Electrochemical Corrosion Behavior of X70 Pipeline Steel in Wall Jet Zone under Jet Impingement at High Temperature and High Pressure CO_2 Environment

摘要

The electrochemical corrosion behavior of X70 pipeline steel in the wall jet zone was investigated at high temperature and high pressure CO_2 environment by jet impingement loop equipment with micro-electrode techniques. The corrosion morphologies and profiles of X70 steel after different corrosion times were characterized by Scanning Electron Microscope (SEM) and 3D optical measurement device. Results showed that the corrosion process of X70 steel was dominated by the anodic dissolution in the first 4 hours (4h), during which the highest corrosion rate was reached. With the increase of the coverage and compactness of corrosion products on the surface, the corrosion resistance of X70 steel increased, and the corrosion rates decreased significantly. The profile of the X70 surface showed gradually increasing tendency with the increasing distance under the influence of wall shear stress. The Open Corrosion Potential (OCP) and Linear Polarization Resistance R_p (LPR) of X70 steel firstly decreased and then increased as a function of corrosion time. At 4h, the Electrochemical Impedance Spectroscopy (EIS) plot was composed of the high-frequency capacitive loop, the medium-frequency capacitive loop and the low-frequency inductive loop. The inductive loop disappeared with the increasing time, which showed that the surface of X70 steel was completely covered by the compact corrosion products. The film resistance R_f and charge transfer resistance R_t increased gradually with time owing to the formation and growth of corrosion scale. The double-layer capacitance C_(di) decreased slowly as a function of time, which is mainly attributed to the increase of the thickness of protective layer. Moreover, the film capacitance C_f increased rapidly in the first 10h and then showed a slowly decreasing tendency.