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 CaCl 2 (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 CaCl 2 (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 O 2 and CO 2 , calcium carbonate (CaCO 3 ) in the form of calcite and aragonite was produced on iron in the presence of CaCl 2 (aq), whereas siderite (FeCO 3 ) was produced on the surface of iron in the presence of NaCl(aq). However, in either solution without gradual O 2 and CO 2 exposure, a heterogeneous mixture of lepidocrocite (γ-FeOOH) and an iron hydroxy carbonate (Fe x (OH) y CO 3 ) was grown on the iron surface. In situ liquid AFM was used to measure the surface roughness in CaCl 2 (aq) and NaCl(aq), as an estimation of the corrosion rate. In CaCl 2 (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 x (OH) y CO 3 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 CO 2 and mineral nucleation. The knowledge gained from these interfacial reactions are important for understanding the connection between surface corrosion, mineral grown, and CO 2 capture for sequestration.

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.

Vibrational spectroscopy techniques have evolved to measure gases, liquids, and solids at surfaces and interfaces. In the field of surface-sensitive vibrational spectroscopy, infrared spectroscopy measures the adsorption on surfaces and changes from reactions. Previous polarized modulated-infrared reflection− absorption spectroscopy (PM-IRRAS) measurements at the gas/ solid interface were developed to observe catalytic reactions near reaction conditions. Other PM-IRRAS measurements use liquid cells where the sample is submerged and compressed against a prism that has traditionally been used for electrochemical reactions. This article presents a new method that is used to observe in situ adsorption of molecules using PM-IRRAS at the gas/liquid/solid interface. We demonstrate the meniscus method by measuring the adsorption of octadecanethiol on gold surfaces. Characterization of self-assembled monolayers (SAMs), the "gold standard" for PM-IRRAS calibration measurements, was measured in ethanol solutions. The condensed-phase (air/liquid) interface in addition to the liquid/solid interface was measured simultaneously in solution. These are compared with liquid attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy measurements to confirm the presence of the SAM and liquid ethanol. A model of the three-phase system is used to approximate the thickness of the liquid ethanol layer and correlate these values to signal attenuation using PM-IRRAS. This proof-of-concept study enables the measurement of reactions at the gas/liquid/solid interface that could be adapted for other reactions at the electrode and electrolyte interfaces with applications in environmental science and heterogeneous catalysis.

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A facile method for selective and spontaneous deposition of iron oxide nanoparticles on graphite surfaces.