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Intermittent oil–water wetting can have a significant effect on the internal corrosion of steel pipelines. This paper presents a combined experimental and molecular modeling study of several influential factors on the surface properties and corrosion behavior of mild steel in CO2 environments. The influence of different model oils (LVT-200 and Aromatic-200) and select surface-active compounds (myristic acid, cyclohexane butyric acid, and oleic acid) on the corrosion behavior of carbon steel during intermittent oil–water wetting was determined by measuring the corrosion rate after intermittent wetting cycles. The interfacial tension measurements were performed to study the incorporation of the oil phase along with surface-active molecules in the protective layer formed on the specimen surface. Results showed that the interfacial tension for an aromatic oil–water interface is lower than that for an aliphatic oil–water interface. To understand this result, molecular dynamics simulations of oil–water interfaces were performed in the presence of surface-active molecules and different oils to analyze the structure of the layer formed at the interface. The simulations supported the hypothesis that aromatic molecules are less structured at the interface, which results in the incorporation of more water molecules into the protective layer formed at the steel surface, causing a higher corrosion rate. On the other hand, the simulations revealed that myristic acid in an aliphatic oil forms a well-aligned structure at the interface, devoid of any water molecules. This is in agreement with the hypothesis that the linear molecular structure of myristic acid favors the alignment of molecules at an aliphatic oil–water interface, resulting in a lower interfacial tension and more effective corrosion mitigation as compared to the other two nonlinear compounds tested. It is concluded that an important factor controlling the corrosion behavior is the molecular structure of the oil–water interface, which is adopted by the steel surface layer through the Langmuir–Blodgett process. 

Electrochemical atomic force microscopy (EC-AFM) experiments, including simultaneous linear polarization resistance (LPR) tests and in situ AFM imaging, under a CO2 atmosphere, were performed to investigate the adsorption characteristics and inhibition effects of a tetradecyldimethylbenzylammonium corrosion inhibitor model compound. When the inhibitor bulk concentration was at 0.5 critical micelle concentration (CMC), in situ AFM results indicated nonuniform tilted monolayer formation on the mica surface and EC-AFM results indicated partial corrosion of the UNS G10180 steel surface. At 2 CMC, a uniform tilted bilayer or perpendicular monolayer was detected on mica, and corrosion with UNS G10180 steel was uniformly retarded. Consistently, simultaneous LPR tests showed that corrosion rates decreased as the inhibitor concentration increased until it reached the surface saturation value (1 and 2 CMC). Molecular simulations have been performed to study the formation of the inhibitor layer and its molecular-level structure. Simulation results showed that at the initiation of the adsorption process, islands of adsorbed inhibitor molecules appear on the surface. These islands grow and coalesce to become a complete self-assembled layer. 

While both field experience and laboratory experiments have shown that the efficiency of adsorbed corrosion inhibitor films improves upon exposure of the aqueous solution to a hydrocarbon phase, a credible explanation of these results is lacking. Using a combination of experiments and molecular simulations, this study examines how exposure to oil molecules affects the nature of adsorbed corrosion inhibitor films on metal surfaces. It is found that oil molecules get coadsorbed in the corrosion inhibitor films, making them more hydrophobic, structurally more ordered, and well packed. Corrosion inhibitor molecules with a bulky polar head adsorb in nonplanar, cylinder-like morphologies. Coadsorption of oil molecules changes the morphology of these films to a planar self-assembled monolayer.