| Anti-corrosion coating is the most widely used,efficient and convenient corrosion protection measure for steel structures in marine environment.Early corrosion under the coating is hidden and dangerous,so efficient,accurate and sensitive corrosion detection technology needs to be studied urgently.In recent years,the emergence of emerging smart materials has made it possible to monitor and detect early localized corrosion under coatings with high sensitivity and convenience,and endow organic coatings with corrosion early warning functions.Corrosion warning intelligent coating based on fluorescent probe is an effective method for high-throughput scanning and precise localization of local corrosion.However,the fluorescent probes that respond to pH or iron ion single factor have the problem of lack of accuracy.Premature exposure of fluorescent probes can lead to failure and fluorescence quenching.Based on this,this thesis innovatively conducts in-depth and systematic research on the realization of fluorescence contrast enhancement,early protection and targeted release of fluorescent probes,and Cl-capture and treatment of corrosion factors,combined with advanced and comprehensive microscopic characterization techniques,and is committed to build a set of high contrast,high sensitivity and high precision early corrosion early warning system under coating.Rhodamine B acylhydrazone(RBA)with no fluorescence activity was prepared by hydrazide reaction and acetone nucleophilic dehydration reaction using the widely used fluorescent agent rhodamine B as the starting point.Experiments show that the complex product of RBA and Fe3+with no fluorescence activity exhibits strong fluorescence emission behavior after being excited by ultraviolet light,and the Fe3+·RBA complex can enhance the fluorescence intensity in an acidic environment,thus realizing the synergy between pH and Fe3+.The"fluorescence switch"mechanism under preferential response overcomes the shortcomings of misjudgment of fluorescence and insufficient localization accuracy of local corrosion under stimulation by a single factor.The hetero-ion experiments show that RBA also exhibits a characteristic response to Fe3+.The electron density distribution,complexation active atoms and complexation reactivity of RBA were clarified by quantum chemical calculation.Compared with RB,the coordination number between RBA and iron ion was smaller.This indicated that the Fe3+responsive sensitivity of RBA was higher.The complexation of RBA with iron ion reduced the overall energy of the molecule,so the tendency of complexation reaction was significant,and the stability of the complex was high.This was conducive to RBA as the Fe3+responsive fluorescent indicator.The behavior of Fe3+·RBA in absorbing photons and emitting fluorescence was mainly attributed to the transition of electrons between HOMO-2 and LUMO+5 spin orbitals.In order to avoid the failure of the fluorescent indicator RBA due to the interference of the complex corrosive environment,two protective carriers,layered double hydroxides(LDHs)and zeolite(ZEO),were used in this paper,based on their anion and cation exchange properties,respectively.Load protection function of RBA.The deprotonated RBA was intercalated between LDHs by a layered self-assembly method to construct an RBA-LDHs supramolecular system to realize the early protection of fluorescent probes.The interlayer spacing of LDHs increased from 0.3342nm to 0.5124nm before and after loading RBA by XRD.Combined with the detection of N and O elements by XPS,the intercalation loading of LDHs on RBA was synergistically verified.The concentration and fluorescence intensity of RBA released from RBA-LDHs in chloride solution were detected by UV-Vis absorption,fluorescence and fluorescence correlation spectroscopy,demonstrating the Cl-responsive intercalation exchange behavior of RBA.At the same time,combined with the detection and calculation of the change of interlayer spacing before and after the release of RBA from LDHs,it was verified that Cl-was trapped by LDHs intercalation,and the purpose of pretreatment of the corrosion zone was achieved.The application of RBA-LDHs to epoxy coatings could remarkably locate the early localized corrosion under the coating and the defect damage area of the coating with fluorescent signal in 3.5wt%Na Cl solutions.Meanwhile,electrochemical impedance spectroscopy experiments showed that the sheet-like morphology and dispersed distribution of RBA-LDHs enhanced the shielding performance of the coating and significantly reduced the diffusion rate coefficient of corrosive media in the coating.Using ZEO to clad RBA in its lattice cage or load it on the surface,realized the protection of RBA,and prevented RBA from prematurely exposed to corrosive environment and caused fluorescence quenching.Characterization analysis by SEM,XPS and XRD proved that RBA was successfully loaded by ZEO.Through the detection and fitting analysis of fluorescence correlation spectrum,it was found that there were two different scales of fluorescent particles.Small-scale fluorescent particles were released surface-adsorbed RBA,which fluoresce when bound to iron ions.The large-scale fluorescent particles were ZEO coated with RBA in the lattice cage.Fe3+could diffuse into the ZEO lattice cage and form fluorescent active complex with the RBA claded in the cage.Combined with the TOC detection experiment,the release of surface-adsorbed RBA was verified,and the concentration of released RBA was calculated.Fluorescence microscopic observation showed that the application of RBA-ZEO to the coating could effectively indicate the occurrence of early corrosion under the coating and accurately locate the corrosion site.Electrochemical impedance spectroscopy experiments showed that nano-scale ZEO(or RBA-ZEO)filled micropores in coatings,which increased the difficulty of diffusion and penetration of corrosive media into the coatings,and significantly improved the barrier and anti-corrosion performance of the coatings.In addition to the self-warning of coating damage,the self-healing function was another important feature of intelligent anti-corrosion coating.Self-healing technology based on corrosion inhibitor was simple and efficient.After adding corrosion inhibitor or molecular carrier loaded with corrosion inhibitor in the coatings,when the coating was damaged and local corrosion occurred,the corrosion inhibitor was released from the coatings to the damaged part and acted directly on the local corrosion process,so as to inhibit the corrosion electrochemical reaction of the exposed metal surface at the damaged part of the coatings.Therefore,the corrosion inhibition performance of RBA on carbon steel was investigated.Electrochemical test results showed that RBA performed significant corrosion inhibition effect on carbon steel.The inhibition efficiency was positively correlated with its concentration.The optimum concentration was about 6 mmol·L-1,and the corrosion inhibition efficiency was always higher than 99%,and the durability was excellent.The inhibition effect of RBA on carbon steel was realized by its adsorption on the surface of carbon steel,and it was mainly based on the adsorption in the anode area and the inhibition of anode reaction.The coverage of RBA on carbon steel increased with the increase of electrode potential,and the potential and coverage showed the positive effect of mutual promotion,so it exhibited excellent anodic corrosion inhibition performance.The adsorption behavior of RBA on carbon steel surface was the combination of physical adsorption and chemical adsorption and conformed to the characteristics of Langmuir adsorption.The results of quantum chemical calculation and molecular dynamics simulation showed that the active atoms adsorbed by RBA on the surface of carbon steel were carbonyl O,and the benzene ring H and C(CH3)2 groups on the accepting group undergone physical adsorption which due to the negative potential of the acceptor group as the whole.The molecular properties of RBA calculated by quantum chemistry theory agreed with the experimental results of corrosion inhibition efficiency.The quantitative structure-activity relationship equation between the molecular structure of RBA and the inhibition efficiency was preliminarily constructed by combining quantum chemical parameters.In conclusion,RBA showed excellent corrosion inhibition performance for carbon steel.The self-warning intelligent anti-corrosion coatings based on LDHs and ZEO loaded RBA fluorescent agents constructed in this paper could be expected to show the corrosion inhibition and self-healing effect. |
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