Studies On Smart Coatings For Anti-corrosion Of Carbon Steel

Carbon steel is widely applied in machinery equipment,transportation,chemical processing,pipelines,mining and construction as a metallic material.However,the corrosion of carbon steel significantly deteriorates its strength,security and appearance,resulting in catastrophic failure or other serious consequences.Organic coatings are commonly used as an effective strategy for protecting steel from corrosion due to its advantages of simple construction,low cost and so on.But these coatings are susceptible to generate micron cracks during servicing,resulting in the aggressive species permeate into the coating/substrate interface.Subsequently,the coatings will fail to prevent the corrosion process.The smart coatings developed in recent years have attracted attentions because the intelligent microcapsules doped into the coatings can response to the environment changes and then release the corrosion inhibitor to prevent further corrosion of the steel substrate.However,the intelligent anti-corrosion coatings still exist some obstacles such as the complex synthesis process of the intelligent microcapsules,the poor performance of the stability,relatively low loading capacity,low sensitivity towards external environment or poor compatibility with the host organic coating,which severely hinder the application and development of the smart coatings.In view of the problems mentioned above,four kinds of p H-sensitive intelligent microcapsules loaded with corrosion inhibitor benzotriazole(BTA)were elaborately designed and prepared.Then the smart anti-corrosion coatings were fabricated via painting the alkyd resin or epoxy resin coating doped with the prepared microcapsules on the carbon steel substrates.Moreover,various properties and performances of the intelligent microcapsules were characterized by scanning electron microscopy(SEM),infrared spectroscopy(FT-IR),thermogravimetry/differential scanning calorimetry(TGA/DSC)and ultraviolet-visible spectroscopy(UV-Vis).The protection properties of the smart coatings were evaluated by electrochemical methods and salt spray experiments.Meanwhile,the mechanism of the anti-corrosion performance of the smart coating has been discussed.The main contributions of this dissertation are described as follows:(1)To solve the problems of the complicated synthesis process,poor stability and compatibility with the host organic coating of the intelligent microcapsules,a novel hybrid hydrogel of BTA@PHVA/PEI loaded with BTA was synthesized through free radical polymerization used vinyltriethoxysilane(VETO),acrylic acid(AA)and hydroxypropyl acrylate(HPA)as monomers.The as-prepared BTA@PHVA/PEI presents high thermal stability,remarkable p H-sensitivity.The amount of the loaded BTA is about 10.12 wt%.The releasing rate of BTA from BTA@PHVA/PEI rises with the increase of the external environment p H values,which follows Fickian diffusion mechanism.Furthermore,a smart coating was fabricated on carbon steel using an alkyd primer doped with BTA@PHVA/PEI.The smart coating shows remarkable anti-corrosion performance for carbon steel.It is because BTA melecules can release from BTA@PHVA/PEI triggered by the the corrosion-induced p H changes.The released BTA is absorbed on carbon steel to forming a layer of adsorptive film,which can retard the further corrosion of carbon steel.(2)In order to solve the problems of the poor p H-sensitivity and spontaneous leakage of the intelligent microcapsules,mesoporous silica nanomaterials(MSNs)was synthesized via base catalyzed sol-gel process used tetraethyl orthosilicate(TEOS)as the reactant.MSNs was modified by carboxyl group and then wrapped by the highly branched polyethyleneimine(PEI)to prepare the intelligent microcapsules of BTA@MSNs-COOH-PEI loaded BTA.The prepared microcapsules exhibit the uniform diameter which is 828 nm.The amount of BTA loaded in the microcapsules is about10.09 wt%.The microcapsules are very sensitive to the p H changes of the environment.The releasing rate of BTA from BTA@MSNs-COOH-PEI can be accelerated under high p H value conditions,and the releasing model of BTA follows Fickian diffusion mechanism.Furthermore,the smart coating,which was verified to have the well-pronounced anti-corrosion performance for carbon steel substrate in the Na Cl corrosion medium,was fabricated based on the epoxy resin primer doped with BTA@MSNs-COOH-PEI.The anti-corrosion mechanism of the smart coating is inferred that the corrosion-induced p H changes could trigger the accelerated release of BTA molecules from intelligent microcapsules doped in the coating.The released BTA could retard the further corrosion of carbon steel substrate.(3)With regard to the as-prepared MSNs,we also prepared a kind of p H-sensitive intelligent microcapsules BTA@MSNs-SO₃H-PDDA loaded BTA by MSNs grafting with-SO₃H and further being wrapped by poly dimethyl diallyl ammonium chloride(PDDA).The prepared microcapsules present near-spherical with narrow size distribution and smooth surface.The average diameter and the specific surface area of the microcapsules are 718 nm and 65.2 m²/g,respectively.The amount of BTA loaded in the microcapsules is about 13.37 wt%.Moreover,the releasing rate of BTA from BTA@MSNs-SO₃H-PDDA can be accelerated under the alkaline conditions,and the releasing model of BTA follows Fickian diffusion mechanism.Furthermore,a smart coating was prepared on carbon steel through the dispersion of BTA@MSNs-SO₃H-PDDA into epoxy resin.The as-prepared smart coating shows remarkable anti-corrosion performance for carbon steel in Na Cl corrosion medium.The anti-corrosion mechanism of this smart coating is agreement with the smart coating doped with BTA@MSNs-COOH-PEI.(4)For solving the problems of the complicated synthesis process,low loading capacity of the intelligent microcapsules,MSNs with large pore size was synthesized through the addition of the pore-expanding agent of 1,3,5-trimethylbenzene(TMB).The prepared MSNs was modified by poly(acrylic acid)(PAA)to obtain the intelligent microcapsules of BTA@MSNs-PAA.BTA@MSNs-PAA presents near-spherical and smooth surface,with a uniform diameter of 320 nm and high specific surface area of121.2 m²/g.BTA@MSNs-PAA can load BTA by electrostatic interaction between PAA and BTA molecules.The amount of BTA loaded in BTA@MSNs-PAA is up to 16.49wt%.The releasing rate of BTA from BTA@MSNs-PAA can be accelerated under the acidic conditions,which follows Fickian diffusion mechanism.Furthermore,the smart coating was fabricated on carbon steel based on the epoxy resin doped with BTA@MSNs-PAA.The prepared coating presents remarkable anti-corrosion performance for carbon steel in Na Cl corrosion medium,because the releasing of BTA from BTA@MSNs-PAA is triggered by corrosion-induced p H decreasing,which could prevent the further corrosion of carbon steel substrate.