Understanding the local pitting corrosion characteristics of carbon steel in CO2 corrosion environment using artificially machined pits

Carbon steel remains the most commonly used material in CO2 – containing oilfield, and other energy applications. Pitting corrosion is a prominent form of corrosion attack on carbon steel, and linked to the formation of non – protective iron carbonate (FeCO3) films and/or their breakdown. The conditions for local breakdown of FeCO3, and other fundamental aspects of pitting corrosion are still not clearly understood; particularly in relation to the evolution of the local chemistry within active pits. Local distribution of corrosion activities/species is likely to influence FeCO3 formation; evolution and properties to support and/or impede pit propagation. This study investigates the local corrosion environment within an artificially machined pits to understand the local pitting corrosion behaviour. It focuses on the local evolution of FeCO3 and pitting corrosion. Pits with specific geometry on X65 carbon steel samples are exposed to two different environments: pH of 4 and 5.9 for 168 h. Tests were performed at atmospheric pressure and 60 °C in 1 M NaCl solution. Linear polarization technique was implemented in combination with a suite of post-experiment surface analysis: Scanning Electron Microscopy, X-ray diffraction, 3D-profilometry and Raman spectroscopy for assessing overall corrosion rate, localised corrosion products and pitting characteristics. Results show that at both pH levels, more crystalline FeCO3 was formed within the pits than at the top surfaces. The size and compactness of FeCO3 decrease from the base of the pits towards the top surfaces, which correlates with the extent of pitting corrosion within the pit and the spatial variation in the local chemistry.