Investigation of Pseudo-Passivation of Mild Steel in CO₂ Corrosion

The iron carbonate corrosion product layer formed on mild steel in 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 electrochemical impedance spectroscopy (EIS) were used. Surface analysis techniques (SEM, XRD, TEM) were employed 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.