Analysis of corrosion scales on X60 steel under high H₂S/CO₂ content environments

Corrosion of X60 steel under high-pressure H₂S/CO₂ coexist environment was investigated using a high-temperature and high-pressure H₂S autoclave. The structure and constitution of corrosion scales were studied through scanning electron microscopy (SEM) and X-ray diffraction (XRD). Corrosion experiments were carried out under situations with the same H₂S/CO₂ partial pressure ratio (1.74) and different H₂S partial pressures from 0.15 MPa to 2.0 MPa. The results show that the corrosion scales mainly consist of ferrous sulfide. The absence of ferrous carbonate in the corrosion scales suggests that the corrosion process was controlled by H₂S. Under low H₂S partial pressure conditions, tetragonal FeS_1-x is the predominant corrosion product. When the H₂S partial pressure was increased to 2.0 MPa, hexagonal FeS, hexagonal Fe_1-xS and cubic FeS₂ could also be observed in the corrosion scales. Localized corrosion observed on X60 steel under a higher HaS partial pressure condition can be attributed to the loose scale with S-rich corrosion products and the local breaking of corrosion scales, which also induced the decreasing of general corrosion rate at a higher H₂S partial pressure level.