Construction Of Functional Layered Double Hydroxide And Study On Anticorrosion Properties Of Organic Composite Coatings

This dissertation focuses on the application of nanomaterials layered double hydroxides in the field of metal anticorrosion.Layered double hydroxides have lamellar structure and are easily agglomerated due to strong electrostatic attraction and non-covalent bond interaction.In order to make full use of the physical properties of layered double hydroxides,they need to be modified.One approach is to reduce the thickness of LDH through physical,chemical and mechanical means,weaken the interlayer interactions,and thus prepare thinner layered double hydroxide nanosheets.Another approach is to consider the properties of layered double hydroxides,which are easily functionalized and composited with other materials such as graphene,they can complement each other.Graphene has high conductivity,while layered double hydroxides have poor conductivity.The combination of the two nanomaterials can reduce the conductivity of graphene.In addition,graphene is chemically inert and difficult to functionalize,layered double hydroxides are connected with a large number of hydroxyl groups,which are easy to produce non-covalent bond interaction with other molecules and easy to functionalize.Meanwhile,graphene has strong barrier ability,which can make up for the lack of barrier ability of layered double hydroxide.Based on the above research ideas,the exfoliation and functionalization of layered double hydroxides,the anion exchange of layered double hydroxides and the preparation of layered double hydroxides/graphene hierarchical nanocomposites are fully utilized in this dissertation.Structural characteristics,excellent ion exchange properties and insulated properties of the layered double hydroxide make it applicable in the field of metal anticorrosion.（1）The combination of acidification and ultrasonic exfoliation in formamide was used to strip LDH.Acid treatment and ion exchange insert caffeic acid ions between layers to replace carbonate.Using the weak interaction between formamide and caffeic acid anions,formamide was introduced into the interlayer faster,the hydrogen bond network between LDH layers was destroyed,and the interlayer water molecules were squeezed to realize the exfoliation of LDH.After being stripped,the thickness was reduced to 3.6 nm,which proved that this method could effectively peel the Mg Al-CO₃^2--LDH.The compatibility between the stripped LDH and waterborne polyurethane was further improved.At the same time,the intercalated caffeic acid ions could also chelate iron ions and accelerate the formation of passive film.（2）The electrification of LDH laminates was changed by carbon dots modification,and the agglomeration of LDH could also be improved by the combination of carbon dots and LDH.After LDH was covered by carbon dots,the Zeta potential in aqueous solution changes from 6.33 m V to-27 m V,which improved the dispersion stability of LDH in aqueous solution,changed the interlayer force and promoted the exfoliation of LDH.The experimental results showed that the LDH-Cdot/epoxy had higher impedance modulus than the blank epoxy,indicating that LDH-Cdot exhibited good compatibility with epoxy resin and enhanced the barrier performance of the composite coating.（3）Mg Al-NO₂^--LDH nanomaterials were prepared by the combination of acidification and ion exchange,and the corrosion protection mechanism was revealed.The controlled release process of interlayer NO₂^-in 3.5 wt.%Na Cl solution was confirmed by UV-Vis spectrum analysis.With the exchange of Cl^-and interlayer NO₂^-in 3.5 wt.%Na Cl solution,the release rate of NO₂^-would gradually slow down and finally the concentration of NO₂^-in the solution tended to be stable.The anti-corrosion mechanism of Mg Al-NO₂^--LDH/epoxy composite coating was studied by LEIS.When cracks or pores appeared in the coating,corrosive chloride ions could penetrate into the coating along the defects.When contacting with LDH dispersed in the coating,the release of interlayer NO₂^-would be triggered.The released NO₂^-had redox reaction with the metal substrate,accelerating the passivation of the metal substrate.（4）G-SPANI-LDH nanohybrid was prepared by the combination ofπ-πinteraction and electrostatic adsorption,and its corrosion protection mechanism was revealed.The conductivity of graphene and G-SPANI-LDH was measured by four-probe conductivity meter.The test results showed that the average conductivity of graphene was 163.9±1.7 S/cm and the average conductivity of G-SPANI-LDH was 3.0±0.1 S/cm,the conductivity decreased obviously.The long-term electrochemical results showed that the corrosion resistance of G-SPANI-LDH/epoxy was improved,which was attributed to the decrease of graphene conductivity and the enhanced barrier performance of G-SPANI-LDH.The study on the anti-corrosion mechanism of the defective coating shows that the resistance of the defective G-SPANI-LDH/epoxy was significantly lower than that of the blank epoxy and G/epoxy,indicating that the conductive SPANI has a passivation effect on the metal substrate.In the Raman distribution of the corrosion product,it can also be found that G-SPANI-LDH/epoxy has the highest concentration of the corrosion product Fe₃O₄.（5）rGO-LDH-Asp hierarchical nanocomposites were prepared by in-situ coprecipitation and electrostatic interaction.By thermogravimetric analysis,the loading amount of L-aspartic acid anions adsorbed on the surface was 40.5 wt.%.SEM and TEM characterized the dispersion of the filler in the resin.It was found that rGO-LDH-Asp had good compatibility with waterborne epoxy,which was attributed to the L-aspartic acid anions adsorbed on rGO-LDH-Asp,which improved its compatibility with waterborne epoxy.From the electrochemical impedance spectroscopy,it is found that rGO-LDH-Asp/epoxy coating has good corrosion protection characteristics,and the coating resistance could be maintained at 1.231×10⁹Ωcm² after being soaked in 3.5 wt.%Na Cl solution for 90 days.The anti-corrosion mechanism of the coating was explored through micro-area electrochemistry.The L-aspartic acid anion adsorbed on the surface of rGO-LDH can accelerate the passivation of metal substrate and delay the corrosion process.