Effect of Cl^- on Electrochemical Behavior of X80 Pipeline Steel in HCO^-₃ Solutions

The electrochemical behavior of X80 pipeline steel in NaHCO3/NaCl solutions was studied using potentiodynamic polarization curves, electrochemical impedance spectroscopy(EIS) and SEM. The results show that the passivation region becomes narrower, the pitting potential decreases and the passivation current density increases gradually with the increase of Cl- concentration, and the corrosion resistance of X80 pipeline steel lowers. In HCO3-/Cl- solutions, Cl- competes with HCO3- for adsorption on the surface of X80 pipeline,which results in two kinds of effects: the passivation effect of HCO3- and the aggressive effect of Cl-. The passive film thickens and the anodic current density decreases because of the former effect, which keeps the metal in passive state. On the contrary, the latter effect makes the passive film thinned or the solution conductance increased, which leads to the increase of anodic current density and to the loss of metal passivity. The point defect model was used to explain the initiation and breakdown process of passive film. The corrosion resistance of X80 pipeline steel in HCO3-/Cl- solutions system is strongly relevant to the concentration ratio of HCO3- to Cl-. The corrosion resistance of X80 pipeline steel is strengthened with the increase of HCO3- concentration and weakened with the increase of HCO3- concentration.