Study on under-deposit corrosion behavior of X65 steel in simulated CO2 saturated produced water

Under-deposit corrosion is a key issue in various processes of oil and natural gas production. In this paper, the corrosion behavior of X65 steel under CaCO3 scales was investigated, which are commonly occurred in such environments. To simulate different conditions of CaCO3 scaling, accelerated scaling was achieved through cathodic polarization on X65 steel samples. Subsequently, chemical immersion of weight loss test was performed over 72 h in a simulated produced water at 40°C with a CO2 partial pressure of 0.5 MPa (total pressure of 1 MPa). The results indicated that pre-scaling duration significantly influenced the corrosion rate of X65 steel. Notably, samples scaled for 6 h exhibited a corrosion rate of 0.1572 mm/a, markedly higher than 0.0371 mm/a of unscaled samples. High-temperature and high-pressure in situ electrochemical measurements were carried out to reveal the electrochemical behavior of X65 steel with varying pre-scaling time, including open circuit potential, linear polarization resistance, electrochemical impedance spectroscopy and polarization curves. Additionally, scanning electron microscopy was used to observe the surface morphology and cross-sectional microstructures before and after product layer removal, which facilitates a discussion on the mechanisms of under-deposit corrosion.