Understanding Pitting Corrosion Behavior of X65 Carbon Steel in CO2-Saturated Environments: The Temperature Effect

A systematic study examining the pitting corrosion behavior of X65 carbon steel (UNS K03014) in CO2-saturated 10 wt% NaCl brine is presented. This paper examines the impact of changes in a key environmental parameter (temperature) on the pitting corrosion processes of carbon steel. Pit propagation studies were conducted in static conditions at different operational temperatures. The evolution of corrosion products on carbon steel was followed over 168 h of immersion and their characteristics were related to initiation and growth of pits on the steel surface. The developed corrosion products were studied through the use of x-ray diffraction and scanning electron microscopy. The extent of pitting was evaluated through the implementation of surface interferometry to study discrete pit geometry, namely, the size and depth, while the general corrosion rate was evaluated using linear polarization resistance measurements. The results indicate that pitting corrosion dominates the general thickness loss of the material in conditions where the morphology of the corrosion product layer was predominantly cementite (Fe3C) and/or a form of iron carbonate (FeCO3) often referred to in literature as “amorphous” FeCO3. At 30°C and 50°C, pit propagation continued unhindered despite the growth of an amorphous FeCO3 layer. However, at 80°C, results suggest that a “self-healing” effect involving the precipitation of substantial levels of nano-scale polycrystalline FeCO3 is capable of hindering the growth of active pits in the conditions evaluated. The results also reflect that in instances where high general corrosion rates of the steel surface were observed, the growth rate of pits were heavily underestimated by profilometry measurements (which recorded pit depths relative to the corroded surface). The concept of “relative” pitting and “absolute” pitting rates/depths are consequently introduced and the discrepancies between both interpretations are discussed.