| As the lightest metal structural material,magnesium alloy has high specific strength and specific stiffness.However,the poor corrosion resistance of magnesium alloys limits its wide application.Electrodepositing Al plating on the surface is a viable magnesium alloy corrosion protection technology.Especially in some aerospace and aviation equipment as the core components,not only can effectively reduce the weight of the equipment,reduce the loss,and extend the life of the equipment,while electrodepositing the A1 coating ensures the excellent performance of the magnesium alloy itself.In this paper,ionic liquid(BMIC-AlCl3)prepared by using 1-butyl-4methylimidazolium chloride and AlCl3 was used as electrolyte to study zinc immersion in magnesium alloy and electrodeposition of Al and Al-Mn coating processes in ionic liquids.The direct electrodeposited aluminum coating on the surface of the magnesium alloy has a poor bonding force and usually needs to be immersed in zinc.The effects of time and temperature on the properties of magnesium alloy zinc coating and the adhesion of aluminum coating were investigated.The surface morphology and elemental composition of the zinc-impregnated layer were analyzed by scanning electron microscopy(SEM)and its accompanying X-ray energy spectrum(EDS).The influence of zinc immersion process on the adhesion of A1 coating was studied by pull-out test.The phase composition of the zinc-impregnated layer was characterized by X-ray diffractometry(XRD).The corrosion resistance of the zinc-impregnated layer was characterized by electrochemical polarization curves in 3.5%NaCl solution.It was found that Zn was unevenly deposited on the surface of the magnesium substrate during zinc immersion and preferentially deposited on the Mg-rich a phase.When the immersion time is 1 min,the coating layer cannot completely cover the surface of the substrate,resulting in a final adhesion of A1 5.7 MPa to the substrate.When the immersion time is 3?5 min,the Zn layer can completely cover the surface of the magnesium alloy,and the adhesion between the coating and the substrate can reach 9.5 MPa.It is found that the adhesion of the Al coating to the substrate after immersion in magnesium at 75? can reach 9.7 MPa;when immersion in 85?,the adhesion of the electrodeposited A1 coating to the substrate is only 4.7 MPa.XRD analysis showed that the main component of the surface of the magnesium alloy after zinc immersion was Zn.However,the zinc immersion treatment does not greatly improve the corrosion resistance of the magnesium alloy.The magnesium alloy was shown to be an electrodeposited Al coating in an ionic liquid after zinc immersion.Exploring the pulse frequency,pulse duty cycle,current density and electrodeposition time of the pulse current on the quality of the Al coating.The experimental results show that:(1)In the pulse current electrodeposition process,when the pulse frequency is 500 Hz,the surface of the Al coating obtained by electrodeposition is more flat,and the corrosion current density is reduced to 4.13×10-5 A·cm-2,showing good corrosion resistance.(2)When the pulse duty ratio is 70%,The bonding strength of the Al plating between substrate reaches 10.7 MPa.(3)When the pulse current density is 15 mA·cm-2,the adhesion of the coating particles is better.The corrosion potential of the magnesium alloy substrate covered by Al plating is moving to-1.506 V,and the corrosion current density is decreased to 4.37×10-5 mA.·cm-2.(4)When the electrodeposition time is 60 min,the Al plating layer has not only a smooth surface but also a uniform thickness,and the thickness is 15 ?m to 20 ?m.In the BMIC-AlCl3 ionic liquid,MnCl2 was added as an electrolyte to co-deposit the Al-Mn coating on the surface of the ZM5 magnesium alloy after zinc immersion.By adding different molar contents of MnCl2 in the ionic liquid(BMIC-AlCl3),the Al-Mn coating with different amounts of Mn was obtained.The micro-hardness test was used to characterize the difference in physical properties between Al-Mn and Al-plated layers and the Motto-Schotty curve of the passivation film on the surface of the coating to analyze the properties of the passivation film on the surface of Al-Mn coating,and the linear sweep voltammetry Al and Mn co-deposition in BMIC-AlCl3 ionic liquids for analysis.The results show that when the BMIC-AlCl3 ionic liquid is electrodeposited with Al-Mn,the microstructure of the coating is mushroom-like,and the corrosion current density can be reduced to 2.907 × 10-5 A·cm-2.The microhardness can reach 101.8 HV50.The passivation film on the surface of Al-Mn coating exhibits N-type semiconductor properties in 3.5%NaCl solution,and the carrier concentration of passivation film of Al-Mn coating obtained in an ionic liquid containing 0.2 mol·L-1 MnCl2 is lower.The linear sweep voltammetry curve indicates that the addition of MnCl2 in the ionic liquid will cause the reduction peaks of Al and Mn in deposition to move negatively.As the amount of addition increases,the reduction peak will move further negatively. |
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