Preparation And Properties Of Ni-P-TiO₂ Electroless Composite Coating On Magnesium Alloy

Magnesium alloys are more and more widely used in aviation,automobile and electronics industries due to their excellent properties,such as low density,high specific strength and specific stiffiness.However,the standard electrode potential of magnesium is only-2.37V,which is vulnerable to corrosion.In order to improve the corrosion resistance of magnesium alloy,relevant research has been carried out both at home and abroad.Electroless plating has attracted much attention due to its advantages such as simple process,low cost,uniform thickness and good corrosion resistance.In this paper,the bath formula of electroless plating Ni-P coating was optimized by orthogonal test,and the composite coating of nano-TiO2 particles with high dispersion was prepared on AZ31 magnesium alloy substrate by electroless plating and sol method.The performance of the coating was further optimized by heat treatment and chromium-free passivation.The Ni-W-P-TiO2 composite coating was prepared by adding sodium tungstate,and the effect of adding different content of sodium tungstate on the composite coating was studied.Double layer Ni-P/Ni-P-TiO2 composite coating was prepared on magnesium substrate,Ni-P and Ni-P-TiO2 composite coating as bottom and top layer,respectively.The morphology,structure and properties of the double layer coating were studied and compared with that of the single layer Ni-P,Ni-P-TiO2 coating.On the basis of the analysis and discussion,the following conclusions are drawn:(1)With the increase of TiO2 sol content,the hardness and corrosion resistance of Ni-P-TiO2 composite coating increased at first and then decreased.In this paper,when the content of TiO2 sol is 8ml/L,the hardness and corrosion resistance of Ni-P-TiO2 composite coating is the best.The structure of the composite coating is dense without obvious defects and cracks.TiO2 particles are dispersed in the coating and Ni grains in the composite coating are refined at the same time.(2)After heat treatment,the hardness of Ni-P-TiO2 composite coating increases with the increase of heat treatment temperature,and the corrosion resistance increases first and then decreases.The comprehensive analysis shows that the hardness and corrosion resistance of the composite coating are the best when the heat treatment temperature is 250 ?.After passivation with silicate,the hardness and corrosion resistance of the coating can be improved by adding appropriate sodium silicate.In this paper,when the content of sodium silicate is 20g/L,the corrosion resistance and hardness of the coating are good.(3)The Ni-W-P-TiO2 composite coating was prepared by adding sodium tungstate into Ni-P-TiO2 bath.The results show that with the increase of sodium tungstate content,the size of cellular particles in Ni-W-P-TiO2 composite coatings shows a tendency of decreasing first and then increasing,and the hardness and corrosion resistance of the coatings increase first and then decrease.(4)Compared with the Ni-P,Ni-P-TiO2 coating,the hardness of the double-layer Ni-P/Ni-P-TiO2 composite coating is increased to 363HV.The porosity decreases obviously,the current density of self-corrosion decreases to the order of 10-7,the pitting potential,the self-corrosion potential also shift positively,and the corrosion resistance increases obviously,which indicates that the introduction of double-layer interface is beneficial to improve the hardness and corrosion resistance of the coating.