Research On Electroless Plating And Corrosion Resistance Of Ni-Co-P Coating On AZ31 Magnesium Alloy

In order to improve the poor corrosion resistance of magnesium alloys, Ni-Co-P alloy coating is tried to deposit on AZ31 magnesium alloys by electroless plating in present paper.The effects of the concentration of NaH2PO2 , NiSO4, CoSO4and temperature in affecting corrosion resistance is studied in orthogonal experiments. The microstructure, surface morphology and composition of the formed coating are analyzed by X-ray diffraction(XRD), scanning electron microscopy(SEM) and energy dispersive spectrometer(EDS), respectively, and the corrosion resistance of the coating is measured by drip drops and Tafel curves. The coating's porosity is defined by attaching filter papers. The main conclusions are:(1)Orthogonal experiments indicates: the factors that is affecting the resistance time by level sequence are: NiSO4,NaH2PO2.H2O,CoSO4 and Temperature. It is benefical for resistance time by increasing NiSO4,NaH2PO2.H2O,CoSO4 and reducing temperature. Optimized parameters in this orthogonal experiment is NaH2PO2.H2O 20g/L,NiSO4 20g/L, CoSO4 12g/L,sodium acetate 13g/L,40% HF12g/L,NH4HF2 8g/L,thiourea 1mg/L,pH=6, 77℃,reacting time 2 hours. For this condition the formed coating is 10μm thick, resistance time in drip drop test is 465 second, the porosity is almost disappear.(2)The formed Ni-Co-P coating is amorphous cell shape; 9.41% of Phosphorus content is far more than the coating formed in alkaline solutions, meanwhile Co content is lower(3) The corrosion resistance of the sample is obviously improved ,as the drip drop testing time of the sample is extended from 1 minute to 10～30 minutes after plating, the corrosion current decreased by three orders of magnitude after plating, 7.943×10-2mA/cm2change to 6.310×10-5mA/cm2, so the corrosion rate reduced ;the corrosion potential of the sample has transformation from -1.48V to -0.47V, tendency of corrosion is decreased.(4)The average growth rate of Ni-P and Ni-Co-P coating are 26.67μm/hr(0.44μm/min)和20μm/hr(0.32μm/min) in this work.The growth rate of Ni-Co-P coating is decreased by increasing CoSO4 in solutions; porosity is reduced along with growing of coating; the growth rate of Ni-P coating is beyond the Ni-Co-P coating while its density is lower.