Adsorption equilibrium at the metal-oxide film interface in the oxidation reactions of Ni,Cr, and their alloys

An adsorption thermodynamic model of the oxidation of Ni-Cr alloys is proposed. According to this model, the adsorption of the alloy component with a lower surface energy (Ni) at the alloy-oxide film interface shifts the equilibrium of the solid-phase reaction 3NiO + 2Cr = Cr₂O₃ + 3Ni (1) toward the enrichment of the oxide film in NiO. It was demonstrated that the total Gibbs energy change for reaction (1) can be presented as ΔG _{T, S} = ΔG _T + ΔG _S, where ΔG _T < 0 is the contribution from the Gibbs energy of the thermochemical reaction of oxidation of Ni and Cr atoms and ΔG _S > 0 is the contribution from the surface Gibbs energy of formation of the alloy associated with the replenishment of the surface layer of the alloy during its oxidation. Calculations of ΔG _S are based on the published data on the surface energy of the pure metal ΔG _S ^o and results of authors’ theoretical studies. It was found that the dependence of \({{a_{NiO}^3 } \mathord{\left/ {\vphantom {{a_{NiO}^3 } {a_{Cr_2 O_3 } }}} \right. \kern-\nulldelimiterspace} {a_{Cr_2 O_3 } }}\) on the content of Cr in the alloy determined from calculated equilibrium characteristic of reaction (1) at 1373 K proved to be in satisfactory agreement with the available experimental data on the composition of the oxide film on Ni-Cr alloys. In addition, the values of the potentials of metal-oxide Ni and Cr electrodes in an aqueous solution at 298 K are calculated, which nearly coincide with the published values of the Flade potential for the passivation of these metals.     

Computational insights into the molecular mechanisms for chromium passivation of stainless-steel surfaces

First principles DFT calculations are used to gain insights into the molecular mechanism of Cr passivation of FeCr alloy surfaces. The systems studied represent early stages of oxidation of FeCr alloys when the oxide layers extend just a few atomic layers into the bulk. A Monte-Carlo atom-swapping algorithm was developed to efficiently explore possible atomic positions and identify the most promising structures that yield overall energy lowering. Analysis of the resulting low energy structures show that the surface oxide layer is rich in chromium while there is a reduction in chromium in the metallic phase near the alloy-oxide interface. Furthermore, there is an increased concentration of Fe near the oxide-air surface. Analysis of the molecular structure of the oxide layers found that oxidized Cr was predominantly in the Cr₂O₃ phase, while oxidized Fe was present as both FeO and Fe₂O₃. We propose that the oxidative variability of Fe facilitates O diffusion in the iron-rich phases because of the range of geometries available for accommodating the O atom. In contrast, O diffusion is less facile in Cr, which has little variability in oxidation state.     

Influence of minor alloying elements on the initial stages of oxidation of austenitic stainless steel materials

Surface oxidation of Fe‐19Cr‐17Ni, Fe‐19Cr‐18Ni‐1Al and TiC‐enriched Fe‐19Cr‐18Ni‐1Al alloys was investigated by photoelectron spectroscopy (PES). The experiments were conducted at 323 K in pure O₂ (2.7 × 10^?6 mbar). Composition and morphology of the nanoscale surface oxides were determined quantitatively by inelastic electron background analysis. Moreover, use of synchrotron radiation facilities were necessary to obtain improved sensitivity for studying minor alloying elements such as Al and Si. The results indicate oxygen‐induced segregation of Al, which significantly hinders the oxidation of the major alloying elements Fe and Cr. Ti remains in its inert carbide form. The relative concentration of Fe within the oxide layer was found to increase with the oxide‐layer thickness, indicating greater mobility of Fe relative to other alloying elements. Copyright © 2008 John Wiley & Sons, Ltd.     

