Corrosion Behaviors Of X70 Pipeline Steels With Coating Holidays

Studying the corrosion behaviors of X70 pipeline steel with coating defects and wedge-shaped simulated crevice in the simulated solutions that are base on the Baotou area soil in order to replenish the blank of pipeline steels causticity investigation in Inner Mongolia Autonomous Region .Electrochemical behaviors of X70 steel were studied in the simulated solutions.X70 steel's polarization curves of measurement result indicate : in open system, the corrosion degree of X70 steel increases with the increasing of HCO₃^- concentration in the solution. Fixed the Cl- and HCO₃^- concentration, with the SO₄^2- joining, the corrosion degree of the steel increases. comparing in the simulate solutions and the soil lixivium, corrosion degree of X70 steel is bad in the simulate solutions.The X70 pipeline steel as the body, The samples with the different sort and the different size circle defect which was made with epoxy coal pitch were immerged in the simulated solution, corrosion happened in the defects. Result of Ecorr and EIS diagram indicate the corrosion degree of X70 steel's samples with disbonded and broken coating defects was most slight, disbond take second place, it is worst that the corrosion degree of X70 steel's samples with broken coating defect.In the several different sizes of circle broken defects, following the diameter of defects increasing, the corrosion degree worse and worse. Corrosion medium was diffused from the joint between coating and base metal.A self-made wedge-shaped simulated crevice was used to investigate the solution chemistry and the electrochemical behavior of the metal in the crevice. Measured distribution of pole potential and pH value of the solution in the crevice under the natural corrosive state. Measured distribution of pole potential, current and pH value of the solution in the crevice forcing different cathodic polarization potential.The result shows: in all of simulated solutions, the electrode potential turned more and more negative , the pH value turned more and more small from the mouth of the crevice to the tip of the crevice under the natural corrosive state. the total trend is to the sensitivity of decay more and more strong.Under cathodic polarization condition,following the polarization potential negative, the electrode potential corresponds to negative , the current density gradual enlarge and pH value rise in the crevice. When the polarization potential is less than -925mVSCE, the every sample in the crevice can get protection, don't take place the gap corrosion; While the polarization potential is -775mVSCE, miss to the protective potential, bring the gap decay.