Testing of inhibitors for CO2-corrosion at elevated temperatures is often performed so as to delay or avoid formation of a protective iron carbonate layer. Inhibitor performance in the presence of FeCO3 (partly or fully covering the steel surface), however, also deserves attention. This is relevant for parts of pipelines where rapid iron carbonate formation is expected, or when formation water breakthrough requires switching from pH stabilization to inhibition. From a methodological viewpoint such studies may indicate whether experiments in the presence of FeCO3 layers should be included in routine inhibitor performance testing. Investigating and comparing the behavior of corrosion inhibitors in the presence of iron carbonate layers of varying degrees of protectiveness was the objective of a series of experiments within the framework of a Joint Industry Project focusing on high temperature inhibition, carried out in our laboratories.

This contribution presents the results obtained with an imidazoline type generic inhibitor in high salinity brines with pH above 6, with 0.5 and 1 bar CO2 partial pressure. Inhibitor performance was studied by means of electrochemical measurements in glass cells at 80°C as well as in a pressurized jet impingement apparatus at 100°C and 120°C. Timing of the inhibitor injection was based on the development of the iron carbonate layer.

The inhibitor effectively reduced the corrosion rate when protective FeCO3 formation was not yet complete. Measurements of the Fe2+ concentration also revealed that the inhibiting effect hindered the increase of the supersaturation required for effective FeCO3 precipitation, slowing down or completely halting the film formation process. In contrast, once a fully protective iron carbonate layer has developed, no additional corrosion rate reduction can be observed upon inhibitor addition.

The results provide insight into how the inhibitor effect manifests itself when applied at various stages of iron carbonate formation on a steel surface. The experiments performed under enhanced convection illustrate the important role mass transport plays in the formation of iron carbonate layers and on inhibition. Both aspects are important for an improved understanding of inhibition of CO2-corrosion in oil and gas pipelines.

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