Study On Preparation,anti-corrosion And Self-healing Properties Of Amino Acid Intercalated MgAl-LDHs Coatings On Mg Alloy Surface

Magnesium alloy has good application prospects in the fields of automobiles,aviation,communications and portable microelectronics due to their many advantages such as high specific strength,light weight,strong castability,strong shock absorption ability,good thermal conductivity and electrical conductivity.However,the magnesium alloy that has a lively chemical nature and a low corrosion potential can susceptible to rapidly corrosion by corrosive ions such as chloride ions,which greatly limits the application of magnesium alloys.In recently years,researchers have found that if layered double hydroxides（LDHs）are deposited on a metal substrate,the unique layered structure and interlayer anion exchange ability of LDHs can be used to reduce the contact between the etching liquid and the substrate and capture corrosion Ion（Cl^-）,thus,the corrosion resistance of the base material can be improved.MgAl-LDHs and magnesium alloy have common elements,which is easier to grow densely on the surface of magnesium alloy.If a corrosion inhibitor can be inserted between MgAl-LDH layers,a protective coating with autonomous healing function is expected.Amino acids,a non-toxic,easily degradable and environmentally friendly green corrosion inhibitors,have attracted much attention.However,there are many types of amino acid ions,it is necessary to systematically study the corrosion resistance differences of LDH coatings inserted with different amino acid ions and explore the mechanism.In this paper,magnesium alloy is used as the substrate.Aspartic acid（ASP）intercalated MgAl-LDHs coating was first prepared by one-step hydrothermal method.The effects of hydrothermal growth time on the morphology and corrosion resistance of MgAl-ASP-LDH coating were mainly studied.What’s more,the self-healing performance and autonomous healing mechanism were explored.Then,four different amino acid（phenylalanine（Phe）,cysteine（Cys）,threonine（Thr）and glycine（Gly））intercalated MgAl-LDHs coatings were prepared by one-step hydrothermal method.The effects of different amino acid intercalation ions on the morphology and corrosion resistance of MgAl-LDHs coating were studied,and the corrosion resistance mechanism was preliminary analyzed.The main conclusions are listed as follows:（1）MgAl-ASP-LDHs and MgAl-NO₃^--LDHs coatings were prepared by one-step hydrothermal method on the surface of AZ31 magnesium alloy and the morphology and corrosion resistance of coatings were compared.It was found that the size and specific surface area of MgAl-ASP-LDH nanosheets are larger,which can capture more corrosive ions（Cl^-）in the corrosive solution and release aspartic acid ions with corrosion inhibition,showing better corrosion resistance.（2）The effect of growth time on the morphology and corrosion resistance of MgAl-ASP-LDH coating was studied.It was found that the surface of the coating grown for 6 h had grown dense three-dimensional rose LDH.The surface was rough,the contact angle reached 92°and it was already hydrophobic.Within 12h,the corrosion resistance of the coating is enhanced as the growth time increases.The coating grown for 12h has the highest pore coverage（effectively blocks corrosive ions:Cl^-）and still maintain the rosette structure（conducive to the capture of Cl^-）.It exhibits the best corrosion resistance,with a corrosion potential of 0.116V and a corrosion current density of 0.022μA/cm².（3）The self-healing properties of the MgAl-ASP-LDH coating were explored.The scratched MgAl-ASP-LDH coating was soaked in 3.5 wt.%Na Cl solution for 2 days to10 days,the corrosion resistance of the sample continued to increase and eventually reached the corrosion resistance level of the original film,indicating that MgAl-ASP-LDH coating has good self-healing properties.The mechanism is attributed to the chemical adsorption of ASP ions in the scratched area and the dissolution/recrystallization of LDH.（4）The MgAl-Phe-LDH,MgAl-Cys-LDH,MgAl-Thr-LDH,MgAl-Gly-LDH coatings were prepared by one-step hydrothermal method.The coating morphology and corrosion resistance were studied,and it was found that MgAl-LDH coatings with different amino acid intercalations were uniformly and densely grown on the magnesium alloy substrate,showing a typical LDH sheet structure.The corrosion current density of the MgAl-Phe/Cys/Thr/Gly-LDH coating is 3 orders of magnitude lower than that of the magnesium alloy substrate,which greatly improves the corrosion resistance of the Mg alloy.Among them the MgAl-Cys-LDH coating shows the best corrosion resistance,the corrosion potential is 0.137V and the corrosion current density is 0.018μA/cm².（5）The anti-corrosion mechanism of different amino acids intercalated MgAl-LDH coatings was studied.It was found that the difference in corrosion protection of MgAl-Phe/Cys/Thr/Gly/Asp-LDH coating depends on the number of lone pair electrons and conjugatedπelectrons in the amino acid polar group.The molecular chain of Cysteine（Cys）contains the polar groups with S,O,N as the central atom,which provide the most lone pairs electrons and exhibit the strongest ability to adsorbed on the substrate.Therefore,MgAl-Cys-LDH shows the best corrosion resistance among the five coatings（corrosion resistance ranking:MgAl-Cys-LDH>MgAl-Asp-LDH>MgAl-Phe-LDH>MgAl-Thr-LDH>MgAl-Gly-LDH）.