Synergistic Effects Between 2-Mercaptoethanol and 2-Methyl-imidazoline as Corrosion Inhibitors in CO2-Saturated NaCl Solution: DFT and Experiments

Abstract CO2-induced corrosion of carbon steel in brines is a pervasive and costly challenge in oil and gas pipelines. While imidazoline derivatives have been traditional inhibitors, their film formation may be compromised under CO2 corrosion conditions, requiring high dosage and offering limited coverage. This study is motivated by the need to design low-dosage, high-efficiency inhibitors by integrating small sulfur-containing molecules in synergy with imidazolines. We have evaluated the individual and combined performance of 2-mercaptoethanol (ME) and 2-methyl-imidazoline (MI) using density functional theory (DFT) and experiments. ME-MI (1:1) exhibits a lower HOMO-LUMO gap (Egap) and higher adsorption energy (in magnitude) than the individual molecules indicative of a synergistic effect between them. For individual molecules, the surface-coverage of ME shows higher adsorption energy and better coverage compared to MI, suggesting better inhibition performance of ME over MI even though the individual Egap and adsorption energy values suggest otherwise. Electron density difference (EDD) plots further suggest that both ME and MI strongly adsorb on Fe3C surface. The experimental results in CO2-saturated 3% NaCl confirm that while ME outperforms MI in terms of individual performance, a 1:1 ME-MI blend delivers a higher efficiency thus indicating synergistic effects as evidenced by DFT calculations.