dipping process, was investigated instead of the traditional pretreatment, that is, the chromium acid pickling and fluoridization. The new method could make the processing easier and the environmental pollution less.(4) One blaster processing was used to modify the surface of the magnesium alloy before this novel pretreatment, and the results showed the adhesion between the substrate and coatings was improved. Finally an efficient, safe and reproducible processing of the Ni-P-W coatings was proposed.