| Magnesium alloy has low density, high strength-to-weight ratio and othergood physical and mechanical characteristics. However, the application ofmagnesium alloys has been limited due to their undesirable properties includingpoor corrosion and low wear resistance. Thus, the formation of anticorrosion andhigh wear-resistance coatings on the surface of Mg or Mg alloys is necessary inpractical applications. Since magnesium is one of the most electrochemicallyactive metal, ordinary coatings, such as nickel, copper and zinc coatings, can onlyprovide a physical barrier to corrosion attack of magnesium substrate. So, anycoatings on magnesium alloys should be as uniform, adherent and porosity free aspossible.The preparing technologies, characteristics and anticorrosion mechanismshave been investigated in the paper. In the paper zinc phosphating process wereused to replace chromate treatment for the pretreatment of magnesium alloy.The relations among the characteristics of organic coating and themicrostructure, composition, phase structure and wettability of phosphate filmwere studied. It is shown that molybdate complex zinc phosphate film exhibits theoptimal anticorrosion characteristics and can be used as the base for electro-coatand powder coating of magnesium alloy.The complex zinc phosphate film is also suitable to be the base film of theelectroless nickel deposit because there are nanocrystalline zinc particlesdispersed the phosphate film, which can provide the active particles for nickelnucleus. The anticorrosion characteristics of multilayer electroless deposit (Ni-P/Ni-W-P) are much better than single electroless nickel deposit.In the present work, the technology of organic coating and the electrolessNi-P deposition on AZ91D magnesium alloy was proposed. Several contributionsshoald be mentioned in the follows1. Characteristics of organic coating of magnesium alloy are related with phasestructure, anticorrosion characteristics and wettability of phosphate films.The denser of the phosphate film and the more content of (311) phases ofhopeite, the better of the anticorrosion and acid resistance and alkaliresistance and wettability of phosphate films;and the better of the adhesionand resist corrosion of the organic coating on magnesium alloy.2. The anticorrosion performance of the organic coatings based thenon-chromate phosphate coating was proved better by the salt spray test andadhesion and appearance performance. A new environment-friendly surfacetreatment method for magnesium alloy parts was provided in the study. Thetechnology of " molybdenum modified zinc phosphate coating+electro-coat+paint" and "molybdenum modified zinc phosphate coating+powder paintof magnesium alloy were developed. They can satisfy many kinds ofinternational and national quality standards.3. Direct electroless Ni plating of magnesium alloy was obtained from nickelsulfate plating for the first time. The magnesium alloy samples were etchedfirst in a solution of chromate and nitric acid and than soaked in HF solutionto form a conversion film before electroless nickel deposition. But chromiumcompounds are carcinogenic substance, which should abstain to use.4. The non-chromium zinc phosphate coating was used as pretreatment coatingof electroless Ni–P deposits obtained from the acid bath containing nickelsulphate for the first time. The microstructure of the phosphate coatings wasobserved by using SEM and the phase compositions were analyzed by theusing XRD. Chemical compositions in the phosphating coating wereZn3(PO4)2·4H2O and Zn. Metal zinc in the phosphating coating becomes thecatalytic particles for the nickel deposition. The addition of sodiummetanitrobenzene sulphonate greatly increased the micro cathode sites of thephosphate coating and made the coatings fine greatly. It was proved that theelectroless Ni–P deposits plus phosphating coating obtained from the bathcontaining sodium metanitrobenzene sulphonate exhibited good anticorrosionperformance.5. In this study, AZ91D magnesium alloy samples were pretreated in a zincphosphating bath to obtain a phosphate coating. Then, the electroless Ni-Pdeposition on the phosphate coating was undertaken by using the plating bathcontaining sulfate nickel. The phosphate coating on the AZ91D magnesiumalloy was obtained from the bath containing mainly phosphoric acid and zincoxide. There was zinc in the phosphate coatings and the addition of Na2MoO4in the phosphating bath resulted in the increase of zinc in the coating. Theexistence of metallic zinc in the phosphate coating makes it possible to be asthe pretreatment layer for further electroless Ni-P deposition. The Ni-Pcoatings with dense and fine microstructure were obtained on the phosphatecoatings on AZ91D alloy obtained from the phosphating bath with additionof 2.0~2.5g/L Na2MoO4.The Ni-P plus phosphate coatings on the AZ91Dmagnesium alloy exhibited good performance. Salt spray test showed thatthey could withstand about 150 hours without corrosion. The surfacehardness of the Ni-P coating is about 670Hv, and its hardness was increasedto about 915Hv through the heat treatment of 2 hours at temperature of180°C.6. An available method to obtain electrless Ni-P/Ni-W-P coatings on AZ91Dmagnesium alloy was discussed. The microstructure of the coating wasanalyzed by scanning electron microscope (SEM) and X-ray diffractmeter(XRD). The corrosion resistance of the coatings was estimated byelectrochemical polarization measurements and 10%HCl solutionimmergence test. The electrless Ni-P/Ni-W-P coatings can stand in 10%HClsolution for 170 minutes without corrosion. The hardness of the coating is622HKV, and the coating has good adhesions. With the proof of theexperiment, this technique can offer reliable protection for AZ91Dmagnesium alloy.
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