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.
In aqueous carbon dioxide (CO2)-saturated environments, such as those found in geothermal energy, oil and gas and carbon abatement industries, various naturally occurring layers can be found on the internal surface of carbon steel infrastructure, such as pipelines, as they corrode in the mildly acidic conditions. Amongst the most commonly found layers are iron carbonate (FeCO3)1-7, iron carbide (Fe3C)8, 9 and magnetite (Fe3O4).10, 11 FeCO3 can offer corrosion protection to the underlying steel when formed under certain conditions, as too can Fe3O4. Fe3C is typically associated with enhancement of electrochemical activity of carbon steel and is revealed due to preferential dissolution of ferrite in the steel microstructure – through the formation of a porous network at the steel surface. Each of these layers play a fundamental role in the uniform and localized corrosion of the underlying carbon steel.