Study On The Long-lasting Anti-corrosion Performance And Mechanism Of MOFs@g-C₃N₄ Nanosheet Modified Epoxy Composite Coating

Epoxy resins（EPs）have been widely used in corrosion protection due to their good adhesion,excellent mechanical properties and chemical resistance.However,the main disadvantage of epoxy coatings was the loss of corrosion resistance after prolonged immersion in water.In addition,UV light was absorbed by the epoxy matrix,leading to photo-oxidation reactions that altered its chemical structure and could eventually lead to the complete degradation of the material.Therefore,in addition to improving the long-term corrosion resistance of epoxy coatings,it was also essential to improve the ability of epoxy coatings to resist UV aging.In this thesis,MOFs@g-C₃N₄ nanoparticles were successfully prepared by modifying g-C₃N₄ nanosheets with alkaline-responsive NH₂-MIL-101 and acid-responsive ZIF-8,respectively,and then applied to epoxy coatings to successfully construct a tri-functional integrated smart coating with excellent barrier properties,activity inhibition,and UV resistance.The specific research of this thesis was as follows.（1）The g-C₃N₄ nanosheets were prepared by polymerization of melamine on the surface of sodium chloride crystals.Characterized by FT-IR,XRD,TEM,and SEM,the results showed that the prepared g-C₃N₄ nanosheets were thin in the micron range of transverse size and exhibited excellent absorption in the UV region.（2）NH₂-MIL-101@g-C₃N₄ nanoparticles（MCN）were prepared by loading NH₂-MIL-101 onto the surface of g-C₃N₄ nanosheets using a hot solvent method.FT-IR,XRD,SEM,TEM,and XPS indicated the successful synthesis of MCN nanoparticles.The water stability of MCN nanoparticles was studied by ICP/TOC in acidic（p H=3）,neutral（p H=7）,and alkaline（p H=11）3.5wt%Na Cl solutions and the results showed that MCN nanoparticles were alkaline p H responsive.The active corrosion inhibition of MCN nanoparticles was studied by polarization test（Tafel）and electrochemical impedance spectroscopy（EIS）.The corrosion inhibition efficiency of MCN nanoparticles on carbon steel was 80.82%,and the total resistance（R_t）reached 5743.7Ω·cm²after 8hours of immersion.SEM and water contact angle tests on the carbon steel surface confirmed the formation of the inhibited film.（3）ZIF-8@g-C₃N₄ nanoparticles（ZCN）were prepared by loading ZIF-8 onto the surface of g-C₃N₄ nanosheets using a normal temperature synthesis method.FT-IR,XRD,SEM,TEM,and XPS indicated the successful synthesis of ZCN nanoparticles.The water stability of ZCN nanoparticles was studied by ICP/TOC in acidic（p H=3）,neutral（p H=7）,and alkaline（p H=11）3.5wt%Na Cl solutions and the results showed that ZCN nanoparticles were acidic p H responsive.The active corrosion inhibition of ZCN nanoparticles was studied by polarization test（Tafel）and electrochemical impedance spectroscopy（EIS）.The corrosion inhibition efficiency of ZCN nanoparticles on carbon steel was 87.2%,and the total resistance（R_t）reached 7036Ω·cm²after 5 hours of immersion.SEM and water contact angle tests on the carbon steel surface confirmed the formation of the inhibited film.（4）Smart coatings were prepared by adding MCN nanoparticles to epoxy resin.The scratching experiment showed that the constructed coatings had active activity inhibition,and the Log|Z|_{0.01 Hz} value increased from 4.46 to 4.85Ω·cm² after 8 hours of immersion,and the protective film formed at the defects prevented further corrosion.The barrier performance experiment showed that the MCN/EP smart coating maintained a high Log|Z|_{0.01 Hz} value（10.51Ω·cm²）after 40 days of immersion in 3.5wt%Na Cl solution,demonstrating excellent corrosion resistance.The UV aging test confirmed the excellent UV resistance of the MCN/EP smart coating.After 300 hours of UV exposure,it had a water contact angle of 71.7°,a tensile strength of 24.52 MPa,and an energy storage modulus of 1943 MPa.The water absorption of the MCN/EP coating after 168hours of immersion in water was 2.18%,and the Log|Z|0.01 Hz value of the MCN/EP coating after 20 days of immersion in 3.5wt%Na Cl solution was 10.43Ω·cm².In conclusion,the hydrophilicity and water absorption of the smart coating increased only slightly,with no major degradation in tensile strength,corrosion resistance,and thermo-mechanical properties,which would further improve the lifetime of the coatings.（5）Smart coatings were prepared by adding ZCN nanoparticles to epoxy resin.The scratching experiment showed that the constructed coatings had active activity inhibition,and the Log|Z|_{0.01 Hz} value increased from 4.48 to 4.76Ω·cm² after 8 hours of immersion,and the protective film formed at the defects prevented further corrosion.The barrier performance experiment showed that the ZCN/EP smart coating maintained a high Log|Z|_{0.01 Hz} value（10.47Ω·cm²）after 40 days of immersion in 3.5wt%Na Cl solution,demonstrating excellent corrosion resistance.The UV aging test confirmed the excellent UV resistance of the ZCN/EP smart coating.After 300 hours of UV exposure,it had a water contact angle of 85.3°,a tensile strength of 22.32 MPa,and an energy storage modulus of 2083 MPa.The water absorption of the ZCN/EP coating after 168 hours of immersion in water was 3.11%,and the Log|Z|0.01 Hz value of the ZCN/EP coating after20 days of immersion in 3.5wt%Na Cl solution was 10.44Ω·cm².In conclusion,the hydrophilicity and water absorption of the smart coating increased only slightly,with no major degradation in tensile strength,corrosion resistance,and thermo-mechanical properties,which would further improve the lifetime of the coatings.