Investigation of Iron Dissolution Mechanism in Acidic Solutions with and without Dissolved CO₂—Part I: Electrochemical Impedance Spectroscopy Measurements Available to Purchase

Aqueous CO₂ corrosion of mild steel is one of the major problems in the oil and gas industry. While current understanding primarily focuses on cathodic reaction mechanisms, less attention has been given to the impact of aqueous CO₂ on the anodic iron dissolution reaction. In contrast, the mechanism of iron dissolution in strongly acidic environments has been thoroughly investigated. Among the reaction mechanisms found in the open literature, a multipath mechanism was identified that could explain the iron dissolution in strong acidic sulfate solution; both in terms of steady-state polarization sweeps and impedance data at various pH values and current densities. However, the role of aqueous CO₂ in solutions containing chlorides on the mechanism of iron dissolution remained an open question. The present study used electrochemical impedance spectroscopy (EIS) as the main technique, to study the mechanism of iron dissolution in strong acid chloride solution with and without the presence of CO₂. Results showed that the presence of chloride ions (0.5 M) decreased the rate of iron dissolution by competing with hydroxide ions to adsorb on the metal surface, forming chloride-containing intermediate species that participate in the iron dissolution reaction. The resulting decrease in the availability of hydroxide intermediates, which are more effective at enhancing the reaction rate compared to chloride-containing intermediates, leads to an overall decrease in the rate of iron dissolution. While the presence of CO₂ increases anodic current density, EIS investigation revealed that neither aqueous CO₂ nor other carbonic species directly react on the bare metal surface to form adsorbed intermediates involved in the anodic reaction. EIS investigation suggested that aqueous CO₂ may induce changes in the chemical composition of adsorbed species, rate constants, and surface coverage, thereby altering the kinetics of the underlying reactions.