Abstract: 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 Epit 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 SO¬42- ions, it was found SO¬42- ions were able to inhibit corrosion aroused from Cl-1 ions, accordingly a model was set up to describe the process. In addition, to further explore the inhibited effect of SO¬42- ions, EIS was used in solutions of different Cl- and SO¬42- concentrations, the results revealed that the electrochemical resistance has a relation with the [SO42-]/[Cl-], that was, the bigger the value of [SO42-]/[Cl-], the greater the electrochemical resistance.

Key words: X70 steel, Chloride ions, Sulphate ions, Pitting, General corrosion