The Protection System Construction And Mechanism Research Of Functional Corrosion Inhibition-Coating Based On Layered Double Hydroxides

In recent years,the corrosion protection technology with multifunctionality has garnered considerable attention with the emerging of new materials and technology,in which the micro/nano materials have play a very important role.Layered double hydroxides(LDH)is a class of 2D intercalation anionic clay,combines numerous advantages,especially the intrinsic anion exchange capacity,which endow it with natural advantage for corrosion protection in the marine environment that rich in chloride.Therefor,LDH has became an ideal nanomaterial for smart anticorrosion technology.However,in the field of corrosion protection,there is little research on the synthesis and structural characteristics of LDH,particularly in combination with inorganic or organic corrosion inhibitors.The traditional anion exchange method is time-consuming and labor-intensive,and the intercalation of corrosion inhibitors in LDH is also inefficient.In addition,the intrinsic incompatibility between nanoclay and organic matrix leads to poor dispersion and weak interfacial interactions.Herein,this paper focuses on the synthesis of LDH,the loading of corrosion inhibitor and the interface interaction between LDH and organic coating matrix,combined with advanced and comprehensive characterization technology,to conducts in-depth and systematic research.The aim is to build a high performances of inhibition-coating protection system with green,efficient and functional properties.The main contributions are as follows:(1)In terms of the controllable synthesis and growth mechanism of LDH,the effects of reaction atmosphere,growth temperature and time,p H as well as the interlayer anions on the morphology,structure and composition of LDH were discussed according to the reaction mechanism of LDH.The results show that CO₂ has a great influence on the interlayer anions of LDH.CO₂ from solution and air can able to participates in the formation of LDH through generating CO₃^2-and intercalated into the interlayer of LDH.Besides,the binding priority with LDH of CO₃^2-is significantly higher than that of NO₃^-.The growth temperature and time mainly affect the grain size of LDH.The size of LDH would increase several times(from several hundred nanometers to several microns)when increase of temperature(65??110?).p H(the concentration of OH^-)plays a very important role in the formation of LDH.In a certain p H range(10±0.5),a typical lamellar LDH can be obtained.However,the products are mainly composed of irregular metal oxides and exhibit obvious aggregation and stacking when the p H is too low or too high.In addition,the small inorganic anionic inhibitors such as NO₂^-and Mo O₄^2-can be directly intercalated into the interlayer of LDH via the coprecipitation method,and the obtained products are all single-phase structure.(2)In order to improve the loading capacity of LDH to organic corrosion inhibitor,a new method combining delamination and reconstruction was applied to obtain the LDH with high loading of the organic corrosion inhibitor-5-Methyl-1,3,4-thiadiazole-2-thiol(MTT)according to the memory effect of LDH.The results show that the reconstructed LDH exhibits typical layered structure,and the interlayer anions is composed of pure MTT-.However,the traditional anion exchange method can not achieve complete exchange substitution,and the obtained LDH is polyphase structure.Besides,the corrosion inhibitor loading capacity of the reconstructed LDH improved is increased by about 3 times compared with the product prepared by anion-exchange,and the LDH also has a rapid Cl^-–responsive release performance.The electrochemical results demonstrate that the LDH-MTT-obtained by the new strategy shows excellent corrosion inhibition for mild steel in 3.5 wt%Na Cl solution.And the maximum corrosion inhibition efficiency is about 94%,which is more than 30%higher than that of LDH-MTT-prepared by ion exchange method.(3)In order to improve the interaction and dispersion between LDH and coating matrix,three efficient methods were proposed:Firstly,graphite fiber(GF)was used for the first time to improve the interaction between LDH and coating matrix:the highly ordered layered double hydroxides(LDHs)nanosheet arrays was in-situ grown on oxidized graphite fiber(OGF)via a facile one-step hydrothermal approach,and then it was incorporated into the polyvinyl butyral(PVB)to prepare highly dispersed nanocomposite coating.The results show that the LDH filled PVB coating presented low-dispersed and defective features due to the agglomeration of LDH,while the OGF/LDH were distributed uniformly within PVB coating,and present interlaced distribution features.The corrosion experiments indicate that although the addition of LDH in PVB coating can improve the barrier performance,it is more prone to pitting corrosion due to the presence of defects;on the contrary,the PVB/OGF/LDH coating exhibits excellent corrosion resistance,the impedance value changes slightly and remains at a high level(10⁸?10⁹?cm²)after 30 days'immersion in 3.5 wt.%Na Cl solution.Secondly,a bioinspired multilayer hybrid film based on LDH monolayer nanosheets(LDH-NS)was fabricated:a large amount of LDH-NS with uniform features were directly synthesized via the bottom-up strategy,and the as-prepared nanosheets was used in a hybrid film with nacre-like structure via one-step coassembly process.The results show that LDH-NS has ultra-thin structure(0.71 nm)and high surface activity,and exhibits good interface interaction(hydrogen bond)with PVB molecules.Besides,the LDH-NS was well aligned and stacked along the substrate surface under flow and gravity induction.The corrosion experiments reveal that the multilayer PVB/LDH-NS composite film can effectively inhibit the diffusion of corrosive species and improve the corrosion resistance.Finally,on basis of the above research,we have developed a nacre–inspired layered hybrid coating based on LDH nanoplatelets and PVB combining excellent barrier and self-healing performances:LDH intercalated with inorganic inhibitor(molybdate)was directly synthesized by coprecipitation method,and the obtained LDH was modified by silane successfully.Then the PVB/LDH composite coating with pearl-like structure was fabricated by combining LB and spin coating method.The results show that the composite coating(containing 3.4 wt.%LDH nanoplatelets)exhibits excellent barrier performance,the impedance value maintains at a high level(10¹⁰?10¹¹?cm²)after immersion of 50 days,which is 5 orders of magnitude higher than that of pure PVB coating.In addition,when corrosion species(Cl^-)penetrate into coating and reach to the surface of mild steel,the intercalated inhibitor Mo O4^2–will be released responsively from the incorporated LDH and react with corrosion products of mild steel to form a dense passivation layer(Fe₂(Mo O₄)₃ and Fe Mo O₄)to cover the exposed surface,which could effectively prevent the aggressive medium from further erosion.That is to say,the coating has good self-healing performance.