Iron oxides are earth-abundant minerals, soil and atmospheric dust that comprise a large percentage of the Martian soil and on Earth. Iron and oxide interfaces participate in reactions in the water cycle through redox processes. The evolution and mechanism of iron to minerals and complex oxides is unknown. These reactions begin with surface corrosion, a redox process, that is governed by water and O2 balancing and catalyzed by ions. The transformation of iron to minerals occurs at complex interfaces, where ions in electrolytes, O2, CO2 gases highly influence its oxidation. In this study, the surface reaction of iron is presented for understanding the interfacial corrosion and oxidation of iron surfaces into minerals. In situ polarized modulated-infrared reflection absorption spectroscopy (PM-IRRAS) measured the interfacial corrosion reactions of Fe in MgCl2(aq) or KCl(aq) electrolytes at the air/electrolyte/iron interface under ambient conditions. PM-IRRAS was used to measure the variation of the compositional changes at the air/electrolyte/Fe interface as a function of time and atmospheric O2 and CO2 adsorption. In KCl(aq) and MgCl2(aq), different types of interfacial carbonates (CO3) and bicarbonates (HCO3) were produced. The submerged areas of both types of Fe samples evidenced the formation of a heterogeneous mixture of minerals, different than that of the interface region. The chemical properties of K+ and Mg2+ along with atmospheric gases (O2 and CO2) influence the composition of corrosion products at the air/electrolyte/Fe interface compared to the region submerged in solution with little atmospheric O2 and CO2 exposure. These results were further corroborated with in situ AFM and ex situ ATR-FTIR, XPS, and AFM analysis. This study is important for understanding the influence of cations on surface corrosion and fundamental chemical and physical processes in mineral growth relevant for planetary and conversion processes at complex interfaces.

Role of mono (KCl) and divalent (MgCl2) chlorides on iron corrosion and mineral formation at air/electrolyte/iron interface

ACS Fall 2021

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