Electrochemical Corrosion Behavior of X70 Pipeline Steel in Turbulence Zone under Jet Impingement at High Temperature and High Pressure CO₂ Environment

The corrosion behavior of X70 pipeline steel in the turbulent zone was investigated in situ with a micro-electrode technique using loop jet impingement under high temperature and high pressure CO₂ environment. The morphology of the corrosion product formed on the surface and corrosion behavior of X70 steel after different periods were investigated by scanning electron microscopy and in situ electrochemical methods, respectively. The electrochemical behavior of X70 steel was closely related to the evolution of corrosion scales on the steel surface. The surface of the steel changed gradually from the presence of both substrate and corrosion product to loose, porous corrosion scales during the first 12 h. After 12 h, the corrosion scales were mainly composed of inner and outer scales. Because of the effect of high wall shear stress in the turbulent zone, the porous, less-protective outer scale was thinned and then removed from the steel surface. Consequently, the surface was increasingly covered by the compact inner scale, which decreases the corrosion rate of the steel considerably. Correspondingly, during the first 12 h, the corrosion potential E _corr and linear polarization resistance R _p of the sample decreased continuously. Meanwhile, electrochemical impedance spectroscopy (EIS) exhibited high- and medium-frequency capacitive loops and a low-frequency inductive loop. Analysis of EIS revealed that the resistance R _f of the corrosion film increased slowly and charge transfer resistance R _t decreased steadily, while the double-layer capacitance C _dl and corrosion film capacitance C _f decreased rapidly. After 12 h, the protectiveness of the corrosion scales improved with time, and thus the E _corr and R _p increased. As the inductive component weakened with time and finally disappeared at 48 h, EIS changed to double capacitive loops. The R _f, Rr, and C _dl increased quickly. Furthermore, the C _f stabilized.