Investigation of pitting corrosion of carbon steel in sweet and sour oilfield corrosion conditions: a parametric study

The challenges in managing localised corrosion failures in oilfields are large. In environments containing both CO2 and H2S gas, pitting corrosion of carbon steel is considered to be a common occurrence and particularly complex. The consequences of pitting corrosion failures are usually severe in terms of the health and safety or the environment as well as economic cost to the operators.
The actual mechanisms and sequence of electrochemical activities for pitting corrosion in these environments is still not fully understood. The film formation characteristics and morphology in CO2 and H2S-containing systems are also known to influence the general and pitting corrosion behavior of carbon steel. However, questions still remain as to how different oilfield process parameters such as temperature, chloride ion concentration, in-situ pH of corrosion environment and the combined presence of CO2 and H2S gas could be influencing the corrosion kinetics and pitting corrosion process; pit initiation and pit propagation of carbon steel materials exposed to oilfield corrosion environments.
A comprehensive and systematic investigation of the pitting corrosion behaviour of X65 carbon steel in sweet and sour corrosion environment has been carried out using a combination of electrochemical techniques and a robust set of post-experiment surface analysis techniques. The results confirms the notion that pitting corrosion process of carbon steel is very different from pitting corrosion in passive alloys and that the evolution of pitting corrosion of active materials in sweet and sour corrosion environments is governed by; the uniform corrosion contribution, the local environments; corrosion products and active ionic species and environmental parameters.
In sour corrosion environments, pitting corrosion characterisation using the NPFLEX 3D interferometer and extensive analysis of corrosion product morphologies using the transmission electron microscope was able to show that FeS corrosion product formation occurs faster than FeCO3 in H2S-CO2-containing corrosion system leading to pit initiation and also that in scenarios where competitive precipitation of FeCO3 and FeS is likely, the risk of pitting corrosion is increased.