Effect of Cations on the Oxidation and Atmospheric Corrosion of Iron Interfaces to Minerals

Surface corrosion involves a series of redox reactions that are catalyzed by the presence of ions. On infrastructure surfaces and in complex and natural environments, iron surfaces readily undergo redox reactions, impacting chemical processes. In this study, the effect of how cations influence the formation of the mineral scale on iron surfaces and its connection to surface corrosion was investigated in CaCl₂(aq) and NaCl(aq) electrolytes. Polarized modulated-infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to measure the oxidation and formation of carbonates at the air/electrolyte/iron interface, which confirmed that the iron surface oxidized faster in CaCl₂(aq) than in NaCl(aq). PM-IRRAS, attenuated total reflectance-Fourier transformed infrared spectroscopy, and X-ray photoelectron spectroscopy show that after the adsorption of atmospheric O₂ and CO₂, calcium carbonate (CaCO₃) in the form of calcite and aragonite was produced on iron in the presence of CaCl₂(aq), whereas siderite (FeCO₃) was produced on the surface of iron in the presence of NaCl(aq). However, in either solution without gradual O₂ and CO₂ exposure, a heterogeneous mixture of lepidocrocite (γ-FeOOH) and an iron hydroxy carbonate (Fe(OH)CO₃) was grown on the iron surface. In situ liquid AFM was used to measure the surface roughness in CaCl₂(aq) and NaCl(aq), as an estimation of the corrosion rate. In CaCl₂(aq), Fe was found to corrode faster than Fe in NaCl(aq) due to more ions at equimolar concentrations. Surface physical changes, as measured by ex situ AFM, confirmed the presence of a heterogeneous mixture of γ-FeOOH and an Fe(OH)CO₃ in the submerged region. This indicates that the cation does not affect the type of mineral grown on the Fe surface in the region completely submerged in the electrolyte. These results suggest that the cations play a unique role in the initial stages of corrosion at the interface region, influencing the uptake of atmospheric CO₂ and mineral nucleation. The knowledge gained from these interfacial reactions are important for understanding the connection between surface corrosion, mineral grown, and CO₂ capture for sequestration.