| With rapid industrialization,improving the anticorrosion performance of marine-based steel is of great significance.At present,water-based zinc-rich epoxy coating is one of the most widely used and most effective marine steel protective materials,because of its excellent anticorrosion performance,physical properties,and economic advantages,among others.At present,added functional conductive fillers are considered potential solutions to extend the anticorrosion time.However,in the water-based zinc-rich epoxy coating,steam escape pores are easily formed between the filler particles and the resin during the drying process,and this void layer is easily broken or collapsed due to local corrosion.This results in the exposure of the metal surface to the external corrosive environment.As a result,the corrosion protection of the zinc-rich coatings fails over a large area.Meawhile,the use of uneven and oversized conductive self-healing filler materials in water-based zinc-rich epoxy coatings directly affects its dispersibility and filling ability,resulting in limited anticorrosion performance.Moreover,for the common self-healing coatings,the release of the active filler is usually a physical process that requires additional physical forces and other conditions.Therefore,when the base material is chemically corroded under natural conditions,it cannot achieve precise conveying and filling effects,resulting in poor repair capabilities for vacancies.In summary,a small size,conductive,intelligent self-healing anticorrosive filler is the ideal choice for water-based zinc-rich epoxy coatings.Herein,based on the reversible ring-opening reaction of the epoxy group of the tertiary amino group-rich epoxide cationic polyacrylamide(ECP)at the beginning of hydrothermal process(acidic environment)and the irreversible ring-opening(crosslinking reactions)at the late hydrothermal period(alkaline environment),47 nm-sized ZnS@Cs were prepared via a one-pot hydrothermal process.During this process,the covalent bonds formed between the ZnS core and elastic carbon shell significantly improved the mechanical and chemical stabilities of ZnS@C.Moreover,via this universal synthesis strategy,a series of TMS@Cs,such as MoS2@C,NiS@C,and CuS@C were developed with excellent mechanical and chemical stability.With uniform carbon-coated ZnS(ZnS@C)nanoballs as the smart active release filler,we propose an anticorrosive and self-healing zinc-rich maleic anhydride epoxy coating.Due to the high pore filling efficiency of the nanoballs,the water vapor transmission rate of the coating.The carbon-shell of the nanoball increases electron transmission paths in the coating,which effectively activates more Zn-sites and extends the cathodic protection time.Moreover,once the steel-substrate undergoes regional corrosion,the SO42-hydrolyzes from the ZnS-core of the nanoball and reacts with iron ions on the corroded area accurately and intelligently to fill the gap and self-heals into a new densebarrier Fe2(SO4)3 layer,which significantly improves the shielding protection ability during the long-term usage of the coating.The specific conclusions are as follows:(1)The bisphenol A epoxy resin E20 and the unsaturated fatty acid linoleic acid were used as raw materials for preliminary esterification,and then further esterification with maleic anhydride was carried out to introduce a large number of carboxyl groups into the molecule.Finally,the maleic anhydride epoxy ester is self-emulsified with water to form MAE epoxy emulsion.Infrared spectroscopy(FTIR)analysis of synthetic raw materials and products showed that the synthesized polymer MAE was consistent with the experimental design.By varying the amount of maleic anhydride,its effect on the particle size and stability of the emulsion was studied.The optimal addition of maleic anhydride was 10%,the average particle size of the MAE emulsion was 148.2 nm,the PDI was 0.119,and the stable storage period was over 100 days.The emulsion dispersion stability index TSI value was 6.6.The MAE varnish coating was prepared by the preparation method of water-based coating and used as the anti-corrosion protection coating of steel plate.By observing the AFM picture of the MAE resin varnish film,it can be concluded that the surface of the coating is relatively flat,there is no obvious defect,and there is no air hole caused by the overflow of water vapor.The maximum drop of the coating surface is 154.4 nm,and the water vapor transmission rate of the film is 142.2 g/m2 d.Electrochemical impedance spectroscopy(EIS)and polarization curves(Tafel)were used to evaluate the protective ability of the coatings.From the Tafel curve,the corrosion voltage of MAE coating is-0.4989 V,and the corrosion current is 5.24E-7 A/cm2.The increase of the corrosion voltage and the decrease of the corrosion current by two orders of magnitude indicate that the MAE coating can effectively protect the iron plate and delay the corrosion.The polarization resistance Rp=163.12 kΩ.cm2,and the corrosion rate CR=0.12 mm/year.Compared with bare iron,the anticorrosion efficiency is increased by 95.76%,showing excellent anticorrosion performance.Electrochemical impedance spectroscopy(EIS)shows that the low-frequency impedance of the MAE coating before soaking is |Z|f=10mHz=1.35E5 Ω.cm2,indicating that the coating has excellent shielding performance.According to the standard salt spray test,within the 180-hour standard salt spray test,the MAE varnish coating has good corrosion protection to the steel plate.