Formation mechanism of corrosion scales of carbon steel by CO₂ corrosion under high temperature and high pressure

The corrosion behaviors of carbon steel exposed to CO₂-containing water under high temperature and high pressure for different intervals of time were measured by using an autoclave. The morphology and microstructure of the CO₂ corrosion scale were investigated by SEM, EDS and XRD, and its formation mechanism was discussed. The results indicate that the corrosion scale is mainly composed of Fe₃C and a little FeCO₃ in the beginning period (8 h), and with the process of corrosion (after 72 h) it changes to FeCO₃. The corrosion scale consists of inner and outer scales. HCO₃^- penetrates the scale, reaches the scale/substrate interface and reacts with substrate to form the inner scale, while the formation of the outer scale is attributed to the concentration of Fe²⁺ and CO₃^2- in the solution exceeding the solubility of FeCO₃. The grain size of the outer scale is smaller and denser than that of the inner scale. The adhesion intensity between the inner and outer scales is weak but that between the inner scale and substrate is strong.