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J. Phys. Chem. C 2010, 114, 4436–4444
First Principles Assessment of Carbon Absorption into FeAl and Fe3Si: Toward Prevention of Cementite Formation and Metal Dusting of Steels Donald F. Johnson† and Emily A. Carter*,‡ Department of Chemistry and Department of Mechanical and Aerospace Engineering and Program in Applied and Computational Mathematics, Princeton UniVersity, Princeton, New Jersey 08544-5263 ReceiVed: August 14, 2009; ReVised Manuscript ReceiVed: January 26, 2010
We assess two alloys, FeAl and Fe3Si, in terms of their ability to inhibit ingress of carbon into steels using periodic density functional theory to model alloy thin films. Absorption of carbon into Fe3Si via its (100) and (110) surfaces is predicted to be considerably more endothermic compared to absorption into FeAl or pure Fe. Predicted energy barriers for surface to subsurface diffusion are ∼1 eV or larger for all four surfaces studied. A very endothermic dissolution enthalpy (1.65 eV) and large diffusion activation energies (e.g., 1.29 eV) suggest Fe3Si should inhibit carbon uptake into bulk steel and slow bulk diffusion. Combined with the results of other recent work [Johnson, D. F.; Carter, E. A. Acta Mater. 2010, 58, 638], we expect that a protective coating of Fe3Si should be effective at preventing steel degradation by hydrogen and carbon containing gases. 1. Introduction Corrosion of steel is a considerable burden on the economy due to steel’s ubiquitous use in industry. In harsh environments with high concentrations of carbon-containing gases, steel will degrade via two common mechanisms: carburization and metal dusting.1–3 Both mechanisms involve adsorption of a C precursor, e.g., CO, which dissociates on the metal surface. The adsorbed C atom then penetrates into bulk steel. Carburization typically refers to the formation of brittle carbides within steel and usually occurs at temperatures above 1100 K. Metal dusting occurs when methane or CO attack steel, forming cementite (Fe3C) that subsequently decomposes into fine particles of iron, graphite, or carbon filaments.2–4 While metal dusting is usually associated with lower temperature environments (650-1100 K), graphitization of steel also can occur under prolonged exposure to high temperatures.5 Because the rate of carbon corrosion depends on ingress and bulk diffusion of carbon, it might be possible to alleviate these degradation processes by inhibiting or preventing C uptake into steel via protective coatings. Thin protective coatings are often employed to provide thermal protection, increase wear resistance, and prevent chemical attack. At low temperatures, stainless steel (Cr modified) is usually sufficient for protecting against rusting. Chromium can also be applied as a thin layer on top of the steel resulting in formation of a Cr2O3 passivation layer that prevents further chemical attack.6 However, Cr deposition and oxidation during use can produce toxic Cr(VI) and therefore it would be desirable to find a nontoxic, more environmentally friendly coating. We therefore turn to Al and Si as possible nontoxic alloying agents, because their presence can lead to formation of protective oxide scales. Two iron alloys, FeAl and Fe3Si, both exhibit corrosion resistance to carburization and sulfidation,7–9 which was attributed to formation of Al2O3 and SiO2, respectively. However, an oxide scale might not be present in nonoxidizing environ* Corresponding author. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Mechanical and Aerospace Engineering and Program in Applied and Computational Mathematics.
ments or under thermomechanical conditions where the oxide scale has spalled or worn off. In these cases, the underlying metal alloy could be left vulnerable to corrosive elements such as H and C. For this reason, it is important to examine H/C penetration into the pure alloy surfaces. Previous periodic density functional theory (DFT) calculations within the generalized gradient approximation (GGA) by Jiang and Carter10 examined H and C precursors (H2S, CO) on surfaces of the two metal alloys mentioned above, namely FeAl and Fe3Si. While it was determined that neither alloy could simultaneously inhibit surface dissociation of H2S and CO, it still might be possible that these alloys could slow ingress of the adsorbed atoms produced. We recently examined hydrogen absorption and diffusion into these alloys11 using periodic DFTGGA and the results suggested that Fe3Si would effectively limit uptake of H. It remains to be seen whether the same alloys limit C uptake or slow bulk C diffusion. Thus, here we turn our attention toward C absorption and bulk diffusion in FeAl and Fe3Si to complete our assessment of these alloys as possible protective layers for preventing carburization and hydrogen embrittlement of steel. The adsorption and surface chemistry of CO on metals has been a subject of great interest, particularly as it relates to Fischer-Tropsch catalysis. Previous DFT-GGA calculations have investigated adsorption and dissociation of CO on surfaces of pure Fe10,12–19 as well as on carbide surfaces: Fe3C,20 Fe4C,21 and Fe5C2.22 Atomic carbon adsorbs to the 4-fold hollow site on Fe(100), as observed with low-energy electron diffraction (LEED) measurements23,24 and confirmed by DFT-GGA calculations.13,25 On the Fe(110) surface, DFT-GGA calculations25 predict that carbon sits at the long-bridge site while LEED measurements find a more complex pattern.26 As mentioned above, the previous work that inspired the current study characterized the dissociation of H2S and CO and adsorption of the resultant atomic species on four iron alloy surfaces: FeAl(100), FeAl(110), Fe3Si(100), and Fe3Si(110). Carbon was predicted to bind to surface Fe atoms at high coordination sites, with the exception of the Al-terminated FeAl(100) surface where it is 4-fold coordinated to Al.10 A much