In situ study on the effects of Ni and Mo on the passive film formed on Fe–20Cr alloys by photoelectrochemical and Mott–Schottky techniques

Effects of alloying elements (Ni and Mo) on the structure of passive film formed on Fe–20Cr alloys in pH 8.5 buffer solution were explored by analyzing the in situ electronic properties measured using the photoelectrochemical technique and Mott–Schottky analysis. The passive film formed on Fe–20Cr–10Ni was found to be mainly composed of Cr-substituted γ-Fe₂O₃ from similarities in photocurrent response for the passive films formed on the alloy and Fe–20Cr. On the other hand, the photocurrent spectra for the passive films of Fe–20Cr–15Ni–(0, 4)Mo alloys exhibited the spectral components associated with NiO and Mo oxide (MoO₂ and/or MoO₃) in addition to that induced by Cr-substituted γ-Fe₂O₃. Mott–Schottky plots for the passive films formed on Fe–20Cr–(10, 15)Ni and Fe–20Cr–15Ni–4Mo confirmed that the passive films on Fe–20Cr–(10, 15)Ni–(0, 4)Mo alloys have a base structure of Cr-substituted γ-Fe₂O₃ with variation of densities of shallow and deep donors depending on the Ni and Mo contents in the alloys. We suggest that the passive film formed on Fe–20Cr–(10, 15)Ni and Fe–20Cr–15Ni–4Mo alloys are composed of (Cr, Ni, Mo)-substituted γ-Fe₂O₃ when the concentrations of Ni and Mo are below critical values. However, NiO and Mo oxide (MoO₂ and/or MoO₃) would be precipitated in the passive films when the concentrations of Ni and Mo exceed critical values.     

AES- und XPS-Untersuchungen zum Einflu? von Chrom, Nickel und Molybd?n auf das Korrosionsverhalten von rostfreien St?hlen in S?uren

Summary The alloys Fe17.8Cr, Fe16Cr2.4Mo and Fe18Cr14Ni2.5Mo (at%) were polarized in 0.5 mol/l H₂SO₄ or in 0.1 mol/l HC1 + 0.4 mol/l NaCl. The composition of the oxide layer and of the metallic layer beneath the oxide and the kinetics of the passive layer formation were determined by AES and XPS. In the active region, selective dissolution of Fe leads to an enrichment of Cr, Ni and Mo at the metal/electrolyte interface. In the passive region, the thickness of the rapidly formed passive layer is determined by the potential. The chromium content of the passive layer approaches a stationary, high value. The passive layer essentially consists of the anions O^2- and OH^– and of the cations of Cr, Fe, Mo, whereas Ni — and less pronounced Mo — are enriched below the layer.     

In Situ Study of the Surface Oxidation of FeCr Alloys Using Grazing Incidence X‐ray Absorption Spectroscopy (GIXAS)

The surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe₂O₃ and from b.c.c. Cr to corundum type Cr₂O₃. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in.     

Passive and transpassive films formed on a nickel alloy in ammoniacal solution

The surface of alloy 625 (Ni‐22Cr‐9Mo) was characterized with XPS and AES depth profiling after exposure to mildly alkaline ammoniacal solution at open circuit and after potentiostatic treatments at various potentials. It was determined that the passive surface film was a Cr and Mo oxide that was depleted in Ni with respect to the bulk alloy. Increased solution temperatures or oxygen concentrations decreased the Ni to Cr ratio in the passive film. The alloy's transpassive film was characterized using SEM and EDX as well as XPS. The transpassive film was heavily depleted in Ni and Cr and consisted mainly of an oxide of Fe. Copyright © 2009 John Wiley & Sons, Ltd.     

Adsorption of phenyl phosphate on Ni-Cr alloy surface: Experimental and theoretical investigations

The adsorption behaviour of phenyl phosphate, which is an available biomolecule, on NiCr alloys was investigated. Atomic flame spectroscopy was used to characterize the elemental dissolution during immersion in neutral aqueous solution of 0.09 w% sodium chloride, 37°C. Phenyl phosphate is shown to reduce the release of both Ni²⁺ and Cr³⁺ ions. XPS analyses evidence the formation of a passive film which is mainly consisted in dichromium trioxide and an ultrafilm layer of phenyl phosphate is adsorbed at the passive film surface. DFT+U calculations show that the phenyl phosphate self-assembling at a Cr₂O₃ surface is thermodynamically favoured, with calculated adsorption energy of 2.9 eV. The first half of this value is due to the interaction with the surface, and the second one is due to self-assembling. This study suggests that phenyl phosphate has an important capacity to prevent, in neutral liquid environment, the release of Cr-Ni surface ions thanks to self-assembling in an inner sphere adsorption on the passive film surface. The phosphate group is covalently anchored to the surface. However, the phenyl ring has two roles: (i) it strongly contributes to the self-assembling and (ii) it acts as a hydrophobic function.     

