A Systematic Study of Imidazole and Other Related Molecular Corrosion Inhibitors

Abstract

Among the many approaches commonly used to address the challenge of corrosion, the use of molecular corrosion inhibitors is commonplace in the realm of oil and gas wells and pipelines. Most experimental studies on these molecular inhibitors study a single species for a specific environment. We sought to develop a more general and widely-applicable systematic approach to the study and evaluation of these inhibitors utilizing both electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS). Focusing on small, heterocyclic aromatic molecules we evaluated four sets of closely related molecules under standardized conditions. All potential inhibitors were evaluated for effectiveness against iron corrosion in brine under both acidic and neutral conditions, achieved by continuously bubbling through CO2 and Ar respectively. Our sets of prospective inhibitors included: imidazole vs. pyrrole vs. pyridine, pyrrole vs. furan vs. thiophene, imidazole vs. pyrazole, and pyrazine vs. pyrimidine vs. pyridazine. In addition, we scrutinized the selection of the equivalent circuit used in EIS, and the importance of understanding its connection to the physical system under study.

From our studies of these species we established several key areas to consider when selecting or designing a molecular corrosion inhibitor. First, as evidenced by the increased inhibition by imidazole relative to either pyridine or pyrrole, there is likely an as yet un-explained cooperative advantage when two heteroatoms are present. Second, selection of an inhibitor is largely dependent on the types of surface interactions available, which depends on the surface species present. Third, the most important consideration is how pH of the system will change the state of the inhibitor and therefore its ability to interact with the corroding surface and decrease corrosion. By working within a systematic framework where only one variable is considered at a time, we propose not specific rules for selecting the most effective inhibitor, but rather the most important factors to consider in molecular inhibitor selection and design.