Early stages of pitting corrosion of UNS K03014 carbon steel in sour corrosion environments: The influence of CO2, H2S and temperature

The challenges in managing localized corrosion failures in oilfields are of serious concern. In environments containing both CO2 and H2S gas, pitting corrosion of carbon steel is considered to be a common occurrence and particularly complex. 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 is also known to influence the pitting corrosion behavior of carbon steel. However, questions still remain as to how the combined presence of CO2 and H2S gas both influence the corrosion mechanisms, as well as pit initiation and propagation. This paper presents part of an ongoing parametric study to investigate pitting corrosion behavior of carbon steel in CO2/H2S-containing environments. The work presented explores the impact of changes in two key process parameters (namely; temperature, absence/presence of 10% H2S and absence/presence of CO2 in the gas phase) on the early process of pitting corrosionon carbon steel in NaCl brines. A pit initiation study is conducted up to 7 hours based on changes in temperature and presence/absence of H2S gas. Corrosion film properties and morphology are studied through a combination of electrochemical and surface analysis techniques which include scanning electron microscopy (SEM) and X-ray diffraction (XRD). The extent of corrosion damage of the carbon steel is evaluated through the implementation of surface interferometry to study discrete pit geometry; namely, the size and depth. The results show that the depths of pits initiated after 7 hours were higher in H2S-containing environments (10% H2S in the gas phase), while general corrosion rates dominate the degradation mechanism when CO2 gas is solely present.