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

Abstract

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.

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

“…Metal iron (iron­(II), 99.99%, Allied Metals Corp. and Puratronic, 99.995% metals basis, Alfa Aesar) was obtained in the form of a metal plate and cut into small sample pieces (2.0 cm × 1.0 cm × 0.20 cm). One side of each sample was polished using a metal polishing instrument (Struers, LaboPol-1) in a four-step metal polishing method as described previously to a mirror finish. This was necessary to produce a highly reflective surface for PM-IRRAS measurements and a defect-free surface for AFM measurements.…”

“…An iS50R Fourier transform infrared spectrometer (Thermo Scientific, Inc.) instrument with aluminum mirrors was used to collect PM-IRRAS infrared reflectance absorbance spectra and ATR–FTIR spectra of the samples. An in-house built (by the Perrine research group) optical set up was coupled to the same instrument to collect PM-IRRAS, as described previously. , The polished iron samples were attached to the sample holder in a vertical orientation, as shown in Figure A. A photoelastic modulator (PEM, Hinds, Inc.) with a ZnSe polarizer was placed in the beam path to modulate the signal between p and s polarized light.…”

“…The optical alignment onto the iron surface and the optimization of the peak-to-peak (p/p) signal were carried out as described previously. , Several types of spectra were collected under the different stages of corrosion of iron with either 10 mM CaCl 2 (aq) or 10 mM NaCl­(aq) electrolytes. First, a spectrum of the dry, polished iron sample was collected before starting the corrosion reaction.…”

“…Inorganic thin films are a complex mixture of minerals produced on the surface of metals from corrosion and other complex mechanisms. , The chemical composition of the mineral depends upon the metal and the chemical composition of solutions, solution pH, temperature, and gases in the environment. − Atmospheric dust and other particulate matter that settle on metal surfaces can also lead to corrosion, such as in the crevice phenomenon, where the small space between dust and the surface of the corroding species traps water easily, and fluctuations in relative humidity cause dissolved ions to deposit onto the surface of the substrate, increasing corrosion rates. , Corrosion reactions and mineral formation are complex in nature as gases and ions in electrolytes come in contact with metal surfaces, producing oxides, hydroxides, and carbonates. − …”

“…Iron interfaces are ideal for studying fundamental corrosion processes as they comprise not only material infrastructure but also soils and atmospheric dust present in the mineral cycle, which undergo spontaneous redox reactions in aqueous ambient environments. − Although there are many studies pertaining to the corrosion process of iron, ,− there are few studies connecting how the chemistry in complex mixtures from ions, gases, and solid surfaces impacts surface corrosion, chemistry, and mineral growth. Our previous studies have shown that siderite (FeCO 3 ) is produced from exposure of Fe to aqueous NaCl­(aq) and atmospheric CO 2 . In HCl­(aq), lepidocrocite (γ-FeOOH) was produced at the iron interface, suggesting that only O 2 from air exposure influenced the mineral product.…”