Synergistic effect of Cl^- and HCO₃^- on the anodic electrochemical behavior of N80 steel

The anode electrochemical behavior of N80 steel in different concentrations of chloride and bicarbonate solutions was studied by potentiodynamic polarization curves and dynamic electrochemical impedance spectroscopy (DEIS). It is found that N80 steel is passivated without Cl^- when the concentration of HCO₃^- exceeds 0.03 mol·L^-1. Cl^- can greatly improve the critical value of passive concentration of HCO₃^- and the passive film is seriously damaged meanwhile. In the anode region, the pitting potential drops, the passive current density increases, and the passive potential region narrows with Cl^- addition in the solution with the same HCO₃^- concentration. DEIS results correspond with the potentiodynamic polarization curves. According to the relationship between the polarization potential and electrical components in the equivalent circuit, the roughness and compactness of the passive film decrease with Cl^- addition, and the increase in passive current density depends on the decrease in charge transfer resistance mainly.