Improving The Corrosion Protection Ability Of Epoxy Coatings Using Ca-based LDH Intercalated With Different Inhibitors As Pigments On Q235 Steel Substrate

Corrosion is a big global issue impacting our everyday lives,the manufacturing content and the destruction of millions of dollars in production per year.The impact of corrosion is very huge on the country’s economy,and it consumes at least 3-4%of the gross domestic product（GDP）.Therefore,it is very necessary to mitigate the corrosion.Structural materials such as steel are widely used in various fields i.e.,automotive,marine,aerospace,and electronic applications.But steel usually corrodes rapidly due to its highly oxidative nature.It readily forms the oxide layer and causes material loss when comes in contact with aggressive environments.Several anticorrosion methods have been observed so far,while coating techniques have taken precedence during the previous few decades due to their ease of application.Organic coatings,among other types of coatings,have been widely used in metal protection owing to their ease of operation and efficacy,as well as their ability to protect against corrosion.Micropore flaws and other intrinsic problems,on the other hand,are nearly unavoidable in these coatings due to the nature of the material.Because of these flaws,corrosive species are capable of penetrating from the coating to the metal substrate.To address this issue,pigments or fillers are being utilized in these coatings,which can considerably increase the corrosion protection performance of coatings and also extend their service life due to their excellent self-healing characteristics.In the case of self-healing coatings,the capacity to repair coating defects and recover their corrosion protection performance with little or no external interference is frequently cited as a characteristic.Layered double hydroxides（LDHs）are one of the few nanocontainers that show remarkable properties such as self-healing,ion exchange,superior adhesiveness and hydrophobicity.The inhibitor-loaded LDHs are used as pigments to enhance corrosion protection.This work is divided into two projects directed at appraising the corrosion resistance and self-healing performance of the epoxy-based composite coatings on the Q235steel substrate.In the first project,the CaAl LDH loaded with different concentrations of 2-mercaptobenzothiazole（MBT）was prepared by an in-situ intercalation hydrothermal approach in the form of both coating and powder.Initially,the optimum concentration of MBT was investigated using the electrochemical method.The synthesized CaAl-MBT LDH powder with optimized inhibitor concentration was then incorporated into epoxy resin as a pigment at various concentrations to improve the corrosion protection capabilities of Q235 steel.The 2%-added CaAl-MBT LDH revealed a considerable improvement in corrosion protection efficiency.The increase in corrosion resistance can be attributed to the formation of a barrier between the steel and the corrosive medium,as well as the capture of Cl ions by LDH,which was followed by the release of MBT ions from the LDH layer,which could react with Fe2+and form a chelate network over the steel surface,preventing the infusion of electrolyte.The composite coating also had high adherence to the substrate（5.83 MPa）and represented self-healing capabilities when probed through the LEIS technique.In the second project,the CaFe-TTA LDH nanoplates and CaFe-TTA LDH@g-C₃N₄ nanohybrids were successfully prepared using the facile approaches.The prepared CaFe-TTA LDH nanoplates/CaFe-TTA LDH@g-C₃N₄ nanohybrids were incorporated into the epoxy resin to develop corrosion-resistant nanocomposite coatings.The resistances at 0.01 Hz(|Z|_0.01)of EP/CaFe-TTA LDH and EP/CaFe-TTA LDH@g-C₃N₄ samples were reduced by one order of magnitude after 60 days of immersion in 3.5%NaCl solution（from 5.49×10⁸Ωcm² to 4.42×10⁷Ωcm² and from 1.18×10⁹Ωcm²to 2.25×10⁸Ωcm² respectively）,compared to the pristine EP coating,which decreased approximately two orders of magnitude（from 2.61×10⁸Ωcm² to7.69×10⁶Ωcm²）.The localized electrochemical impedance spectroscopy（LEIS）and salt spray tests further explored the self-healing activity of the coatings and revealed that the scratched EP/CaFe-TTA LDH@g-C₃N₄coatings maintained excellent corrosion resistance.The pull-off adhesion test showed an improved adhesion strength as compared to the pristine epoxy coating.Moreover,the DFT simulation confirmed the reaction mechanism of the nanofillers during the corrosion process.Hence,the results revealed that the EP/CaFe-TTA LDH@g-C₃N₄-based coatings delivered superior barrier/active inhibition performance due to the labyrinth and self-healing effect of nanofillers,suggesting a promising avenue for the development of a variety of nanocomposite coatings for various functional applications.