Understanding the Role of Diethanolamine-Based Protic Ionic Liquids in Corrosion Inhibition: Electrochemical and Surface Characterization of Carbon Steel in Saline Environments

This study presents an evaluation of the corrosion inhibition behavior of three protic ionic liquids (PILs), 2-hydroxy diethanolamine formate (PIL A: 2-HDEAF), 2-hydroxy diethanolamine propionate (PIL B: 2-HDEAP), and 2-hydroxy diethanolamine pentanoate (PIL C: 2-HDEAPe), on A36 carbon steel in a chloride electrolyte (3.5 wt % NaCl). The emphasis was converged on elucidating interfacial adsorption, film formation, and surface chemistry that reinforce inhibitor efficacy. A complementary set of electrochemical and surface techniques, including weight loss measurements, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), optical microscopy, field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM), was employed to evaluate the electrochemical response and characterize the inhibitor-modified steel surfaces. X-ray diffraction (XRD) was used to perform an identification of the main phases of corrosion products and adsorbed films. The adsorption behavior was quantitatively evaluated using several adsorption isotherm models, including Langmuir, Temkin, and Freundlich. Among them, the Frumkin isotherm provided the best description of the experimental data, yielding an average standard free energy of adsorption (Δ<i>G</i>°_ad) of -20.89 kJ mol^-1, which is indicative of predominantly physical adsorption at the steel/electrolyte interface. Among the PILs studied, PIL A exhibited the highest inhibition efficiency (>75%) and promoted the formation of a dense, protective interfacial film, whereas PILs B and C showed progressively lower performance. Inhibition efficiency correlated positively with inhibitor concentration and followed the trend PIL A > PIL B > PIL C. Surface morphologies demonstrated significant mitigation of chloride damage in the presence of PILs, consistent with electrochemical results. XRD analysis revealed the stabilization of surface films (iron oxides and oxyhydroxides), including goethite, which are indicative of altered interfacial reactions in the inhibited systems. These results accentuate the importance of interfacial adsorption evaluation and film formation mechanisms in governing corrosion inhibition performance, highlighting the potential of tailored PILs for surface protection in chloride-containing media.