The effects of temperature and pH on the characteristics of corrosion product in CO2 corrosion of grade X70 steel

The effects of temperature and the in situ pH on the composition and morphology of corrosion product layers in the CO2 corrosion of X70 steel were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, respectively. The experiments were carried out in a range conditions including temperature 55–85 °C and pH 5.5–6.5 over a period of 72 h. At 55 °C, no corrosion product was formed on the steel surface at any pH conditions. By increasing the temperature to 65 °C, iron carbonate (FeCO3) phase was formed at all pH conditions with an apparently non-compact morphology. Raising the pH increased the compactness of the layer. At 75 and 85 °C, a compact layer was formed at all pH conditions, while the most compactness was seen at pH 6.5 for both of these temperatures. It was also observed that the thickness of FeCO3 layer increased with increasing temperature. Therefore, it could be concluded that the optimum conditions for producing a compact and thick layer of FeCO3 was obtained at temperature of 85 °C and pH 6.5.