Effects of Cl^- and SO₄^2- ions on corrosion behavior of X70 steel

Corrosion behaviors of X70 steel were studied by means of electrochemical experiments and morphology observation, First, through potentiodynamic polarization in solution of various Cl^- ions concentration, it was found that E_pit began to appear in solution of Cl^- concentration above 0.1 mol/L, and there was a critical point of Cl^- concentration between 0.05 mol/L and 0.1 mol/L, below which the extent of pitting and general corrosion were trivial, while in solution of Cl^- concentration above 0.1 mol/L, general and pitting corrosion became greater as the increasing of Cl^- concentration. All of them were confirmed by the SEM observations after anodic polarization. Second, via the potentiodynamic polarization curves of X70 steel in 0.5 mol/L Cl^- solution with 0, 0.05, 0.5 and 1 mol/L SO₄^2- ions, it was found SO₄^2- ions were able to inhibit corrosion aroused from Cl^- ions, accordingly a model was set up to describe the process. In addition, to further explore the inhibited effect of SO₄^2- ions, EIS was used in solutions of different Cl^- and SO₄^2- concentrations, the results revealed that the electrochemical resistance has a relation with the [SO₄^2-]/[CI^-], that was, the bigger the value of [SO₄^2-]/[CI^-], the greater the electrochemical resistance.