For Q235 carbon steels in 0.01 mol L-1 NaNO2-HCl solutions with different pH values, the effects of pH values on the functional mechanisms of nitrite anions (NO2-) were studied. In the pH 1 and pH 2 solutions, the Q235 steels showed the electrochemical behavior of activation, and uniform corrosion (UC) was the main corrosion type on the Q235 surface. The presence of NO2- did not affect the electrochemical behavior and the corrosion type but only accelerated the anodic reaction of Fe oxidation. In the pH 3 and pH 4 solutions, the Q235 steels presented the electrochemical behavior of activation-passivation transition, and intergranular corrosion (IGC) occurred on the Q235 surface. The cathodic reaction of NO2- reduction resulted in the transition of electrochemical behavior from activation to passivation and the change of corrosion type from UC to IGC. In the pH 5 and pH 6 solutions, the Q235 steels exhibited the electrochemical behavior of self passivation-pitting, and pitting corrosion (PC) was present on the Q235 surface. The strong oxidability of NO2- played a critical role in the formation of γ-Fe2O3 passive film on the Q235 surface under the combined action of Fe2+ and NO2-.

For carbon steels immersed in CO2 saturated solutions at different temperatures, the structure and the composition of corrosion product film formed on the steel surface were studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion rate of the steel was evaluated by potentiodynamic polarization, and the relation between the corrosion rate and the film property was discussed. The corrosion rate of the steel was very closely associated with the structure and the composition of corrosion product film, which were affected significantly by the solution temperature. From 30 to 60 ºC, the corrosion product film composed of FeCO3 was porous and poorly adherent, and the corrosion rate increased with the rise of temperature. At 70 and 80 ºC, the corrosion product film was also composed of FeCO3 and presented a compact and dense cubic crystal structure, resulting in the decrease on the corrosion rate. The corrosion rate increased once again when the temperature was up to 90 ºC, which was attributed to the negative effect of high temperature water vapor corrosion on the grain coarsening and the part exfoliation for the FeCO3 film.