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The charge of an Fe atom versus Cr configurations (with the neutral atom, of charge 0 e, as the reference level). The axis labels ‘Pure Fe’, ‘Cr in 1L’, ‘Cr in 2L’ and ‘2Cr in 2L’ refer to pure Fe surface, one Cr atom in the surface layer, one Cr atom in the subsurface layer and two Cr atoms in the subsurface layer at sites 8 and 9 (Fig. 1). The green curve with square markers shows the charge of the subsurface Fe atom nearest to the Cr atom. The blue curve with filled circles gives the charge of the surface Fe atom nearest to the Cr atom. For the ‘2Cr in 2L’ case, the red branch (open circle) shows the charge of the Fe atom at site 5, and the blue branch (filled circle) shows the charge of an Fe atom at sites 2 and 4.
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The adsorption of oxygen on bcc Fe–Cr(100) surfaces with two different alloy concentrations is studied using ab initio density functional calculations. Atomic-scale analysis of oxygen–surface interactions is indispensable for obtaining a comprehensive understanding of macroscopic surface oxidation processes. Up to two chromium atoms are inserted in...
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Citations
... The 55-atom decahedron nanoalloys have hexagonal (111) and square (100) facets, and for these nanoalloys, we tried all nonequivalent oxygen adsorption sites on the surface. The adsorption energy of atomic oxygen on Pt13Ag42 and Pd13Ag42 core-shell nanoalloys is calculated as the difference between the total energies of the combined system (atomic oxygen+nanoalloy) and the separated ones (atomic oxygen and nanoalloy) [72][73][74]; ...
... The adsorption depends on the coverage of oxygen atoms and alloying atoms on the surfaces. [14,18,19] In the bulk, oxygen atoms prefer to solute at octahedral interstitial sites (OCT) of the Fe lattice [15,16] and diffuse quickly to the next OCT site with the energy barrier of 0.54 eV. [17] However, they are easily trapped by vacancy defects and grain boundaries, which also slow down the diffusion of oxygen. ...
Oxidation corrosion of steels usually occurs in contact with the oxygen-contained environment, which is accelerated by high oxygen concentration and irradiation. The oxidation mechanism of steels is investigated by the adsorption/solution of oxygen atoms on/under body-centered-cubic (bcc) iron surfaces, and diffusion of oxygen atoms on the surface and in the near-surface region. Energetic results indicate that oxygen atoms prefer to adsorb at hollow and long-bridge positions on the Fe(100) and (110) surfaces, respectively. As the coverage of oxygen atoms increases, oxygen atoms would repel each other and gradually dissolve in the near-surface and bulk region. As vacancies exist, oxygen atoms are attracted by vacancies, especially in the near-surface and bulk region. Dynamic results indicate that the diffusion of O atoms on surfaces is easier than that into near-surface, which is affected by oxygen coverage and vacancies. Moreover, the effects of oxygen concentration and irradiation on oxygen density in the near-surface and bulk region are estimated by the McLean’s model with a simple hypothesis.
... Importantly, such calculations have shown that the aforementioned critical Cr concentration for the onset of the corrosion resistance in stainless steels closely coincides with an onset of anomalous surface segregation of Cr in Fe-Cr alloys [10,11]. This observation suggests that the segregation of Cr toward the surface plays a crucial role in facilitating the formation of the protective, self-healing layer of iron and chromium oxides that is known to be the reason for the corrosion resistivity of stainless steels [12][13][14][15][16]. ...
To enable accurate molecular dynamics simulations of iron-chromium alloys with surfaces, we develop, based on density-functional-theory (DFT) calculations, a new interatomic Fe-Cr potential in the Tersoff formalism. Contrary to previous potential models, which have been designed for bulk Fe-Cr, we extend our potential fitting database to include not only conventional bulk properties but also surface-segregation energies of Cr in bcc Fe. In terms of reproducing our DFT results for the bulk properties, the new potential is found to be superior to the previously developed Tersoff potential and competitive with the concentration-dependent and two-band embedded-atom-method potentials. For Cr segregation toward the surface of an Fe-Cr alloy, only the new potential agrees with our DFT calculations in predicting preferential segregation of Cr to the topmost surface layer, instead of the second layer preferred by the other potentials. We expect this rectification to foster future research, e.g., on the mechanisms of corrosion resistance of stainless steels at the atomic level.
