Technology And Corrosion Resistance Research Of Electroless Ni-P Gradient Plating On Magnesium Alloy

The poor corrosion resistance hinders the wide application of magnesium alloys,composite technology widen performance of alloys in various field. Conversion coatingelectroless Ni-P platings with excellent corrosion resistance are deposited on magnesium alloy.The second part is that chemical conversion coating- epoxy resin/ SiO₂ hybrid coating are prepared on substrate. The performance of two parts of composite coatings was analyzed.Chemical conversion process based on tin salts as the main component. Then Ni-P gradient coating is prepared on the basis of stannate conversion coating. The microstructure and corrosion mechanism of stannate conversion coating-electroless Ni-P platings were investigated. The surface morphology of stannate conversion coating- epoxy resin/SiO₂ hybrid coating were characterized.?1? It has a great influence on the formation of stannate conversion coating, such as pH fluctuations in the conversion solution and deposition time and processing temperature. It was found that good surface morphology and high corrosion resistance was prepared treated during 55 min and 65 ? in stannate solution of 45 g/L at pH values 12.5. Then, it can be observed that the stannate conversion coating was composed of nearly spherical particles, the main elements of Mg, Sn, O, and it's good for subsequent electroless Ni- P gradient coating combined with stannate conversion coating.?2? On the basis of Ni-P amorphous alloy coating, it is observed that contents variation of NaH₂PO₂·H₂O in the solution of electroless plating results in the formation of Ni-P gradient coating. The experiment reveals that the Ni-P gradient coatings exhibit a better corrosion resistance than the homogeneous Ni-P coatings.?3? The reaction speed of TEOS was quick in acidic catalyst, and was low in alkaline catalyst. On the contrary, it was slowly carried out when basic catalyst was adopted. FT- IR test of acidic and alkaline catalysts prepared SiO₂ sols under analysis further confirmed that the acid catalyst was prepared SiO₂ sol of homogeneous system, and alkaline catalyst wasprepared SiO₂ sol of granular heterogeneous system. When the quality ratio of epoxy resin and SiO₂ sol is 2: 3, the binding force and corrosion resistance is best between epoxy resin and inorganic phase of SiO₂ which greatly improves the corrosion resistance of magnesium alloys.To sum up, this article is successfully prepared the composite coating combined with electroless Ni-P technology and epoxy resin- SiO₂ hybrid coating technology on stannate conversion coating. The composite coatings have good surface morphology and good corrosion resistance, which widen the application field of magnesium alloy.