In Situ Growth Of LDH Film On Magnesium Alloy Under Atmospheric Pressure And Its Corrosion Resistance

Layered double hydroxides（LDH）is a two-dimensional nano material with unique structure and properties.It can be used as a corrosion inhibitor for micro-nano containers to build an efficient samrt protective film on the surface of magnesium alloy,which can effectively improve the poor corrosion resistance of magnesium alloy.However,the in-situ growth of LDH film on the surface of magnesium alloy usually needs to be carried out under high temperature and high pressure,which seriously restricts the industrial application.In this work,from the two aspects of"optimized pretreatment film method"and"direct in-situ growth",it is expected to realize the in-situ synthesis of LDH film on the surface of magnesium alloy under atmospheric pressure.Firstly,pure aluminum pretreated films with excellent properties were prepared by plasma immersion implantation&deposition（PIII&D）,successfully synthesized composite LDH film with multiple protection functions,and established the corrosion protection model of the film.Then,LDH films were grown directly on the surface of magnesium alloy with the assistance of chelating agent.The effects of initial cation concentration on the structure and properties of the films were systematically studied.Furthermore,the organic modification of LDH films was studied based on chelating agent assisted experiment.The results show that the PIII&D pure aluminum film was compact and flat,and the surface roughness was low.The continuous and complete Zn Al/NO₃^--LDH film with high crystallinity could be grown in situ on the surface of pretreated film samples at atmospheric pressure.After superhydrophobic modification,the pure water contact angle of LDH@PIII&D sample could reach 160.5°.Compared with the PIII&D sample,the impedance modulus in the 10^-2 Hz of the composite film samples increased by one order of magnitude,the corrosion current density decreased by four orders of magnitude,the corrosion potential increased by 0.47 V,proving that the corrosion resistance was significantly enhanced.In the 24-hour continuous electrochemical impedance test,the test curve of the composite film sample changed regularly.Combined with all test results,it can be verified that LDH@PIII&D sample has multiple protection capabilities of liquid-repellency,anion corrosion inhibition and physical barrier.On the other hand,with the assistance of Ethylenediamine tetraacetic acid（EDTA）,LDH films were directly grown in situ at atmospheric pressure,which can effectively delay the corrosion of magnesium alloy.Either Mg Al-LDH or Mg（OH）₂/Mg Al-LDH composite films are formed depending on the available aluminum concentration in the treatment bath.LDH-0.02 sample fabricated at appropriate initial cationic concentration（0.02 M Al（NO₃）₃ was added）shows better corrosion resistance and adhesion strength,and its unique network structure of bulges remains stable after immersion in 0.05 M Na Cl solution for five days.This result indicates that the changes of Al³⁺ions concentration and consequent LDH crystallization rate caused by different initial cationic concentrations have a notable effect on the structure and properties of obtained LDH films.In order to further improve the properties of LDH films prepared with chelating agent,2-Mercaptobenzothiazole（MBT）and Sodium dodecyl benzene sulfonate（SDBS）organic anions were added during the synthesis process.The results show that organic anions can promoted the nucleation and growth of LDH and existed in different forms.MBT can be adsorbed on the surface of LDH film to form an insoluble organic complex layer.SDBS anion was successfully inserted into the LDH interlayer,which improved the microstructure and hydrophobic properties of LDH films with self-sealing effect.After organic modification,the electrochemical corrosion properties of MBT@LDH and SDBS@LDH samples were improved.The film on the surface of MBT@LDH sample was still continuous and complete after 120 h neutral salt spray test,showing enhanced structural stability.