Research On Pitting Behaviors Of X120 Pipeline Steel In HCO₃^- +Cl^- Solution

Due to its high-strength and high-ductility, X120 pipeline steel will be widely applied in oil & gas transmission. Pitting corrosion may occur and threaten the service safety of the buried pipeline steel. NaHCO₃+NaCl solution was used as basical corrosion environment, the influences of environmental factors(Cl^- concentration, pH, temperature) and stress on pitting susceptibility of X120 pipeline steel were discussed and its metastable pitting behaviors were investigated in this paper.The pitting resistant of X120 pipeline steel changes a lot in different corrosion environment. The relative influences of Cl^- concentration, pH and temperature on pitting susceptibility of X120 pipeline steels in NaHCO₃+NaCl solution were studied using uniform design experimental method by potentiodynamic scan technique. The results show that: the order of the influence of three factors is as follows, the Cl^- concentration has the gratest impact, then combined action of the Cl^- concentration and the temperature, the third one is pH value, and the temperature has the smallest impact(when Cl^- concentration is 0⁰.02 mol·L^-1, T=0⁵0℃, pH=8.5¹1). X120 pipeline steels has the least pitting susceptibility when the Cl^- concentration is 0, the pH is 8.5 and the temperature is 0℃.The results of EIS and Mott-schotkky tests of X120 pipeline steel in the 0.5 mol·L^-1 NaHCO₃ +0.02 mol·L^-1 solution demonstrated that pitting susceptibility depends on the applied stress. When the applied stress is lower than 100 Mpa, the pitting susceptibility of X120 pipeline steel decreases with the increasing of stress owing to the inhibition effect of corrosion products. When the applied stress raises from 100 Mpa to 700 MPa, the pitting susceptibility of X120 pipeline steel increases because the stress improves the material's activity and makes passive film breakdown.The metastable pitting behaviors of X120 pipeline steel were invested by potentiostat polarization. The results indicated that the applied potential has a great effect on metastable pitting behaviors of X120 pipeline steel: when the applied potential increase from 0.1 V to 0.2 V, the stable passive current density of X120 pipeline steel raises from 0.55 to 0.87μA·cm^-2, the corrosion current density in Cl^- included solution changes from 1.9³ to 3.8⁷μA·cm^-2, the metastable pitting lifetime prolongs from 15 to 25 s, and the radius of metastable pitting enlarges from 0.685 to 1.08μm.