Preparation And Corrosion Resistance Of Electroless Plating Ni-Based Amorphous Coating On Magnesium Alloys

Magnesium alloy has small density,high specific strength,shock absorption and excellent anti-electromagnetic,impact resistance,heat transfer conductivity and machinability,however,poor corrosion resistance greatly limits its wide range of applications in many industrial fields.In order to improve the corrosion resistance of magnesium alloy,this study first Electroplating copper on the surface of magnesium alloy,and then electroless Ni-Cu-P,Ni-W-P,Ni-Mo-P three kinds of nickel-based amorphous coating,The effects of the addition of CuSO4,Na2W04 and Na2Mo04 on the phase composition,corrosion resistance and adhesion of the three coatings were studied.The main contents and results of this paper are as follows:(1)First,electroplating copper on the surface of magnesium alloy as the middle layer.The surface of the pre-plated copper has a fine cell morphology,the thickness is about 2?3?m,and there is no obvious pore and crack between the magnesium alloy matrix and the pre-plated copper,which not only reduces the difficulty of the direct electroless plating on the surface of magnesium alloy,but also improves adhesion between electroless plating layer and magnesium alloy substrate,thereby improving the corrosion resistance of the outer layer coating.(2)The surface of Ni-Cu-P,Ni-W-P,Ni-Mo-P coatings showed different size of the cell structure.The surface of coatings is relatively flat and dense tissue.The three kinds of coatings are well bonded with magnesium alloy.(3)For Ni-Cu-P coating,when the addition of CuSO4 in plating bath is 0.8 g/L,the mass fraction of phosphorus in the coating is 9.06%,and the mass fraction of copper is 4.50%.Ni-Cu-P coating shows amorphous structure.Polarization curve shows that the corrosion potential of Ni-Cu-P amorphous coating is-0.31 V and corrosion current is 0.0039396 A/cm2;Impedance spectrum shows that the coating has the maximum allowable arc radius and |Z|value.In the immersion experiment;corrosion rate of Ni-Cu-P coating with 0.8 g/L CuS04 addition in plating bath is the slowest and organization is the most complete.(3)For Ni-W-P coating,when the addition of Na2WO4 in plating bath is 15 g/L and pH value is 6.5,the coating shows typical amorphous structure.The mass fraction of phosphorus in the coating is 9.63%and the mass fraction of tungsten is 1.14%;Polarization curve shows that corrosion potential(-0.326 V)of Ni-W-P amorphous coating is greatly improved and corrosion current(0.0027105 A/cm)is significantly reduced.Impedance spectrum shows that the coating has the maximum allowable arc radius and |Z| value;The immersion experiment results show that the corrosion rate of Ni-W-P amorphous coating is the lowest and the corrosion resistance is the best when the addition of Na2WO4 is 15 g/L in plating bath.(4)For Ni-Mo-P coating,when the addition of Na2MoO4 in plating bath is 0.4 g/L and pH value is 9.5,the mass fraction of molybdenum in the coating is 9.76%and the mass fraction of phosphorus is 9.29%.The coating behaves as amorphous structure.Polarization curve shows that the corrosion potential of Ni-Mo-P amorphous coating is-0.26 V and corrosion current is 0.0087246 A/cm;Impedance spectrum shows that the coating has the maximum allowable arc radius and |Z| value;The results of immersion experiments show that the corrosion resistance of Ni-Mo-P amorphous coating is the best when the addition of Na2MoO4 in plating bath is 0.4 g/L.