Use of corrosion inhibitors to mitigate pipeline corrosion is common in the oil and gas industry. Despite this, few studies focus on how the presence of corrosion products affect their performance. This work aimed to understand the impact of Fe3O4 on the performance of a commercial primarily imidazolinium-based corrosion inhibitor formulation. A magnetite layer was formed in an autoclave at a high temperature (1 wt.% NaCl, N2 sparged, pH 4, 120°C). The performance of the corrosion inhibitor was investigated with and without the presence of Fe3O4 (5 wt.% NaCl, CO2, pH 4.5, 55°C). Linear polarization resistance (LPR) and potentiodynamic polarization were employed to study the effect of Fe3O4 on corrosion rate (CR) and inhibition efficiency (IE). Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize specimen surfaces. The acquired data showed that the presence of magnetite limited inhibitor performance. Dissolution of the magnetite layer over time in the CO2 environment was also observed. This behavior was expected as the experiments were performed in non-thermodynamically favorable conditions for magnetite formation. Polarization sweeps indicated that the cathodic charge transfer and the limiting current of the H+ reduction reaction were significantly accelerated due to the Fe3O4 layer. This behavior can be explained by the increase in cathodic reaction area due to the conductive nature of magnetite.
Among the techniques disseminated in the industry to protect carbon steel pipelines against internal corrosion, the use of corrosion inhibitors (CIs) is one of the most common. Organic compounds containing nitrogen are commonly employed in the petroleum industry to decrease corrosion rates. The high inhibition efficiency can be attributed to adsorption capacity on the metallic surface, creating a protective film that interferes with the electrochemical reactions involved in the corrosion processes.1–3
The selection of appropriate corrosion inhibitors, however, poses several challenges. Typically, it is done through a lengthy and systematic testing process where the performances of each CI candidate are evaluated in simulated operating conditions. Yet, the corrosion inhibitor performances are commonly determined on carbon steel specimens with freshly polished surfaces – this is done to ensure consistency of the results but omits to consider the actual state of the pipeline internal surface. Under operating conditions, however, the internal pipeline surface is far from being neatly polished, and it may instead be covered by products of corrosion such as iron carbonate, sulfides, oxides (which form by precipitation) and/or carbide (which is the results of the dissolution of the ferrite phase). These layers can affect corrosion inhibition, and it is important to document their role. This study focuses on the role of magnetite in corrosion inhibition. Magnetite is not as commonly encountered in oil and gas production as carbonates or sulfides. However, magnetite can still be present on the steel surface in certain conditions: as a product of the manufacturing process (mill scale),4,5 in cases of oxygen ingress6 or at high-temperature applications (>120°C).7,8 When investigating the influence of surface layers on corrosion inhibition, in this case, magnetite, several effects can be expected:
• Magnetite can act as a mass transfer barrier to species diffusing to and away from the metal surface, preventing CI from reaching the surface,
• Inhibitor molecules can adsorb preferentially on magnetite, decreasing the effective concentration of CI in bulk and limiting the CI efficiency, and possibly leading to surface heterogeneity and localized corrosion,
• Magnetite being a moderate electrical conductor, can create galvanic coupling with the surface and enhance the baseline corrosion rate, which in turn can decrease the CI efficiency,
• Since magnetite can be protective, a synergistic effect with CI is possible.
