Galvanic Interactions between Surface Layers and Bare Carbon Steel in Aqueous CO2 Environments

Iron carbonate (FeCO3), magnetite (Fe3O4) and iron carbide (Fe3C) layers are able to form on carbon steel surfaces in aqueous carbon dioxide (CO2) environments, and suspected to play a critical role in uniform and localized corrosion of the underlying steel. Fe3C and Fe3O4 are known to establish micro-galvanic cells with uncovered regions of the steel surface, due to their conductive and semi-conductive nature respectively, enhancing corrosion rates. To evaluate the significance of the layers' galvanic corrosion in an aqueous CO2 environment, layers of Fe3C, Fe3O4 and FeCO3 were formed on X65 carbon steel surfaces. The coupons were then galvanically coupled to a bare X65 carbon steel coupon at different area ratios (AR = 1 and 10) in a pH 5, 1 wt.% NaCl, CO2-saturated solution at 50°C. Galvanic currents were measured using zero resistance ammetry over 24 h, with similarly high galvanic currents measured for the Fe3C-bare steel and Fe3O4-bare steel couples, enhancing the bare steel corrosion rates. The galvanic current was significantly smaller and reversed for the FeCO3-bare steel couple, enhancing rather the corrosion rate of the FeCO3-covered coupon.