Electrochemical Mechanisms And Properties Of Electrodeposited Aluminum On Magnesium Alloy Surface In Chloroalumiate Ionic Liquid

As the lightest structural metallic material,Mg alloy has the advantages of good thermal conductivity,high specific strength,low absorption cross-sections of thermal neutrons,good castability and workability,and so on.Therefore,the use of Mg alloy has significantly increased in recent years,especially in the aerospace and nuclear industry.Mg alloy can be used in radioisotope batteries to reduce the overall weight and save cost for launching a spacecraft.However,the corrosion failure of Mg alloy is the main difficulty and challenge in its application.Aland Alalloys have the advantages of strong corrosion resistance,good thermal conductivity and high conductivity.Therefore,electrodeposition of Aland Alalloy coatings on Mg alloys in ionic liquids can improve the corrosion resistance of Mg alloys without significant increase the weight of Mg alloys.In adddition,the conductivity and heat dissipation effect of Mg alloy will not be affected.Therefore,the electrodeposition mechanism of Aland Alalloys in ionic liquid was systematically studied in this paper.Furthermore,the corrosion mechanisms of aluminum and aluminum alloy in corrosive medium were revealed by electrochemical experiments.The process of electrodeposition Alon complex Mg alloy in ionic liquid was numerically simulated by finite element method.In addition,the effect of high energy electron irradiation on the microstructure and properties of the coating on the surface of magnesium alloy was investigated,which is crucial to promote the application of magnesium alloy in radioisotope batteries.The detailed contents are as follows:1.In order to determine the optimal molar ratio of BMIC to AlCl₃,the geometric structure,connection mode,and interaction of anions and cations in BMIC/AlCl₃ ionic liquids with different molar ratios were systematically studied by using the m062x functional and 6-31++g（d,p）basis set in Gaussian software.The results indicated that there was not only electrostatic interaction,but also hydrogen bond between anions and cations.The lengths of hydrogen bond in ionic liquids with various molar ratios were different.Due to the strong hydrogen bond between bmim⁺cation and Cl^-anion,the melting point（360.15 K）of BMIC was higher than that of[Bmim][Al₂Cl₇]and[Bmim][AlCl₄].Therefore,BMIC was solid at room temperature,while chloroaluminium ionic liquid was liquid.The nucleation mechanism of Alon the surface of zincated ZM5 Mg alloy in BMIC/AlCl₃ ionic liquid with 1:2 molar ratio was studied.According to the classical nucleation equation and the chronopotentiometry curve,it can be concluded that the current density and electrolyte temperature would affect the nucleation overpotential of Al.The nucleation rate was directly proportional to the overpotential,and inversely affecting the final Algrain size.Results revealed that the regulation of Algrain size could be achieved by changing the current density and electrolyte temperature,which provided theoretical support for optimizing the performance of Alcoating.2.A new three-layer film was fabricated on Mg alloy via electroplating to guard against corrosion in chloride aqueous environment,which consisted of the underlying double-layer Zn/Cu and top Al-Zr layer.It was found that the Al-Zr3 alloy coating electrodeposited in the bath with 0.03 mol/L Zr Cl₄ was uniform,compact,high hardness,and amorphous structure.The surface roughness values（R_a）of the deposited Al-Zr3 coating was 4.37 nm.Thus,Al-Zr3coating had a silvery,mirror-like appearance.Furthermore,the three-layer Zn/Cu/Al-Zr3coating can be used as a physical barrier to prevent the diffusion of corrosive medium when it is intact.When the corrosion pores appeared on the surface of the upper Al-Zr3 coating,chemical corrosion would appear on the Al-Zr3 coating.The corrosion potential of Al-Zr3 layer（-0.924 V）was more negative compared to Cu（-0.337 V）.If the corrosion micropores penetrated the Al-Zr3 coating and contacted with the Cu coating,the corrosion mode would change from the initial longitudinal chemical corrosion to transverse electrochemical corrosion.The Al-Zr3 coating on the top layer was used as a sacrificial anode to provide sufficient protection for the coating and substrate.The corrosion current density declined from 1.461×10^-3 A/cm² of bare Mg to 4.140×10^-7 A/cm² of the Zn/Cu/Al-Zr3 sample.The result showed that the corrosion resistance of Mg alloy had been greatly improved.After 6 days of salt spray test,the surface of the three coatings was still in a relatively intact state.3.The electrochemical behavior of aluminum ions in ionic liquids was studied by cyclic voltammetry and polarization curves.The kinetic parameters of aluminum ions at different temperatures were calculated.The transfer coefficients of anode and cathode at 318.15 K were0.136 and 0.864 respectively.The exchange current density for Al₂Cl₇^-/AlCl₄^-redox process was calculated as 4.6 A/m² at 318.15 K.The diffusion coefficients of Al₂Cl₇^-and AlCl₄^-were calculated as 4.04×10^-12 m²/s and 5.99×10^-12m²/s respectively.Taking the above parameters as the initial values,the local current density and electrodeposition efficiency on complex Mg alloy parts in ionic liquid were calculated by finite element simulation.The error between the simulation results and the experimental results was less than 10.7%,which showed that the numerical simulation could accurately predict the actual coating thickness.4.The Mg alloy with the Zn/Cu/Al-Zr3 three-layer was irradiated by 1.2 Me V electron irradiation to simulate irradiation damage mechanism in the space environment.The electron trajectory and energy loss of 1.2 Me V electrons in the Mg alloy with the Zn/Cu/Al-Zr3 three-layer were simulated by Casino software.The incident depth of electrons with energy of 1.2Me V was about 0.34 cm,indicating that the trajectory of electron beam penetrated three layers and was almost completely immersed in the whole Mg alloy.After 1.2 Me V electron irradiation,part of Alin the upper Al-Zr3 alloy was transformed into Al₂O₃.Compared to the unirradiated sample,the irradiated Al-Zr3 coating observed irradiation swelling behaviour.The hardness and wear resistance of irradiated Al-Zr3 coating decreased.