Pitting and Uniform Corrosion of X65 Carbon Steel in Sour Corrosion Environments: The Influence of CO₂, H₂S, and Temperature

Localized corrosion of carbon steel pipelines in oilfield environments is a serious concern. In environments containing both CO₂ and H₂S gases, pitting corrosion of carbon steel is considered to be a common occurrence which is particularly complex and still not fully understood. This paper presents a parametric study investigating the pitting corrosion behavior of carbon steel in CO₂-H₂S-containing environments. The work presented is divided into two parts. Part one focuses on understanding the impact of changes in three key process parameters (namely: temperature, absence/presence of 10% H₂S, and absence/presence of CO₂ in the gas phase) on FeS film formation process and overall the corrosion behavior of carbon steel with emphasis on general corrosion of carbon steel in a 3.5 wt% NaCl brine at 30°C and 80°C. Part two focuses on understanding the early and later stages of the pitting corrosion process, with consideration afforded to general corrosion and iron sulfide (FeS) film formation mechanisms, kinetics, and characteristics. The experiments were conducted based on short-term (7 h) and long-term tests (168 h) to investigate the early and later stages of evolution of pitting corrosion. Corrosion film properties and morphology are studied through a combination of electrochemical and surface analysis techniques which include scanning electron microscopy and x-ray diffraction. The extent of corrosion damage of the carbon steel is evaluated through the implementation of surface interferometry to study pit depth and geometry. The results show that early stages of pitting corrosion are more likely to occur with the formation of mackinawite at both 30°C and 80°C in H₂S systems than in CO₂ systems after 7 h. However, at 80°C extensive uniform corrosion is dominant, leading to the competing effect of iron sulfide (FeS) formation, continuous evolution of pitting corrosion, and continuous ferrite dissolution in the presence of CO₂. Results from 168 h tests showed a buildup of FeS on the corroding steel surface with time with the increase in temperature and presence of CO₂ in the gas phase. The buildup of FeS in H₂S-CO₂-containing environments is enhanced by ferrite dissolution. The FeS formed after 168 h is composed mainly of mackinawite at 30°C, and mackinawite and pyrrhotite at 80°C after 168 h. The combination of mackinawite (with a different morphology) and pyrrhotite is shown to promote more pitting and localized corrosion at 80°C than for mackinawite alone at 30°C.