Effect of FeCO3 Supersaturation and Carbide Exposure on the CO2 Corrosion Rate of Carbon Steel

The pH stabilization technique is a widely used corrosion protection method for multiphase gas pipelines with glycol as hydrate inhibitor. It implies to increase the pH by addition of HCO₃^- in order to enhance the formation of protective iron carbonate films. At temperatures around 20 °C the conditions for precipitating protective corrosion film are less favourable due to the decreasing supersaturation of FeCO₃ with decreasing temperature. The objective of this work is to clarify if corrosion mitigation of pipelines at low temperature relies on the formation of protective corrosion films or if the corrosion rate is sufficiently lowered by the elevated pH.

This paper discusses the corrosion rate and corrosion potential response observed on carbon steel exposed to varying concentrations of HCO₃^- and Fe²⁺ at 20 °C. Steels with different carbon contents were conditioned by anodic polarization in order to dissolve iron and expose the carbide before adding Fe²⁺ and HCO₃^- to supersaturate the solution. The amount of carbide as well as the FeCO₃ saturation ratio influenced the protective properties of the corrosion film.

Keywords: CO₂ corrosion, pH stabilization, FeCO₃ supersaturation, film formation, precipitation, anodic polarization.