ToF‐SIMS studies of the oxidation of Fe by D2O vapour: comparison with XPS

The oxidation of iron (Fe) by water (D₂O) vapour at low pressures and room temperature was investigated using time‐of‐flight (ToF) SIMS. The results supported those found previously using XPS and the QUASES^? program in that a duplex oxide structure was found containing a thin outer surface hydroxide (Fe(OD)₂) layer over an inner oxide (FeO) layer. The extraordinary depth resolution of the ToF‐SIMS profiles assisted in identifying the two phases; this resolution was achieved by compensation for surface roughness. A substantial concentration of deuterium was found in the subsurface oxide layer. This observation confirmed previous assessments that the formation of FeO was from the reaction of Fe(OD)₂ with outward‐diffusing Fe, leaving deuterium as a reaction product. Copyright © 2005 John Wiley & Sons, Ltd.     

XPS study on surface modification of NiTi alloy by acidic solution immersion and subsequent heating in air

Changes in the surface chemical state of a nearly equiatomic nickel–titanium (NiTi) alloy caused by immersion in aqueous solutions of HNO₃ and H₂SO₄ as well as subsequent heating in air at 723 K were analyzed using X-ray photoelectron spectroscopy (XPS). An XPS analysis using angle-resolved technique and a mathematical deconvolution technique revealed that a passive layer formed in an ambient atmosphere contained TiO₂ as a major state and Ni(OH)₂ and NiO as minor states. The Ni(OH)₂ on the alloy remained in the region even when heated in air at 723 K. Therefore, the resulting layer became a Ti-oxide layer with Ni segregated region at the surface, which was NiO formed via dehydration of Ni(OH)₂. However, immersion in an aqueous solution of HNO₃ or H₂SO₄ enables Ni(OH)₂ state to dissolve in the passive layer of a NiTi alloy; thereby, the Ni segregated region rarely appeared in the oxide layer by heating. The Ni segregated region at the surface becomes an obstacle for the inward diffusion of oxygen; thus, the annihilation of such a segregated region results in an increase in the thickness of the oxide layer.     

A DFT Study on the Structure and Properties of Cu/Cr2O3 Catalyst

Using DFT method, the stable adsorption configurations of Cu₄ cluster on Cr₂O₃ (0001) surface were investigated. The regular tetrahedron structure and the planar structures were considered as the initial adsorption configuration of Cu₄ cluster, respectively. The adsorption energies of the two structures were also calculated. The simulation result indicated that the adsorption energy of the regular tetrahedron structure was higher than that of the planar structure, and thus the regular tetrahedron structure was confirmed to be the stable adsorption configuration for Cu₄ cluster on Cr₂O₃ (0001) surface. Moreover, it was observed that the Cu₄ cluster showed the definite stable adsorption sites on Cr₂O₃ (0001) surface, namely 3‐fold O sites. During the adsorption process of Cu₄ cluster onto Cr₂O₃ (0001) surface, the Cu₄ cluster could bond with more Cr or O atoms on the surface, and the apparent charge transfer also occurred correspondingly. Meanwhile, the Cu₄ cluster and Cr₂O₃ (0001) surface would bond in the form of local polarization to enhance the stability of adsorption configuration.     

Microstructural characterization of oxide layers formed on Ni–20Cr–8Al alloy foam using transmission electron microscopy

Microstructural characterization was carried out during the pre‐oxidation of Ni–20Cr–8Al alloy foam using transmission electron microscopy (TEM). During the pre‐oxidation at 1000 °C for 1, 30, and 60 min in air, the sequential formations of NiO, NiCr₂O₄, Cr₂O₃, and α‐Al₂O₃ 1‐μm‐thick oxide layers were, respectively, characterized. Initially, during pre‐oxidization, the layers formed abnormally in an island growth mode, but they grew to be morphologically uniform after 30 min. Pores were found after only 1 min in the middle region of the oxide layers, near the Cr₂O₃ layer, and then these developed into critical micro‐cracks after 60 min. Copyright © 2017 John Wiley & Sons, Ltd.     

