Surfactant-type organic corrosion inhibitors are widely used in the oil and gas industry to mitigate internal pipeline corrosion. Their molecular structure is comprised of a polar head group and a non-polar alkyl tail, with different lengths. Despite many studies qualitatively associating the alkyl tail length to the corrosion mitigation efficiency, there is are no systematic studies and no clear mechanistic explanation in the literature about how the alkyl tail length affects the corrosion process. Consequently, the goal of this research was to relate inhibitor alkyl tail length to changes in activation energy of the electrochemical process associated with CO2 corrosion of an API-5L-X65 steel at pH 4.0. Four different model compounds were synthesized in-house, and utilized to achieve this goal. Their molecular structures had the same head group, dimethylbenzylammonium, with four different alkyl tail lengths corresponding to butyl (-C4H9), octyl (-C8H17), dodecyl (-C12H25) and hexadecyl (-C16H33). In data analysis, the chemical component of the total activation energy was calculated using an Arrhenius-type relationship and by working at the potential of zero charge (PZC), even if this does entirely eliminate the contribution of the electrical component. A linear relationship between the tail length of the corrosion inhibitor and the change in activation energy of the corrosion process was determined, suggesting that the tail directly affects the chemical component of the total activation energy.
Key words: corrosion inhibitors, alkyl tail length, mitigation, activation energy, CO2 corrosion