Galvanic mechanism of localized corrosion for mild steel in carbon dioxide environments

Historically, the mechanism of localized corrosion in CO2 (sweet) environments has been poorly understood. This shortcoming is an obstacle in the development of corrosion control and protection protocols. The purpose of this PhD project was to explore and understand localized sweet corrosion mechanisms through systematic study. An artificial pit cell was developed in order to directly measure the galvanic current resulting from localized corrosion propagation. Thus, galvanic mechanisms of localized CO2 corrosion were elucidated. It was found that two surfaces coexist as termed anode (bare surface in the pit) and cathode (surrounding surface usually covered by corrosion scales) with open circuit potentials (OCP) for these different surfaces being different under the same bulk environments. This potential difference can be the driving force for localized corrosion propagation when the reactions on the two surface balance (a mixed potential is reached). A "gray zone" criterion was determined through experiments and theory to explain localized CO2 corrosion propagation. It was concluded that localized corrosion propagates when the conditions are near the saturation point for iron carbonate, i.e. in the "gray zone". Under this condition, which is neither highly supersaturated nor undersaturated, the pit area stays scale free while the scale remains on the surrounding cathode surface. Electrochemical studies demonstrated that passivation, especially spontaneous passivation, can occur on the cathode surface and that results in a higher open circuit potential on the cathode. Surface analysis using GIXRD and TEM/EDX showed that beneath an iron carbonate film formed first a passive film is formed due to the local high pH conditions underneath the FeCO3 film. The passive film was identified and confirmed to be magnetite, Fe3O4, under the test conditions using X-ray diffraction (XRD) with grazing incidence, its thickness being at the nanometer level, as detected by TEM/EDX. This passive film is responsible for the spontaneous passivation of the surface and causes the more positive open circuit potential compared with that on the bare surface (pit area). In order to confirm the passivation mechanism, a surface pH probe was developed. The surface pH measurements under simulated iron carbonate scale showed a higher pH value, which was high enough to reach passivation as defined by the Pourbaix diagram. An electrochemical model was constructed based on the described galvanic mechanisms of localized CO2 corrosion, having the capability to predict bare surface uniform corrosion, filmed surface passivation and galvanic effects for localized corrosion propagation, in other words, a steady state "worst case" localized corrosion propagation scenario.