Investigation of Pseudo-Passivation of Mild Steel in CO₂ Corrosion

The iron carbonate corrosion product layer formed on mild steel in carbon dioxide (CO₂) environments is known to retard corrosion. When not fully covering the steel surface, it may also lead to initiation of localized corrosion, due to a galvanic effect. In this work, the stability of a protective iron carbonate layer has been studied at 80°C over a relatively wide range of bulk pH. Experiments were done in a glass cell using a three-electrode system. Electrochemical techniques such as linear polarization resistance (LPR) and potentiodynamic polarization (PP) were used. Surface analysis techniques (scanning electron microscopy [SEM], x-ray diffraction [XRD], and transmission electron microscopy [TEM]) were used to confirm the composition and structure of the protective layer. Experimental results confirmed a pseudo-passive behavior, indicated by a positive shift in the open-circuit potential and a significantly retarded corrosion rate for systems at pH 6.0 and above. However, a stable and protective pseudo-passive layer could not be formed at pH 5.6 or lower.