Efects Of Chemical Composition And Reaction Conditions On The Structure And Properties Of LDH Films On The Surface Of Magnesium Alloys

Due to its small density,high specific strength,good biocompatibility,good shock absorption and resistance,magnesium alloys have wide application prospects in the fields of transportation,aviation and military,electronic appliances,biological materials,etc.But its extremely poor corrosion resistance has become a bottleneck restricting the wide application of magnesium alloys.The protective film is currently an efficient method to improve the corrosion resistance of magnesium alloys.Traditional protective films,such as phosphate films,chromate films,anodized films,fail to meet the more demanding service environment.As a typical representative of the smart films,the layered double hydroxide films,referred to as LDH films,because of its unique physical and chemical properties such as ion exchange,memory recovery,self-repair,become a research hotspots.However,the current research is mainly focused on the film with a single chemical composition,or changed the anion between the layers to study the performance difference of the film layer.However,there are few literatures that systematically study the influence of the chemical composition of the film on its structure and performance.In addition,as the most commonly used method for the preparation of LDH films,there is little research on systematically studying the influence of reaction parameters on the film structure and performance in the in-situ growth method.Therefore,this paper adopts a simple and easy hydrothermal in-situ growth method,by changing the metal elements in the chemical composition of LDH,hydrothermal reaction temperature and p H value in the reaction,using a variety of characterization methods and electrochemical testing methods to study the influence of these conditions on the LDH film structure and corrosion resistance.The following main conclusions are drawn about:（1）For Mg/M-LDH,Zn/M-LDH,Ni/M-LDH and Ca/M-LDH（M=Al,Fe）films,when the trivalent metal cation is Al³⁺,the density of the films is relatively high,the bonding strength is relatively good,the length of the layers is between0.5 and 2.5μm,the crystallinity is good,and the structure is stable.At this time,the impedance and the charge transfer resistance of the films is large,which barrier effect is better.The corrosion current density of the film is smaller,which corrosion resistance is better.When the trivalent metal cation is Fe³⁺,the films are relatively loose and easy to fall off,the length of the films is between 0.3 and 1.25μm,the edge of the sheet is broken,and the structural stability is poor.At this time,the resistance and the charge transfer resistance of the films is small,which barrier effect is poor.And the corrosion current density is relatively large,which corrosion resistance is poor.（2）The order of corrosion resistance of LDH films with different chemical compositions is Ca/M-LDH>Mg/M-LDH>Ni/M-LDH>Zn/M-LDH（M=Al,Fe）.When the trivalent metal cation is Al³⁺,the Mg/Al-LDH film has the largest film resistance R_in among the four films,such as Mg/Al-LDH,Zn/Al-LDH,Ni/Al-LDH and Ca/Al-LDH,which is about 1.14×10⁵Ω·cm²,meaning the best physical barrier effect of the film.But the charge transfer resistance R_ct of the Ca/Al-LDH film is the largest,about 9.93×10⁶Ω·cm²,and the corrosion current density I_corr is the smallest,about 8.0×10^-9 A·cm^-2,which has the best corrosion resistance.（3）The hydrothermal reaction temperature（T=90,100,110,120,130,140,150℃）will cause the thickness,density,morphology and corrosion resistance of the Ca/Al-LDH film to change.With the increase of temperature,the thickness gradually increase,the film gradually become dense from loose,and the cracks between the layers gradually disappear.But when the temperature was too high,defects such as pores appear again in the film.So the corrosion resistance of Ca/Al-LDH film increases first and then decreases with the increase of the hydrothermal reaction temperature.At a temperature of 120℃,the corrosion resistance of the film is the best.In addition,the length of the Ca/Al-LDH sheet,which ranges from 0.1 to 1μm,increases with the hydrothermal reaction temperature gradually increases.Temperature affects the structure and corrosion resistance of the film by affecting the nucleation and growth of LDH.（4）The reaction p H（p H=10,10.5,11,11.5,12,12.5,13）will cause the thickness,the morphology,crystallinity and corrosion resistance of the Ca/Al-LDH film to change.The Ca/Al-LDH films prepared with different reaction p H are relatively dense and not observed loose morphology and obvious defects.But as the reaction p H increases,the thickness of the Ca/Al-LDH films have a tendency to decrease.Due to the influence of p H on the formation reaction and crystallinity of LDH,with the reaction p H increase,the change of Ca/Al-LDH sheet length is divided into three stages:constant,rising and falling.The length of the sheet ranges from 0.3 to 1μm.And as the reaction p H increased,the corrosion resistance of the Ca/Al-LDH film increases first and then decreases.When the p H is 11,the corrosion resistance of the film is the best.The p H value mainly affects the corrosion resistance of the film by affecting the anion loading of the LDH film.