Effect of Trace Amount of H₂S on CO₂ Corrosion Behavior of X65 Pipeline Steel

Corrosion experiments of X65 steel under static condition were performed using high temperature and high pressure autoclaves to study the effect of trace amount of H2S on CO2 corrosion behavior. Corrosion morphology and corrosion scale were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Electrochemical method was also used to investigate the corrosion behavior of X65 steel. Mackinawite (FeS1-x), the main corrosion product in CO2-H2S environments, had higher property to protect the metal matrix than FeCO3. Mackinawite with smaller solubility product in comparison with FeCO3 was more prone to precipitate on the metal surface to form compact corrosion scale with high protective property, which was the fundamental reason for the difference of corrosion rates between CO2 and CO2-H2S corrosion.