The Effects of pH Values on Functional Mechanisms of Nitrite Anions for Q235 Carbon Steels in 0.01 mol L^-1 NaNO₂-HCl Solutions

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^-.

Keywords:
Q235 carbon steel; pH value; NO₂^-; corrosion; passivation; polarization