Electroless Plating Ni-P Process Of Magnesium Alloys, Microstructure And Properties Of Coating

Magnesium alloys have low density,high specific strength and stiffness,excellent sound damping capabilities and impact resistance,good electromagnetic interference resistance and shielding performance,easy performance advantages of casting,machining and recyclability as well.For this reason,magnesium alloy products have been more and more widely used in aerospace,automobile,electronic communications and other large-scale potential industry fields,known as the“twenty-first century green environmental protection and ecological metal structure material”.However,the utilization fields of magnesium alloys have been limited because of the chemical activeness,poor corrosion resistance,low surface hardness and other factors.Electroless plating nickel is one of important methods for surface protection on magnesium alloys because its coating is uniformity,smooth surface and has excellent corrosion resistance,wear resistance.The electroless plating Ni-P were prepared separately on the surface of as-extruded AZ31alloys and Mg-7.5Li-2Zn-1Y Mg-Li alloys.The microstructure of EX-AZ31,microstructure and phase composition of Ni-P coating are investigated with optical microscope,scanning electron microscope?SEM?equipped with an energy-dispersive X-ray spectroscopy?EDS?and X-ray diffraction?XRD?;in addition,the electrochemical properties,the microhardness scale and adhesion between coating and substrate by different temperatures heat treatments of Ni-P coating are investigated with electrochemical workstation,microhardness tester and WS-2005 Scratch Tester.The results show that amorphous Ni-P coating on the surface of EX-AZ31 magnesium alloy can be achieved by electroless deposition with nickel sulfate as main salt.The coating exhibits a smooth surface and the packed cellular structure rank tightly without obvious surface defects.The coatings and substrate have a good adhesion,and the critical load?F_c?is29 N.Moreover,the polarization curves show tha t the coating exhibits excellent corrosion resistance and a good protective effect on EX-AZ31 matrix.The state of coating transforms from amorphous into crystalline by heat treatment.The microhardness and adhesion of coating increase firstly,then tend to decrease with an increase in heat treatment temperature,the peak value reaches 916 HV_0.2 at 350?,while the highest adhesion?31.5 N?is obtained at 300?.The free-corrosion potential of coating decreases with a rise in heat treatment temperatures,and the corrosion resistance of coating reduces correspondingly.However,the coating is still compact after heat treatment,the passivating films do not breakdown,and the coating has good protection for EX-AZ31.For Mg-7.5Li-2Zn-1Y magnesium lithium alloy,the best Mg-Li alloy surface pickling process can be obtained as follows:HNO₃?68%?,110 ml/L;CrO₃,125 g/L;room temperature for 50 s.The morphological and structural analysis on the Mg-Li alloy Ni-P surface coating indicate that the coating is composed of uniform sized Ni-P cell composition,meanwhile,the coating has a good adhesion with the substrates,and the thickness of coating is about 12?m.EDX analysis shows the P content?mass fraction?is 13.72%,the coating is high phosphorus.The coating has a good adhesion with substrate,and the critical load?F_c?is 22 N.The E_corr of Mg-Li alloy Ni-P coating can increase to-1.306 V and its corrosion resistance improves significantly.The average hardness of coating is 747 HV_0.2.The surface morphology of Ni-P coating has higher phosphorus content and the Ni-P spherical particles are more fined,uniform and dense,and the combination between the coating and the matrix is more uniform with good corrosion resistance.