Study On Preparation And Process Of Electroless Ni-P/Ni-Sn-P Duplex Coatings On AZ91D Magnesium Alloy

With low density,high strength,high stiffness,high elasticity modulus,good impact resistance,easy to welding,good dimensional stability,good magnetic interference resistance and easy to recycle,magnesium alloy can replace part of metal materials and plastic products and be widely used in transportation,aerospace,3C products,military equipment and other industries,knowm as "the 21st century green engineering material".With limited corrosion resistance,high chemical and high electrochemical activity,developing tendency and application of magnesium alloy were greatly restricted. Corrosion resistance of magnesium alloy surface can be improved by electroless coating.An orthogonal experimental design was used to optimize the preparation of Ni-P coating.The influence of HF on the first electroless coating Ni-P on the AZ91D magnesium was explored.In order to improve the corrosion resistance further,the second electroless coating Ni-Sn-P on the first coating was studied.The effect of PH,temperature,concentration of Na2SnO3 on electroless were researched. The results showed as follows:The optimized parameters obtained by orthogonal experimental design were gained as follows:NaH2PO2·H2O of 25 g/L,C6H8O7·H2O of 15 g/L,PH value of 7.5 and the temperature of 80℃.The coatings obtained at this condition were homogeneous,compact and smooth,and the corrosion resistance was improved greatly compared with AZ91D magnesium alloy base.The concentration of HF greatly affected the coating rate,base stability,morphology and corrosion resistance.When it was 12 g/L,the surface and the corrosion of the coating was good.The second electroless Ni-Sn-P coating was performed afert the first electroless Ni-P coating on the AZ91D magnesium alloy.The Ni-Sn-P coating had smaller cellular particles, uniform,compact and no major defects with amorphous structure.The corrosion potential of Ni-Sn-P coating was inferior to Ni-P coaing,which formed small potential difference and distributed corrosion current,so as to make Ni-P coating.hard to corrode.The coatings rate,buffering of the solutions,surface microstructure,component and corrosion resistance of second Ni-Sn-P coatings were effected by value of PH and the temperature.With increasing of PH value and temperature,the coating rate increased,the size decreased,and the coating were smooth.When the PH value was 9 and temperature was 90℃,the coatings had uniform,compact and smooth surface with wholly amorphous structure,so to make Ni-P coating hard to corrode and protect AZ91D magnesium alloy well.The second electroless Ni-Sn-P coating with different concentration of Na2SnO3 had so little difference between structure and performance.The coating was compact, uniform, smooth, and no major defects with low porosity. The corrosion resiatance were inferior to Ni-P coaing, which formed small potential difference and distributed corrosion current,so as to make Ni-P coating hard to corrode.But in view of environmental protection,the best choice of Na2SnO3 concentration is 8g/L.During the heat treatment process,the corrosion potential of second electroless Ni-Sn-P coating were enhanced.The corrosion potential rised gradually with certain passivation zone,and were inferior to Ni-P coaing.The Ni-Sn-P coating was amorphous,which processed a trend in transforming to crystalline state during heat treatment.There were small ball material precipitated,and had a tendency to grow up with the rise of temperature.