Influence of CO₂ partial pressure on the protection property of corrosion scale formed on N80 steel

Corrosion tests of N80 steel under different CO₂ partial pressures at 65℃ and 90℃ were carried out by a self-made high-temperature high-pressure autoclave. The mass losses of the steel under different CO₂ partial pressures were measured. The morphologies of the corrosion scale and the descaled N80 steel were observed by SEM, and the thickness of the scale was measured. Electrochemical polarization curve and impedance spectroscopy analysis (EIS) were employed to study the corrosion behavior of N80 pre-corroded under different CO₂ partial pressures at two temperatures. The results show that as the CO₂ partial pressure increases, the protection property of the scale becomes better, but the corrosion rate increases because of the increase in solution corrosivity. The local defects of the corrosion scale are responsible for the pitting corrosion of metal matrix, the increase in CO₂ partial pressure makes for reducing the defects. Compared with 65℃, the protection property of the corrosion scale is more sensitive to the change of CO₂ partial pressure at 90℃.