Effect of Corrosion Inhibitor Head Group on the Electrochemical Processes Governing CO₂ Corrosion

The use of corrosion inhibitors is a common practice used to mitigate internal pipeline corrosion in the oil and gas industry. An organic corrosion inhibitor is a molecule comprised of a head group (hydrophilic part) and an alkyl tail (hydrophobic part). Despite the significant amount of work associating the effect of the head group with the mode of adsorption of the inhibitor (such as bonding, effect on the tilt angle of the tail, and hysteresis), there is a lack of research relating the effect of the head group with the electrochemical processes governing the corrosion of mild steel in CO₂ saturated environments. In order to isolate the effect of the head group on the electrochemical processes underlying corrosion, three different model compounds with the same alkyl tail length (10 carbon atoms) and different head groups were synthesized in-house. The head groups included a Brønsted acid ionic liquid (imidazole-sulfonic acid-type), a zwitterion (imidazole-type) and a quaternary ammonium-type. By performing experiments at pH 4, 30 °C, using an X65 steel rotating cylinder electrode at 1000 RPM, corrosion rate and potentiodynamic polarization curves showed that the head group has a significant impact on the corrosion mechanisms associated with CO₂ corrosion. This was particularly true when it comes to the activation energy of the charge transfer reactions governing the CO₂ corrosion process. Finally, an electrochemical model (based on a modified Butler-Volmer equation) was developed based on the changes in activation energy produced by each corrosion inhibitor.