10.1021/jp907883h 2010 American Chemical Society Published on Web 02/24/2010
Carbon Absorption into FeAl and Fe3Si reduced energy barrier was predicted for CO dissociation on FeAl(100) and a further study found an activated state of CO on Al/Fe(100),27 thereby suggesting CO will dissociate easily on FeAl. By contrast, Fe3Si was predicted to significantly inhibit CO dissociation on one surface facet, as deduced from a much increased dissociative adsorption barrier.10 Unfortunately, Fe3Si did not inhibit H2S dissociation relative to pure Fe. Consequently, it is critical to assess the probability of H and C atom ingress into these alloys, because we cannot rely on dissociative adsorption being inhibited as a means of protection of the steel. Early measurements of C diffusion through Fe found Arrhenius behavior with an activation energy of 0.87 eV for interstitial hopping.28 Recent DFT-GGA calculations confirmed these results with an energy barrier of 0.86 eV.29 This excellent agreement between experiment and first-principles predictions give us confidence that the same methods applied to C diffusion through iron alloys will provide meaningful results. Classical embedded atom method interatomic potentials, though generally less accurate than quantum mechanics simulations, have been used to model carbon interactions with iron defects and Fe/ Fe3C grain boundaries.30 Intermetallics and metal alloys have garnered much interest, but there are still many properties that are unknown. Fe-Albased materials are promising as potential lightweight structural materials and have many intriguing properties.31,32 Due to carbon’s important role in the mechanical properties of steel and the growing interest in iron aluminides, the Fe-Al-C system has been the focus of recent experimental33 and theoretical34 investigations. The addition of carbon to FeAl alloys is observed to cause precipitation of perovskite structures (Fe3AlC and Fe3AlC0.5).35 The perovskite, and the carbonvacancy interactions that might lead to its formation, have been investigated with DFT-GGA,36–38 but diffusion of C through FeAl has not been studied. Iron silicides have a wide range of potential applications due to a diverse set of unusual properties: optical properties of FeSi2 conducive to thin-film electronic devices,39 high hardness and corrosion resistance of Fe3Si,40 and giant magnetoresistance exhibited in Fe3Si/FeSi multilayers.41,42 Ordered iron silicides, including Fe3Si, are known to form on silicon substrates when a thin Fe layer is deposited on a Si(100) surface.43,44 Si segregation in Fe-Si alloys has been observed, which can result in ordered Fe3Si or FeSi at the surface.45 DFT-GGA calculations also predicted Si segregation to Fe3Si surfaces.46 Although the silica layer that forms on top of iron silicides protects against corrosion, little is known about the effects of carbon on the unprotected Fe3Si alloy. In this work, adsorption, absorption, and surface-to-subsurface diffusion kinetics of C into FeAl(100), FeAl(110), Fe3Si(100), and Fe3Si(110) is examined along with bulk dissolution thermodynamics and diffusion kinetics. First we present the calculational details and explain the theory behind the diffusion constant calculations in section 2. Data for the adsorption and absorption thermodynamics and kinetics and bulk dissolution and diffusivities are presented and discussed in section 3. Finally, we analyze our results in section 4 and assess the potential effectiveness of Fe alloy protective coatings. 2. Calculational Details The Vienna ab initio Simulation Package (VASP)47,48 was used to carry out spin-polarized Kohn-Sham DFT calculations. Electron exchange and correlation is treated within the GGA of Perdew, Burke, and Ernzerhof49 and the ion-valence electron interactions are described by the all-electron Projector Aug-
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4437 mented Wave (PAW) method50,51 (where ion refers to the nucleus plus core electrons). The PAW method uses a frozen core approximation and the standard PAW potentials are used to self-consistently treat the four, eight, three, and four valence electrons for C, Fe, Al, and Si, respectively. Setting the kinetic energy cutoff for the planewave basis to 400 eV results in total energies converged to within 1 meV/ atom. The number of k-points was increased to converge total energies to within 3 meV/atom. As a result, the k-meshes used for bulk calculations are 8 × 8 × 8, 4 × 4 × 4, and 2 × 2 × 2 for the 16 atom, 54 atom, and for the 128 atom cells. K-meshes of 6 × 6 × 1 and 6 × 4 × 1 are used for the (100) and (110) surface slab calculations, which contain four and eight atoms/ layer, respectively. This results in k-point spacings
