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

Abstract

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 (Einteract) 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.