Effects of Cl, NO3 and SO4 anions on the anodic behavior of carbon steel in deaerated 0.50 M NaHCO3 solutions

The effects of Cl⁻, NO₃⁻ and SO₄²⁻ aggressive anions on the corrosion and passivation behavior of carbon steel electrode in deaerated 0.50 M NaHCO₃ solutions were studied using potentiodynamic anodic polarization and SEM techniques. It was found that the presence of Cl⁻, NO₃⁻ and SO₄²⁻ anions stimulates the anodic dissolution rate in both the active and the pre-passive potential regions. Moreover, significantly great effects were observed in both the passive and the trans-passive potential regions. Pitting corrosion was observed only in the presence of Cl⁻ anions, while the presence of NO₃⁻ and SO₄²⁻ anions facilitate only passivation by oxygen of water without themselves participating in the cathodic process. Also, it was observed that the effect of NO₃⁻ anion, which is a strong oxidizing agent acting “primarily” as stimulator of the cathodic process and then its reaction product acts “indirectly” retarding the anodic process. On the other hand, the effect of SO₄²⁻ anion, which is a non-oxidizing agent, exerts an “indirect” effect on the cathodic reaction increasing its rate and then “directly” influence on the anodic reaction, retarding it.