In Situ PM-IRRAS at the Air/Electrolyte/Solid Interface Reveals Oxidation of Iron to Distinct Minerals

Iron interfaces undergo redox and catalytic processes in various environments, on the surface of soils, dust, minerals, and materials that comprise industrial infrastructure. Measuring reactions at interfaces in complex environments is challenging, where adsorption of gases and interaction of aqueous species occur at the surface. This is due to the presence of several ionic species in solutions that catalyze surface oxidation and undergo ion exchange between the solution and the surface and from the influx of oxygen and other gases. Corrosion is an electrochemical redox reaction that is affected by the presence of oxygen and water, but accelerated by dissolved ions. Polarized modulated-infrared reflection absorption spectroscopy was used to measure in situ surface oxidation at the air/electrolyte/iron interface in semineutral NaCl(aq) and acidic HCl(aq) solutions using the meniscus method under ambient conditions. The iron interface was exposed to air, primarily oxygen, allowing for surface oxidation, where metallic iron was found to transform to siderite in NaCl(aq) and lepidocrocite in HCl(aq). Mechanisms are suggested for the transformation of iron to these corrosion products, which significantly impact our understanding of redox processes in the water cycle, material degradation, and energy applications.