Research On The Displacement Film Located On The Magnesium Alloy AZ31 For Plating

The magnesium alloy is applying increasingly and extensively because of the small specific gravity, the good mach inability and etc. But the corrosion resistance of the magnesium alloy is very bad. The chemistry nickel-plate has many advantage as high rigidity, good equality and corrosion resistance and some other properties. There will be a film oxidized quickly on the surface of magnesium alloy, because of the high chemistry activity of magnesium alloy. A special processing is needed to be carried on to get the good nickel-plate before the chemistry plateing. Prepare the processing methods to the magnesium metal alloy currently all very trivial, because of violent galvanic corrosion between the magnesium alloy and nickel-plate the adhesion of the coating is not ideal.The experiment investigates to carry on the displacement of the Ni, Cu, Fe, Co and Ni-Co, Ni-Cu, Ni-Fe, Ni-Zn on the magnesium alloy AZ31 surfaces first. And optimize the craftworks by orthogonal experiment. Compare their adhesive force between the coating layer with the different displacement films and magnesium alloy by thermal shock and file experiment. The best displacement film craftwork is:CuSO4·5H2O:5g/L; sodium dodecyl sulfonate:100mg/L; pH=5.3; t=1min.Then study the formation process of the copper displacement film, Carry on tests on surface morphology, hardness, corrosion resistance and self-corrosion electric potential. And get some conclusions:(1) Study the growth process of the copper displacement film by SEM. Starting is the small grain of random growth, then takes these small grains as center to grow on all sides, the end the displacement film covers the magnesium alloy surface completely. But the copper displacement film is asymmetry.(2) The self-corrosion electric potential of the chemistry nickel-plate with copper displacement film enhance 0.9V to that without copper displacement film.(3) The sample without displacement film occurrence corrosion in only 5h and the sample with the copper displacement film occurrence corrosion in 10h in the salt-fog experiment box under the sistuation that the thicknesses of the all chemistry nickel-plate to attain 30μm. Explain the copper displacement film have a very big function to enhance the corrosion resistance.(4) The surface morphology of nickel-plate presents typical model and nonexistent obvious surface blemish. It is neat and fine.And in organizing structure aspect it will have good corrosion resistance.(5) The hardness of nickel-plate with copper displacement film is 530 HV and that without copper displacement film is 520HV. The copper displacement film did not influence the hardness of nickel-plate obviously.