Effect of Molecular Structure of Thio-Chemicals on Corrosion Inhibition in CO2 Corrosive Environments

Carbon dioxide (CO2) is frequently present in oil and gas fields, and its use in CO2 flooding for enhanced oil recovery is growing. However, CO2 is highly corrosive to steel in oilfield fluid. The effective and economical method for controlling corrosion is the addition of corrosion inhibitors for carbon steel materials. Thio-compounds of small size have shown potential as corrosion inhibitors to enhance the performance of imidazoline inhibitors. In this study, several small thio-derivatives inhibitors including mercaptoethanol (ME), thiourea (TU), mercaptoacetic acid (TGA), and 2-mercaptobenzimidazole (MBI) were compared to inhibit the CO2 corrosion. They were used as synergists to enhance corrosion inhibition of oleic imidazoline (OIM) on carbon steel in CO2-saturated brine at 60°C. The corrosion inhibition was evaluated using weight loss and electrochemical techniques, while the surface was characterized using atomic force microscopy (AFM). Additionally, quantum chemical calculations were conducted to investigate the mechanism of corrosion inhibition. The results demonstrate that the MBI, with its aromatic group, exhibited superior corrosion inhibition performance compared with ME, TGA, and TU. The surface characterization revealed no pitting and localized corrosion at 10 ppm of inhibitor. A proposed interaction model suggests that OIM becomes protonated and forms a coadsorption layer with MBI on the carbon steel surface through electrostatic attraction. MBI adsorbs onto iron through a bidentate binding-N-S-bridge connection, effectively preventing carbon steel corrosion in the CO2 environments. This research contributes to establishing a structure-properties relationship for thio-chemicals, aiding in the development of more efficient corrosion inhibitors.