Effect of HCO3 − on electrochemical kinetics of carbon steel corrosion in CO2-saturated brines

Highlights: • HCO₃⁻ acted as an additional hydrogen source for the cathodic reaction. • Active dissolution mechanism was discussed based on anodic Tafel slopes. • Half reactions were studied independently on the effect of HCO₃⁻. • CO₂(aq) was verified to participate in the Fe oxidation reaction. • Corrosion products transitioned from iron carbonate to hydroxide as pH increased. The effect of HCO₃⁻(aq) on CO₂ corrosion of carbon steel was investigated in deaerated 3.5 wt% NaCl solutions at 30 °C from pH 3.96 to 7.15. In the CO₂-saturated solutions, the pH was adjusted with different HCO₃⁻(aq) concentrations, c[HCO₃⁻(aq)]. The corrosion rate decreased by a factor of 2 as the pH and c[HCO₃⁻(aq)] increased. The cathodic current density during polarization increased at higher pH with higher c[HCO₃⁻(aq)], indicating that HCO₃⁻(aq) acted as an additional hydrogen source for the hydrogen evolution reaction. As the pH increased, the active dissolution regions displayed similar anodic Tafel slopes and suggested a modified Bockris mechanism for the Fe oxidation reaction. The exchange current densities for the half reactions were calculated to study kinetics of the anodic and cathodic half reactions independently. The anodic exchange current density (j_0,a) increased by one order of magnitude in the presence of CO₂, indicating the involvement of CO₂(aq) in the Fe oxidation reaction. As the pH and c[HCO₃⁻(aq)] increased, the cathodic exchange current density (j_0,c) decreased by a factor of 50 because the increase in j_0,c[HCO₃⁻(aq)] was not high enough to compensate the decline from the other hydrogen sources, especially j_0,c[H⁺(aq)]; and j_0,a decreased by a factor of 2.4 because HCO₃⁻(aq) may have competed with CO₂(aq) for the surface coverage and the increase in j_0,a[HCO₃⁻(aq)] could not compensate the decrease in j_0,a[CO₂(aq)]. It suggests that the reaction rate constant of HCO₃⁻(aq) was smaller than CO₂(aq) for the anodic half reaction and was smaller than H⁺(aq) for the cathodic half reaction. The XPS results verified that the corrosion products transitioned from iron carbonate to hydroxide as the pH increased while iron carbonate remained the major product. As the pH increased with HCO₃⁻(aq), a second time constant was observed at lower frequencies of the electrochemical impedance spectroscopy (EIS) results.