📖 ABSTRACT/OVERVIEW
Sour gas corrosion by hydrogen sulphide in Niger Delta oilfield environments presents a distinct inhibitor design challenge not addressed by existing adsorption mechanism theories developed for sweet CO2 systems, as hydrogen sulphide alters iron surface chemistry through sulphide scale formation and competitive adsorption. This study conducts a rigorous quantum chemical investigation of the adsorption mechanisms of six heterocyclic corrosion inhibitors, including imidazoline, benzimidazole, quinoline, and thiourea derivatives, on clean iron and iron sulphide surfaces in the presence of hydrogen sulphide and chloride ions using density functional theory implemented in Gaussian 16 at the B3LYP/6-311++G(d,p) level with dispersion correction. Original contributions include the development of a comprehensive inhibitor-surface-environment interaction energy framework that decomposes total adsorption energy into frontier molecular orbital, electrostatic, charge transfer, and dispersion components, enabling rational ranking and selection of inhibitor molecular architectures for sour environments. A novel quantum chemical descriptor, the Environment-Modified Adsorption Efficiency Index, is proposed and validated against weight loss corrosion inhibition efficiency data measured in H2S-saturated brine solutions representative of three sour Niger Delta oilfields. Correlation analysis demonstrated that the new descriptor predicted experimental inhibition efficiency with an R2 of 0.947 across 42 test inhibitor-environment combinations, significantly outperforming existing Fukui function-based descriptors. The study provides a theoretically rigorous and practically applicable molecular design framework for next-generation corrosion inhibitors for Nigerian sour field applications. Keywords: DFT, corrosion inhibitor, adsorption mechanism, H2S corrosion, Niger Delta
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