Effect of alternating current and Cl- ions on corrosion behavior of X70 steel in NaCl solution

The combined effect of alternating current (AC) and Cl- on the corrosion behavior of X70 steel was studied by conducting electrochemical experiments and evaluating corrosion morphology characteristics. The results show that the effect of AC on the corrosion behavior of steel was related to Cl- concentration. The extent of potential that negatively shifted when AC was applied decreased with increasing Cl- concentration. The corrosion rate enhanced as AC increased when the Cl- concentration within 0.3 mol/L, but decreased with AC current density as the concentration of Cl- further increased. The double-layer capacitance increased with the increase in Clconcentration. When the concentration of Cl- exceeded 0.3 mol/L, the charge-transfer resistance increased with AC current density. When the concentration of NaCl was below 0.3 mol/L, the corrosion process was mainly controlled by AC, and when the concentration of NaCl increased to 0.5 mol/L, the corrosion process was mainly controlled by dissolved oxygen. AC may inhibit the diffusion of oxygen when the concentration of NaCl solutions exceeded 0.3 mol/L. This reduced the progress of cathodic reaction, resulting in a decrease of the corrosion rate. Therefore, with the increase in Cl- concentration, the effect of AC on the corrosion process of X70 steel was weakened. In addition, the effect of Cl- concentration and AC on the dissolution and diffusion of oxygen in the solution affected the corrosion process. Moreover, the corrosion morphologies indicate that the pitting of X70 steel was aggravated by both Cl- and AC, but the effect of AC on pitting is more significant than Cl-.