Numerical Study to Investigate the Influence of Porous FeCO₃ Layers on the CO₂ Corrosion Rate of Carbon Steel

The formation of protective iron carbonate (FeCO₃) corrosion product layers on X65 carbon steel has been investigated experimentally in high-temperature, high-pressure autoclave corrosion tests. Corrosion testing was performed in carbon dioxide (CO₂)-saturated, 3 wt.% sodium chloride (NaCl) brine at an operating temperature of 80 °C and pressure of 5 bar. The evolution in corrosion rate and mass transfer processes during the formation of an FeCO₃ layer were monitored using in-situ linear polarization resistance and electrochemical impedance spectroscopy. Over time, the precipitation of FeCO₃ resulted in the formation of a high-density layer that formed an effective diffusion barrier and reduced the corrosion rate of the underlying carbon steel by over an order of magnitude. The restricted corrosion rate in the presence of a porous FeCO₃ layer is subsequently represented in a one-dimensional mechanistic CO₂ corrosion model. A parametric study was conducted to investigate the effects of surface coverage from FeCO₃ and the effective diffusion coefficient through the layer on the predicted corrosion rate of the underlying carbon steel surface.