Removal of Iron Carbide in Turbulent Flow Conditions and Influence of Iron Carbonate Formation in Aqueous CO2 Corrosion of Mild Steel

Abstract

Iron carbide or cementite (Fe3C) is often classified as a “corrosion product” but it is originally found in the material’s microstructure and, unlike iron carbonate (FeCO3), it is not precipitated on the steel surface. Rather, it represents the leftover steel structure once the ferrite phase has corroded away. Various researchers have found that Fe3C acts as a diffusion barrier between iron and carbonate ions, which aids in the precipitation of FeCO3. Previous studies have also considered various material compositions and microstructures favoring FeCO3 formation. However, the effect of flow has not been considered previously although it plays a critical role in Fe3C adherence to the steel surface as it is a fragile leftover layer. In this study, a ferritic-pearlitic UNS G10180 material was exposed to flow velocities (0.4 m/s, 2 m/s, and 6 m/s) and shear stresses (0.8 Pa, 20 Pa, and 100 Pa) in a thin rectangular flow channel at favorable layer formation conditions (T = 80°C, pH 6.6, initial [Fe2+] = 2 ppm, initial S(FeCO3) ≈ 10). A critical velocity for Fe3C removal was identified, which further prevented the formation of FeCO3, although it is fully expected that its value should depend on the operating conditions.