Performance evaluation of amino acid-based ionic liquids as corrosion inhibitors for mild steel: Synthesis, electrochemical, gravimetric, and theoretical aspects

This study investigates the effect of cationic variation in amino acid-based ionic liquids (ILs) on the corrosion inhibition of mild steel (MS) in 1 M HCl. The ILs, [Try][Ch] and [Try][Tpr], were synthesized by neutralizing L-tryptophan ([Try]) with choline hydroxide ([Ch]) and tetrapropylammonium hydroxide ([Tpr]), respectively. Electrochemical impedance spectroscopy (EIS) demonstrated that both [Try][Ch] and [Try][Tpr] ILs exhibit excellent corrosion inhibition performance at 303 K, achieving efficiencies of approximately 97 % for [Try][Ch] and 98 % for [Try][Tpr]. However, at an elevated temperature of 343 K, the anticorrosion performance of [Try][Ch] decreased significantly from 97 % to 90 %. In contrast, [Try][Tpr] showed superior thermal stability, slightly decreasing efficiency from 98.41 % to 98.19 %, demonstrating its effectiveness as a corrosion inhibitor (CI). Anodic and cathodic polarization curve current density reduction in potentiodynamic polarization (PDP) plots indicates that ILs acted as the mixed CIs. They obey the Langmuir adsorption isotherms and the ${∆G}_{\mathrm{ads}}^{°}$ values of [Try][Ch] were 33.1 kJ mol ⁻¹ and [Try][Tpr] was 33.8 kJ mol ⁻¹ indicating that adsorption ILs possess through the physisorption and chemisorption. The anti-corrosion performance of synthesized ILs follows the order [Try][Ch] < [Try][Tpr], consistent with theoretical analysis. The E_HOMO values are −3.94 eV for [Try][Ch] and −3.57 eV for [Try][Tpr], while the E_LUMO values are identical for both ([Try][Ch] and [Try][Tpr]) at −0.31 eV. The lower energy gap (ΔE) of [Try][Tpr] (3.26 eV) compared to [Try][Ch] (3.63 eV) indicates a better interaction capability of [Try][Tpr] with MS.