Ionic liquids with two typical hydrophobic anions as acidic corrosion inhibitors

Two ionic liquids with typical hydrophobic anions, 1-hydroxyethyl-3-2011methylimidazolium hexafluorophosphate ([(HOC₂)MIm]PF₆) and 1-hydroxyethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([(HOC₂)MIm]NTF₂), were investigated as corrosion inhibitors for mild steel in 1.0 M HCl solution at 298 K. Electrochemical tests and scanning electron microscopy demonstrated that [(HOC₂)MIm]PF₆ exhibited better corrosion inhibition effect than that of [(HOC₂)MIm]NTF₂. Thermodynamic calculations and UV–visible spectroscopy analysis indicated that both of the inhibitors were adsorbed on the metal surface through physical interaction. The corrosion inhibition mechanisms were studied by molecular dynamic simulations. The [(HOC₂)MIm]⁺ of [(HOC₂)MIm]PF₆ was adsorbed on the Fe surface in the water system and [PF₆]⁻ stayed above [(HOC₂)MIm]⁺ while the adsorbed group of [(HOC₂)MIm]NTF₂ was [NTF₂]⁻. The plane of [(HOC₂)MIm]⁺ had a certain angle with the plane of [NTF₂]⁻ in the water bulk. [(HOC₂)MIm]NTF₂ owned greater spatial steric resistance, which contributed to its lower inhibition ability.