Electrochemical and theoretical studies of two amino acid ionic liquids as corrosion inhibitors for mild steel in 3.5 wt% NaCl solution

Abstract

The inhibitory effects of two newly synthesized inhibitors, tetraethylammonium threonine amino acid ionic liquids ([N₂₂₂₂][Thr]) and choline threonine amino acid ionic liquids ([Cho][Thr]), on the corrosion of mild steel in 3.5 wt% NaCl solution were investigated. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were conducted for this purpose. Additionally, theoretical chemistry concepts were employed to calculate and analyze the quantum parameters of the molecules using density functional theory (DFT). The results showed that increasing the inhibitor concentration led to a significant reduction in the corrosion rate of mild steel in the 3.5 wt% NaCl solution. The inhibitive efficiency values reached 95.3 % and 96.3 % at an inhibitor concentration of 1 mmol·L⁻¹ for [N₂₂₂₂][Thr] and [Cho][Thr], respectively. The inhibition efficiency increased with higher inhibitor concentrations. Furthermore, the effect of temperature on the corrosion behavior of mild steel in the 3.5 wt% NaCl solution was studied within the temperature range of 298–328 K. Polarization plots showed that [N₂₂₂₂][Thr] and [Cho][Thr] acted as mixed type inhibitors. The adsorption mechanism of the studied inhibitors was consistent with the Langmuir isotherm model. The corroded surface has also been analyzed by SEM and XPS.