Combined EIS/Tof/SIMS Investigation of Iron Dissolution Mechanisms in CO2 Saturated Chloride Solutions
Authors: Nesic Srdjan , Moradighadi Negar , Wang Huiru , Wang Luntao , Seyeux Antoine , Pailleret Alain , Marcus Philippe 
Published: December 2024

Aqueous corrosion of mild steel is one of the major problems in the oil and gas industry. The current understanding of corrosion mechanisms, mainly focused to the cathodic reaction, was used to build electrochemical models to predict the corrosion rate of mild steel. However, the effect of aqueous CO2 on the anodic iron dissolution reaction was less studied. In contrast, the mechanism of iron dissolution in strong acidic environments has been thoroughly investigated over many decades. Among the proposed mechanisms in the literature, a broadly applicable multi-path mechanism was identified that could explain the behavior of anodic dissolution of iron 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 anodic reaction had remained an open question.
The present study used EIS measurements and ToF-SIMS in-depth profiling and 3D imaging, as the main techniques, to study the mechanism of iron dissolution in strong acid chloride solution with and without the presence of . EIS results showed that the presence of chloride ions (0.5 M) decreases the rate of the anodic iron dissolution and results in the formation of an additional adsorbed intermediate species which participates in the anodic reaction multi-path mechanism in parallel with the other oxide/hydroxide intermediates. The presence of increases the anodic current density mainly in transition and pre-passivation regions, however, EIS investigation of this mechanism showed that aqueous and other carbonic species do not react directly on the bare metal surface and do not form an additional adsorbed intermediate species that is involved in the anodic reaction. Based on EIS results, it is suggested that the presence of aqueous might effect a change of the chemical composition of the adsorbed species (including hydroxide and chloride intermediates), value of kinetic constants and extent of surface coverage by different adsorbed intermediates and ions, leading to the change in the kinetics of the underlying reactions.
ToF-SIMS 3D mapping was used to provide supporting evidence for the anodic iron dissolution mechanisms of mild steel in chloride containing aqueous CO2 environments. The technique detected adsorbed hydroxide and chloride intermediates formed during corrosion process, which is consistent with the proposed multi-path reaction mechanism for anodic iron dissolution reaction. Despite the presence of aqueous carbonic species and their observed effect on the kinetics of iron dissolution, no additional adsorbed intermediates have been detected in aqueous environments, indicating that carbonic species do not directly participate in the iron dissolution reaction. ToF-SIMS 3D mapping results further suggest that one role of aqueous carbonic species could be to accelerate the adsorption of chloride ions and formation of chloride intermediates.