Preparation And Corrosion Resistance Of LDH Film On LA103Z Magnesium-Lithium Alloy

Magnesium-lithium（Mg-Li）alloy is the lightest metal structural material.It is widely used in many fields such as automobiles,aerospace,3C electronics,and military equipment,due to its high specific strength,low elastic modulus,excellent noise reduction and shock absorption performance and electromagnetic shielding performance.However,the low electronegativity and corrosion resistance of Mg-Li alloy greatly limits its further application and development.At present,the most economical and effective methods to improve the corrosion resistance of magnesium alloys are thin film and coating technology.Layered double hydroxide（LDH）is a kind of layered material formed by octahedral structures,and its interlayer anion exchangeability makes it show great potential in the anti-corrosion field.In this study,LA103Z Mg-Li alloy with high lithium content was used as the substrate.Different LDH films were prepared in situ on its surface by one-step hydrothermal method via adjusting the parameters of hydrothermal reaction time,temperature,solution concentration and solution type.The surface,cross-section morphologies and phase compositions of films were detected by scanning electron microscopy（SEM）,energy-dispersive spectrometry（EDS）,and X-ray diffraction（XRD）and fourier transform infrared spectrometer（FT-IR）,respectively.The corrosion resistance was characterized by electrochemical impedance spectroscopy（EIS）,potentiodynamic polarization curve and hydrogen evolution test.The purpose was to explore the influence of the above parameters on microstructure and corrosion resistance of the LDH film,reveal the film formation mechanism and corrosion resistance mechanism,and optimize the process parameters for the preparation of LDH film with excellent corrosion resistance.The results show that LDH films can be prepared on the surface of LA103Z Mg alloy by in-situ hydrothermal method,which contains two phases LDH and Mg（OH）₂.The main laminate of LDH is composed of octahedral structure including Mg,Al,（Co）and O,and the interlayer anion is NO₃^-.The LDH grows on the outer surface of the film in a flake shape,and most nano-sheets tend to grow vertically in the recessed areas,while a few grow parallel along the raised areas.With the extension of hydrothermal time,the size and quantity of LDH nanosheets increased,and the film was thickened.After 24 h of hydrothermal treatment,the LDH nanosheets with uniform and compact distribution covered the film surface basically,and the LDH film was thick and dense,which exhibited the best corrosion resistance.When the hydrothermal time rised to 30 h,the thickness of the film further increased,but the corrosion resistance was poor owing to the fine nanosheet size and loose film structure.Therefore,an appropriate extension of the hydrothermal time can promote the nucleation and growth of LDH and improve the corrosion resistance of the film.When the hydrothermal time exceed 24 h,the LDH begins to dissolve,and the corrosion resistance of the film decreases.With the increase of hydrothermal temperature,the size,number and coverage of nano-sheets on the surface gradually increased.In addition,the film thickness and the LDH content in the main film also showed an increasing trend.There were many interspaces in tbe LDH films obtained at high hydrothermal temperatures（100℃,110℃）,and the long-term corrosion resistance was worse than that of the film at 900℃.It can be seen that proper increase of the hydrothermal temperature can increase the content of LDH in the main film and promote the growth of LDH nanosheets on the outer surface of the film,thus improving the corrosion resistance of the LDH film,but too high hydrothermal temperature（over 90℃）will reduce the long-term corrosion resistance.The concentration of Al³⁺in hydrothermal solution affects the formation of LDH and the binding capacity between the film and substrate.Improper concentration of Al（NO₃）₃（0.01 M）will prevent the formation of the LDH in the main film,and the surface LDH nano-sheets are few and fine.Too high solution concentration（0.09 M）will lead to the separation of the film from the substrate,and the film thickness will be reduced.Only in proper solution concentration（0.05 M）,the thick and dense LDH film with large LDH size and excellent corrosion resistance can be obtained.Compared with the Mg-Al LDH film,the size and quantity of nanosheets on the surface of the Mg-Al-Co LDH film obtained in Al（NO₃）₃/Co（NO₃）₂ mixed solution decrease,and its corrosion resistance is slightly reduced.Mechanism of LDH film formation:The Mg in the LA103Z substrate dissolves into the solution,and the Mg（OH）₂ precursor is formed in the alkaline environment.Al³⁺(Co²⁺)in the solution replaces Mg²⁺in Mg（OH）₂,and NO₃^-is inserted into the precursor plates,initially forming Mg-Al LDH flake structures.Subsequently,Mg（OH）₂ and LDH nanosheets are successively formed,and finally covers the substrate surface.However,after a long period of hydrothermal reaction,the high temperature hydrothermal solution will penetrate the LDH on the film surface and infiltrate into the interior of the Mg（OH）₂/LDH main film,forming interspaces.The protective effect of LDH film on the substrate can be mainly attributed to two aspects:the physical barrier effect and interlayer anion exchange of LDH.