Imidazolium Ionic Liquids as Corrosion Inhibitors: Insights into Film Impermeability to Corrosive Species

The quantitative evaluation of permeability resistance remains a major challenge in the assessment of IL corrosion inhibitors. Here, we presented a morphology-based methodology that combined electrochemical impedance spectroscopy for inhibition coverage with confocal microscopy three-dimensional analysis to quantify surface roughness (S_a), thereby establishing a dual-criterion framework. At high inhibition efficiency (>73%), surface roughness ranking at identical concentrations directly reflected permeability resistance, whereas under insufficient efficiency, concentration-gradient experiments effectively eliminated coverage interference. Application to three chemically different imidazolium-based ILs ([C₃mim][OTf], [C₃mim][NO₃], and [C₃mim][Br]), which were studied at three different concentrations (10, 30, and 50 mM), revealed a nonlinear relationship between inhibition efficiency and surface roughness, with the nitrate system exhibiting the most favorable permeability resistance. This strategy provided a critical dimension for the quantitative evaluation of IL corrosion inhibitors and advanced the understanding of their protective mechanisms.