There have been increasing concerns related to the challenges posed by hydrogen sulfide (H₂S) corrosion to the integrity of oilfield pipeline steels. In environments containing variable quantities of both carbon dioxide (CO₂) and H₂S gas, the corrosion behavior of carbon steel can be particularly complex. There is still no universal understanding of the changes in the mechanisms, sequence of electrochemical reactions and impact on the integrity of carbon steel materials as a result of changes in H₂S-CO₂ gas ratio. The film formation process, film characteristics, and morphology in CO₂- and H₂S-containing systems are also known to be different depending upon the environmental and physical conditions and this influences the rates of both general and pitting corrosion. Questions still remain as to how the combined presence of CO₂ and H₂S gases at different partial pressure ratios influence the corrosion mechanisms, as well as initiation and propagation of surface pits. This paper presents an investigation into the overall (i.e., general and pitting) corrosion behavior of carbon steel in CO₂-H₂S-containing environments. The work explores the impact of changes in ratios of CO₂ and H₂S partial pressures at both 30°C and 80°C in a 3.5 wt% NaCl solution. All experiments are performed at atmospheric pressure, while H₂S gas content is varied at 0 ppm (0 mol%), 100 ppm (0.01 mol%), 1,000 ppm (0.1 mol%), 10,000 ppm (1 mol%), and 100,000 ppm (10 mol%) in H₂S-CO₂ corrosion environments. Corrosion film properties and morphology are studied through a combination of scanning electron microscopy and x-ray diffraction. The results show that the morphology and composition of iron sulfide formed changes with H₂S gas concentration because of the continuous interaction of the corrosion interface with the corrosion media even in the presence of initially formed FeS (mainly mackinawite). This often leads to the formation of a different morphology of mackinawite as well as different polymorphs of FeS. This also has the impact of either increasing or decreasing the uniform corrosion rate at low and higher concentration of H₂S gas depending on the temperature. Pitting corrosion is also evaluated after 168 h to determine the impact of increasing H₂S content on the extent and morphology of pitting corrosion attack. The results from the pitting corrosion investigation show that increased and severe pitting corrosion attack occurs at higher H₂S concentration and temperature. The morphology of pitting corrosion attack is also linked to the changes in the H₂S content with an indication of a critical concentration range at which the nature of attack changes from narrow and small diameter pits to severe localized attack. The critical concentration threshold for such transition is shown in this study to reduce with increasing temperature.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)