Study of Electrochemical Corrosion Behavior on X70 Natural Gas Pipeline Steel under CO₂ System

Abstract

The electrochemical corrosion behavior of X70 pipeline steel under four different CO2 pressures was investigated by electrochemical test methods and scanning electron microscope analysis, based on the simulated environment of the extracted water of natural gas transmission pipeline in an oilfield of Xinjiang. Results showed that in simulated extracted water solutions with high chloride ion content, the electrochemical impedance spectra(EIS) and polarization curves indicated that the corrosion of X70 steel specimens became increasingly severe with increasing CO2 pressure(0.5, 1.0, 2.0, 3.0 MPa). The results of the loss-in-weight experiments showed that the corrosion rate of the X70 steel specimens increased with increasing CO2 pressure, with a maximum corrosion rate of 2.573 9 mm/a at a pressure of 3 MPa. In addition. the XRD results indicated that the corrosion products were mainly FeCO3. The SEM results showed that the pitting on the surface of the specimen became increasingly severe with increasing CO2 pressure, and the surface of the specimen became increasingly rough. These results were attributed to the effect of high pressure CO2 and pitting exacerbated by high Cl- concentrations. Hence, it could be found that high CO2 and Cl- had a serious damaging effect on X70 pipeline steel. This work provided reference and guidance for the prevention of this type of corrosion.

Keywords

• x70 steel; natural gas pipelines; carbon dioxide corrosion; pitting; chloride ions