Iron Carbide and Its Influence on the Formation of Protective Iron Carbonate in CO₂ Corrosion of Mild Steel

The present research studied the role porous iron carbide layers (Fe₃C) on the formation of protective iron carbonate (FeCO₃). Two mild steels with similar carbon content and different microstructure were chosen. API⁽¹⁾ 5L X65 and UNS⁽²⁾ G10180 steels with tempered martensite and ferrite - pearlite microstructures, respectively, were exposed to a 3 wt.% NaCl-CO₂ saturated solution at pH 6 and 80°C in laminar flow. The corrosion behavior was monitored by linear polarization resistance (LPR) during long term experiments. Electrochemical measurements were supported by surface analytical techniques (Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS)). A porous Fe₃C layer was observed for both steels during the first days of exposure, although morphology was different due to the steel microstructure. With time, the iron carbide layers observed for both steels acted as a diffusion barrier for ferrous and carbonate ions, resulting in a high surface pH, so that when the solubility limit of FeCO₃ was exceeded, a protective FeCO₃ scale formed within the porous layer regardless of the steel microstructure.