The influence of 0.01 mol L-1 NaNO2 addition on the corrosion and passivation of X65 carbon steel in pH 1 to 5 HNO3 solutions was investigated and compared by electrochemical methods and microstructural techniques. In the pH 1 to 5 solutions without NaNO2, the X65 steel presented the electrochemical characteristic of active dissolution, and the corrosion rate of X65 carbon steel decreased gradually with the raise of pH value. By contrast, with the addition of 0.01 mol L-1 NaNO2 in pH 1 to 5 HNO3 solutions, the electrochemical characteristic of X65 carbon steel transferred from the active dissolution in the pH 1 to 5 solutions without NaNO2 to the anodic passivation in the corresponding pH solutions with NaNO2. For the X65 steel in the pH 1 to 5 solutions with NaNO2, with the raise of pH value, the corrosion rate also decreased gradually but the passivation capability strengthened obviously. The corrosion and passivation of X65 carbon steel in pH 1 to 5 HNO3 solutions without and with 0.01 mol L-1 NaNO2 were related to the cathodic reactions of H+ reduction, O2 reduction and NO2-/HNO2 reduction.

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-.