Effects of Medium Condition on CO₂ Corrosion of X65 Pipeline Steel and Its Welded Joint

The effects of medium condition on CO₂ corrosion of X65 pipeline steel and different zones of its welded joint were studied in 3.5% NaCl solution with saturated CO₂ by open-circuit potential measurement, electrochemical impedance spectroscopy testing and scanning electron microscopy (SEM) observation. The results show that a dense corrosion product film of FeCO₃ was difficult to form on the surface of X65 pipeline steel in low pH environment. But it could form in high pH environment even at a low temperature, the higher the temperature, the denser the corrosion product film. And with the increase of temperature the average corrosion rate of X65 pipeline steel decreased and the pitting corrosion resistance became worse. In the different zones of X65 pipeline steel welded joint, the base metal formed a dense corrosion product film first, followed by the heat affected zone, and finally the weld zone. The product film formed on the weld zone was denser than that formed on the base metal and heat affected zone.