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

## TL;DR

This study investigates how diethanolamine-based ionic liquids protect carbon steel from corrosion in salty environments by forming protective films.

## Contribution

The study introduces a new class of PILs for corrosion inhibition and identifies Frumkin isotherm as the best model for their adsorption behavior.

## Key findings

- PIL A showed the highest inhibition efficiency (>75%) and formed a dense protective film.
- Frumkin isotherm best described the adsorption behavior with ΔG°ad of −20.89 kJ mol–1.
- XRD analysis showed stabilization of iron oxides like goethite on the steel surface.

## Abstract

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 (ΔG°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.