An investigation on the synthesis, characterization and anti-corrosion properties of choline based ionic liquids as novel and environmentally friendly inhibitors for mild steel corrosion in 5% HCl

The development of green corrosion inhibitors is a challenging task as it has to comply with strict environmental regulations. Ionic liquids (ILs) have recently been proposed as promising corrosion inhibitors. The present paper reports on two ILs designed to act as green and efficient high-temperature corrosion inhibitors. The prepared ILs, namely, choline formate (ChF) and choline acetate (ChA), are composed of biologically active ions. To elucidate their structure and corrosion inhibition effect on mild steel in 5% HCl the ILs were subjected to characterization tests like proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR) and Fourier Transform infra-red (FT-IR) spectroscopy and corrosion tests like weight loss measurements, potentiodynamic polarization measurements (PDP), and electrochemical impedance spectroscopy (EIS). The effectiveness of the inhibition (%IE) increased with increasing concentrations and temperature up to 50 °C. ChF and ChA exhibited the highest inhibition efficacies of 96.9% and 99.5%, respectively at a temperature of 50 °C and concentration of 2 × 10-3 M. Above 50 °C their inhibition performance diminished, displaying an efficacy of 77.6% for ChF and 79.3% for ChA at 80 °C. The results of polarization measurements suggested mixed type behavior of inhibitors, and adsorption followed Langmuir adsorption isotherm. Furthermore, surface studies like scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) revealed protecting capability of the investigated inhibitors. FT-IR, and Raman spectroscopic studies revealed the adsorption of ILs on the Fe surface, and an ultra-violet visible (UV-vis.) spectroscopy study confirms the formation of Fe2+- ILs complex. X-ray Photoelectron Spectroscopy (XPS) was conducted to study the formation of corrosion products and protective film over the mild steel surface. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations were also done to understand the inhibition mechanism of ILs.