Electrochemical Behavior Research Of X80 Pipeline Steel Stress Corrosion

Recently, with the preliminary application of X80 pipeline steel in the west-east gas pipeline project, the buried pipeline will be suffered the conbined action of external environment and stress in the service process, the service safety and integrity of pipeline steels will be seriously affected. The synergies of environmental factors and stress have larger influence on pipeline steel corrosion as soil environment is complicated and the influence factors are numerous. Therefore, it is extremely positive significance to carry out the relevant basic research that corrosion characteristics and stress corrosion behavior of X80 pipeline steel in simulated soil solution which be influenced by environmental factor and stress.This paper selects the Na HCO_3-Na2CO₃ and NS4 solution, as the basis of simulated soil solution. Electrochemical testing methods such as potentiodynamic polarization and electrochemical impedance spectroscopy use to study the influence law of corrosion behavior of X80 pipeline steel, which the temperature and concentration change in in two kinds of simulated solution. Meanwhile, temperature and ion concentration changes were studied on the influence law of the stress corrosion behavior by four-point bending loading methods. The main conclusions are as followed.The corrosion process X80 pipeline steel in Na₂CO₃ solution was self-passivation mechanism. The corrosion process was active-passive mechanism when added 1mol/L Na HCO₃, and two anodic peak appeared. The passive film protection incresed with the concentration of CO₃^2-, and corrosion rate reduced. The corrosion process of X80 pipeline steel in Na HCO₃ solution was active-passive mechanism. Concentration of HCO₃- impacted anodic peak number in transition passivation zone. Corrosion rate increased with concentration of HCO₃-.The influence of stress change on its corrosion potential and corrosion current was not big in high p H solution. With or without applied stress, there will be a stable passivation range when the concentration of Cl^- less than 0.1mol/L. Stable passivation dispeared when the concentration Cl^- greater than 0.3 mol/L.The corrosion behavior of X80 pipeline steel in NS4 solution changed at different atmosphere conditions. Corrosion rate of X80 pipeline steel increased after introducing O₂ and CO₂, and corrosion rate increases with CO₂ concentration. The increase of SO₄^2-and Cl^- had little impact on corrosion potential, the electrode surface passive when the concentration of HCO₃- increases to 0.087mol/L.The stress changes had no significant effect to E_corr in NS4 solution. The corrosion potential obviously changed with Cl^-, SO₄^2- and HCO₃^- at different concentration under applied stress. Compare the presence or not of applied stress, the concentration of Cland SO₄^2- for pipeline steel had a greater influence on the stress corrosion tendency.