Inhibition performances of imidazole derivatives with increasing fluorine atom contents in anions against carbon steel corrosion in 1 M HCl

This work tests the anti-corrosion ability of three imidazolium-based ionic liquids with different anions—bis(fluorosulfon)imide ([FSI]⁻), bis(trifluoromethylsulfonyl)imide ([NTF₂]⁻) and bis(pentafluoroethylsulfon)imide ([BETI]⁻)—on carbon steels in 1 M HCl solution at 298 K. Electrochemical impedance spectroscopy (EIS) indicated that increasing the number of –CF₂ groups in the anion vastly enhanced the corrosion-inhibition efficiency (η). Even at small concentrations (1.25 mM), [(OHC₂)MIm][BETI] achieved an η of 86%. The polarisation curve verified that [(OHC₂)MIm][BETI] acted as a mixed-type inhibitor and that its adsorption followed the EI-Away adsorption model. Scanning electron microscope (SEM) imaging intuitively confirmed the superior inhibition performance of [(OHC₂)MIm][BETI]. X-ray photoelectron spectroscopy (XPS) showed that [(OHC₂)MIm][BETI] was stably absorbed on the carbon-steel surface. Contact angle measurements indicated that adding the –CF₂ groups effectively decreased the hydrophilicity and surface energy of carbon steel from 69.39 to 19.93 mJ/m², hindering the adsorption of water molecules. Moreover, a frontier molecular orbital theory (FMOT) study confirmed that the –CF₂ groups enhanced the donation and drawing of electrons, and molecular dynamics (MD) simulations revealed that they reduced the interaction energy (E_interact) values between the corrosion inhibitor molecule and the iron surface (from −321.08 to −612.74 kJ·mol⁻¹). It was inferred that among the three ionic liquids, [(OHC₂)MIm][BETI] absorbed most strongly to the carbon-steel surface.