(2)Based on the reversible ring-opening reaction in acidic conditions and the irreversible ring-opening crosslinking reaction in an alkaline environment of the epoxy group of the tertiary amino group-rich ECP,47 nm-sized N-doped carbon-coated ZnSnanospheres were prepared via a simple one-pot hydrothermal synthesis process.At the beginning of the hydrothermal reaction,the aqueous synthesis solution was acidic(pH=5.2),owing to the strong acid and weak base salt of ZnCl2.In an acidic environment,the reversible ring-opening reaction of the epoxy group under acidic conditions enhanced oil-in-water micelles and realized the formation of homogeneous ZnSnanospheres.As the synthesis progressed,the solution became weakly alkaline(pH=8.7)with the continued release of NH3 as a by-product.The irreversible crosslinking reaction of the epoxy group,with the carbonization treatment,caused an elastic N-doped carbon shell to form on the surface of ZnSnanospheres with C-S covalent bonds between the ZnS core and carbon shell.The effects of changing the amount of ECP on the growth of ZnS and the thickness of the carbon shell were revealed by SEM and TEM.The porous and high specific surface area structure of ZnS@C not only provides ion diffusion channels for the inner core,but also increases the contact area between the material and the electrolyte,which is expected to achieve fast ion and electron transport kinetics.The ID/IG ratio was calculated to be about 1.0 in Raman spectroscopy,indicating the high degree of graphitization of the carbon shell in ZnS@C,which lays the foundation for the formation of a structure with high electronic conductivity.The XPS results indicate that there is a C-S covalent bond between the ZnS core and the carbon shell,which is beneficial to suppress the expansion of ZnS@C caused by ion and electron shuttle.Benefiting from the nanometer size,fast charge transfer,and stable core-shell structure,ZnS nanospheres are expected to be ideal conductive fillers for Zn-rich epoxy coatings.(3)With uniform carbon-coated ZnS(ZnS@C)nanoballs as the smart active release filler,we propose an anticorrosive and self-healing zinc-rich maleic anhydride epoxy coating.Due to the high pore filling efficiency of the nanoballs,the water vapor transmission rate of the coating with an initial corrosion efficiency of 99.92%and a low-frequency impedance of |Z|f=10mHz=3.88 × 1010 Ω·cm2,was reduced by 52%.The carbon-shell of the nanoball increases electron transmission paths in the coating and improves conductivity by nearly two orders of magnitude,which effectively activates more Zn-sites and extends the cathodic protection time.Moreover,once the steel-substrate undergoes regional corrosion,the SO42-hydrolyzes from the ZnS-core of the nanoball and reacts with iron ions on the corroded area accurately and intelligently to fill the gap and self-heals into a new densebarrier Fe2(SO4)3 layer,which significantly improves the shielding protection ability during the long-term usage of the coating.The effective anticorrosion time of the proposed coating could be up to 3400 hours.(4)Based on a simple one-pot hydrothermal synthesis of carbon-coated ZnS nanospheres(Reversible ring-opening reaction of ECP’s epoxy group under acidic conditions and irreversible ring-opening cross-linking reaction under alkaline environment),the applicability of this method for the synthesis of other metal sulfides(MoS2@C,NiS@C,and CuS@C,etc.)was extended.By studying the organic coating reaction mechanism in the hydrothermal reaction,NH3,the by-product of thiourea hydrolysis,is the key factor to change the pH value of the environment.NH3 is the driving force for the irreversible ring opening of the epoxy group because NH3 has no-bonding lone pairs and acts as a strong nucleophile to attack the central carbon atom of the epoxy group.This promotes the cross-linking between linear ECP molecules,forming an insoluble network polymeric ECP(P-ECP)encapsulates TMS through more stable ether bonds.Theoretically,the synthesized raw materials must produce by-products containing strong nucleophiles,which can promote the ring-opening reaction of epoxy resins,etc.,thereby promoting the formation of organic cross-linked coated nanoparticles.Therefore,this method is applicable to all series of sulfides synthesized with thiourea as the sulfur source.In addition,the synthesis of two-dimensional metal sulfide materials(using MoS2 as an example)was studied,concluding that this method is not only suitable for the synthesis of three-dimensional metal sulfides,but also for two-dimensional metal sulfides.The high specific surface area and porous structure of nanoscale TMS@C not only increase the contact area between the material and the electrolyte,but also provide ion diffusion channels for the inner core,enabling fast ion and electron transport kinetics.The high degree of graphitization of the outer carbon shell lays the foundation for the formation of a structure with high electronic conductivity.The XPS results indicate that there is a C-S covalent bond between the TMS core and the carbon shell,which is beneficial to suppress the expansion caused by ion and electron shuttle. |
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