Effects of Temperature on the Corrosion Behaviour of X70 Steel in CO2-Containing Formation Water

Abstract: The corrosion behaviour of X70 pipeline steel in CO2-containing formation water at various temperatures was studied in a high-temperature and high-pressure reactor via weight loss measurements, scanning electron microscope, X-ray diffraction, energy dispersive spectroscopy, and electrochemical methods. The concentrations of CO2 and ions in formation water at 30∼150°C were calculated. The equilibrium potentials of anodic and cathodic reactions were determined by thermodynamic equations, and the main cathode reaction was hydrogen ion reduction. X70 steel showed the uniform corrosion in the temperature range of 30 °C - 90 °C and the localized corrosion at 120 °C and 150 °C. The supersaturation (S) was not proportional to the protection effect of FeCO3 scale. At 30°C, the deposition rate of FeCO3 was too slow to form a protective film, so the corrosion rate was high. At 60 °C, the deposition rate of the FeCO3 was lower than the rate of film undermining and the corrosion product scale was non-protective. At 90 °C, although the S was the smallest, the FeCO3 scale was dense and protective. At 120 °C, the size of FeCO3 crystals increased significantly. A void space was formed between the grown grains and the matrix surface, whereas the immature grains still showed the good density and had the protection effect on the matrix. At 150 °C, the rapid growth of FeCO3 crystals led to the increase in the stress in the FeCO3 scale and the scale finally ruptured. H+ produced in the ionization reactions of H2CO3 and HCO3- resulted in local acidification and pitting corrosion.