Comparison of Corrosion Behavior of X65 Pipeline Steel in CO₂-H₂S and CO₂ Environment at Different Flow Velocities

CO₂ corrosion experiments of X65 steel with and without trace amount of H₂S under three flow velocities were performed using high temperature and high pressure autoclaves. Corrosion morphology and corrosion scale were investigated by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD), respectively. Linear polarization electrochemical method was also used to investigate the corrosion behaviors of X65 steel in the two kinds of environment. The results indicated that CO₂-H₂S corrosion rates were remarkably smaller than those of CO₂ corrosion. CO₂ corrosion scale was loose and prone to localized destruction which in turn presented uniform corrosion, pitting corrosion and mesa corrosion with the increase of flow velocities; CO₂-H₂S corrosion scale was compact and had high protective property to the metal matrix as well as high resistance to the flow impact which resulted in uniform corrosion at various flow velocities. Mackinawite (FeS_1-x), the main corrosion product in the CO₂-H₂S environments, had higher protective property to 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 and morphologies between CO₂ and CO₂-H₂S corrosion.