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

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.