As iron powder nowadays attracts research attention as a carbon-free, circular energy carrier, molecular dynamics (MD) simulations can be used to better understand the mechanisms of liquid iron oxidation at elevated temperatures. However, prudence must be practiced in the selection of a reactive force field. This work investigates the influence of currently available reactive force fields (ReaxFFs) on a number of properties of the liquid iron–oxygen (Fe–O) system derived (or resulting) from MD simulations. Liquid Fe–O systems are considered over a range of oxidation degrees ZO, which represents the molar ratio of O/(O + Fe), with 0 < ZO < 0.6 and at a constant temperature of 2000 K, which is representative of the combustion temperature of micrometric iron particles burning in air. The investigated properties include the minimum energy path, system structure, (im)miscibility, transport properties, and the mass and thermal accommodation coefficients. The properties are compared to experimental values and thermodynamic calculation results if available. Results show that there are significant differences in the properties obtained with MD using the various ReaxFF parameter sets. Based on the available experimental data and equilibrium calculation results, an improved ReaxFF is required to better capture the properties of a liquid Fe–O system.
This work presents the use of density functional theory to study the adsorption/dissociation mechanism of the H2S molecule at the Cr-doped iron (Fe(100)) surface. It is observed that H2S is weakly adsorbed on Cr-doped Fe; however, the dissociated products are strongly chemisorbed. The most feasible path for disassociation of HS is favorable at Fe compared to Cr-doped Fe. This study also shows that H2S dissociation is a kinetically facile process, and the hydrogen diffusion follows the tortuous path. This study helps better understand the sulfide corrosion mechanism and its impact, which would help design effective corrosion prevention coatings.
To enable accurate molecular dynamics simulations of iron–chromium alloys with surfaces, we develop, based on density-functional-theory (DFT) calculations, a new interatomic Fe–Cr potential in the Tersoff formalism. Contrary to previous potential models, which have been designed for bulk Fe–Cr, we extend our potential fitting database to include not only conventional bulk properties but also surface-segregation energies of Cr in bcc Fe. In terms of reproducing our DFT results for the bulk properties, the new potential is found to be superior to the previously developed Tersoff potential and competitive with the concentration-dependent and two-band embedded-atom-method potentials. For Cr segregation toward the (100) surface of an Fe–Cr alloy, only the new potential agrees with our DFT calculations in predicting preferential segregation of Cr to the topmost surface layer, instead of the second layer preferred by the other potentials. We expect this rectification to foster future research, e.g., on the mechanisms of corrosion resistance of stainless steels at the atomic level.
ZnMg alloys of certain compositions in the Zn-rich side of the phase diagram are particularly efficient, and widely used, as anticorrosive coatings, but a sound understanding of the physico-chemical properties behind such quality is still far from being achieved. The present work focuses on the first stage of the corrosion process, namely the initial growth of a sacrificial surface oxide layer, whose characteristics will condition the next stages of the corrosion. A comprehensive ab-initio study, based on the density functional theory, is carried out on ZnMg nanoalloys with 20 atoms and different compositions, which serve as model systems to simulate the complex processes that occur in extended granular surfaces. The structural and electronic properties, when progressive oxidation of the nanoalloys takes place, are analyzed in detail with the help of structural descriptors, energetic descriptors such as the oxygen adsorption energies and excess adsorption energies, as well as with electronic ones based on the topological analysis of the electron density and the electron localization function, from which a detailed analysis of the bonding patterns is extracted. We explain why small amounts of Mg create a very positive synergy between Zn and Mg that increases the reactivity to oxygen while reducing, at the same time, the stress induced on the cluster substrate, both facts working in favor of promoting the growth of the oxide crust whilst protecting the core. Moreover, we also show that stoichiometries close to the Mg 2 Zn 11 and MgZn 2 compositions are the best candidates to optimize the protection against corrosion in Zn-Mg alloys, in agreement with the experimental observations.
