Elemental sulfur corrosion of nickel-based alloy 825 in CO₂–H₂S-containing environment at high temperature and high pressure

The elemental sulfur corrosion resistance and corrosion mechanisms of nickel-based alloy 825 at high temperature and high pressure are still missing. This work investigates the elemental sulfur corrosion of nickel-based alloy 825 in CO₂–H₂S-containing environment at high temperature and high pressure using weight loss measurement, surface characterization and in-situ electrochemical techniques including electrochemical noise, electrochemical impedance spectroscopy as well as polarization curves. The results show that in the absence of elemental sulfur nickel-based alloy 825 hardly corrodes in the environment containing 0.5 MPa CO₂ and 1.6 MPa H₂S at 100 °C, since the passive film on the substrate could offer an enough protective performance. However, with the increasing elemental sulfur content, the corrosion of nickel-based alloy 825 becomes more and more obvious. Under the condition of full coverage of elemental sulfur, the uniform corrosion rate of the specimen can be as high as 0.327 mm/y. Moreover, under such a condition, serious localized corrosion could also be observed on the specimen surface. Based on results of electrochemical measurement and surface analysis, it is considered that underneath the elemental sulfur deposit the formation of a strong acidic environment generated from the hydrolysis of elemental sulfur largely destroys the passive film, leading to serious corrosion. Therefore, nickel-based alloy 825 used in high sulfur-containing environment should pay close attention to safety risk from elemental sulfur corrosion. The results of this work could provide a reference for the material selection in high sulfur natural gas field.