In the oil and gas industry, internal corrosion of carbon steel pipelines is commonly encountered during production and transportation. Iron carbonate is the main corrosion product layer in a CO2 corrosion environment. The formation of a protective iron carbonate layer can protect the steel from further corrosion by acting as a diffusion barrier and also by covering portions of the steel surface. Partial removal of the protective iron carbonate layer can lead to severe localized corrosion by the galvanic effect established between layer-covered and layer-free areas. Therefore, it is very important to understand the mechanisms of protective iron carbonate layer removal. In the current study, two possible removal mechanisms were examined by experimental studies: mechanical removal by flow and chemical removal by dissolution.
Three types of experimental setups were used in order to examine whether the protective iron carbonate layer could be removed by flow. Small scale experiments were conducted in a glass cell with a rotating cylinder electrode setup and jet impingement setup. Although two different types of flow pattern were used, results showed that the protective iron carbonate layer was not affected by the flow and a thin yet adherent layer remained on the steel surface and protected the steel from corrosion. Furthermore, a medium scale thin channel flow cell system was designed and constructed, in order to conduct tests under more realistic flow conditions. It was once again proven that the iron carbonate layer remained protective under the enhanced flow condition. In addition, the mechanical strength of the protective layer was characterized in tensile strength experiments. It appeared that the measured strength necessary to separate the protective iron carbonate layer from the steel substrate was on the order of 106 Pa. This value was a few orders of magnitude higher than the wall shear stress encountered in most realistic flow systems, which demonstrated that with only mechanical force exerted by flow, the protective iron carbonate layer cannot be damaged.
A qualitative study of iron carbonate dissolution was made using scanning electron microscopy. It was shown that the dissolution of the iron carbonate layer led to exposure of the underlying steel. Plate shaped iron carbonate was dissolved preferably as compared to prism shaped iron carbonate. Chemical dissolution of the protective iron carbonate layer was first tested in a glass cell with a rotating cylinder electrode setup. It was observed that the corrosion rate of the underlying steel increased as a consequence of the dissolution of the protective iron carbonate layer due to exposure to an undersaturated solution. With the capability of in situ measurement of mass change on the surface, the electrochemical quartz crystal microbalance was employed in the current study to monitor the iron carbonate dissolution rate directly. Quartz crystals coated with iron, gold and platinum were used. Finally a dissolution mechanism of iron carbonate was proposed.