The obtaining of NiCr2O4 nanoparticles by unconventional synthesis methods

This paper presents a study regarding the obtaining of NiCr₂O₄ by two new unconventional synthesis methods: (i) the first method is based on the formation of Cr(III) and Ni(II) carboxylate-type precursors in the redox reaction between the nitrate ion and 1,3-propanediol. The thermal decomposition of these complex combinations, at ~300 °C, leads to an oxide mixture of Cr₂O_3+x and NiO, with advanced homogeneity, small particles and high reactivity. On heating this mixture at 500 °C, Cr₂O₃ reacts with NiO to form NiCr₂O₄, which was evidenced by FT-IR and X-ray diffractometry (XRD) analysis; (ii) the second method starts from a mechanical mixture of (NH₄)₂Cr₂O₇ and Ni(NO₃)₂·6H₂O. On heating this mixture, a violent decomposition at 240 °C with formation of an oxides mixture (Cr₂O₃ + CrO₃) and NiO takes place. On thermal treatment up to 500 °C, an intermediary phase NiCrO₄ is formed, which by decomposition at ~700 °C leads to NiCr₂O₄, evidenced by FT-IR and XRD analysis. NiCr₂O₄ is formed, in both cases, starting with a temperature higher than 400 °C, when the non-stoichiometric chromium oxide (Cr₂O_3+x ) loses the oxygen excess and turns to stoichiometric chromium oxide (Cr₂O₃), which further reacts with NiO.     

Comparison of the specific adsorption of sulfate (HSO<Subscript>4</Subscript><Superscript>−</Superscript>) ions on Cr and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>

The specific adsorption of sulfate ions on powdered Cr was studied by a radiotracer technique using ³⁵S-labeled sulfuric acid in low concentration (c<10^–3 mol dm^–3) in the presence of a large excess of perchlorate supporting electrolyte. The pH and concentration dependence were determined. On the basis of a comparison of the results obtained for Cr₂O₃ and Cr, it can be assumed that, similar to other metals, the overall sorption behavior of Cr is determined by the protective oxide film present on the surface.     

FeO/Pt(111)与FeO2/Pt(111)的几何、电子结构及表面氧活性的第一性原理研究

采用密度泛函理论系统研究了超薄氧化物膜/金属体系FeO/Pt和FeO₂/Pt及其表面不同区域(FCC,HCP和TOP)的几何结构、电子性质及氧的活性.研究发现,表面O-Fe高度差δ_z作为一个重要的特征结构参数直接影响局域表面静电势和表面氧的结合能: δ_z越大,静电势越大,氧的结合能越弱.计算发现,在FeO/Pt体系中,δ_z顺序为FCC > HCP > TOP,而FeO₂/Pt中是FCC > TOP > HCP.此外,在FeO/Pt中,电荷转移方向是从氧化物膜到衬底,Fe的表观价态为+2.36,表面功函较纯Pt(111)的变化可忽略; 而FeO₂/Pt中,电荷转移的方向是从衬底到氧化物,Fe的表观价态为+2.95,表面功函较纯Pt增加1.24 eV.进一步分析了电荷转移和表面偶极对电子性质的作用机制.这些研究结果对于认识超薄氧化物薄膜对表面几何结构、电子性质、表面氧活性的调制具有重要的启示意义.     

The site preference and doping effect on mechanical properties of Ni3Al-based γ′ phase in superalloys by combing first-principles calculations and thermodynamic model

The fundamental aspects of site preference of alloying elements on sublattice of the strengthen γ′ phase with L1₂ structure have not been well understood, which hinders the optimized design of advanced Ni-based high-temperature alloys. In this contribution, the temperature- and composition-dependent site occupying preferences of the binary, ternary, and quaternary of Ni₃Al-based γ′ phase alloyed with M_i where M_i represents the additional transitional metals Co, Cr, Cu, Fe, Mn, Mo, Re, Ta, Ti, V, or W atoms (arranged in alphabetical order) chosen frequently, were studied using a two-sublattice thermodynamic model (Ni, Al, M_i)_1a(Ni, Al, M_i)_3c. The site occupying fractions (SOFs) were calculated based on a thermodynamic database established in this work, where the thermodynamic data of the end-members involved were obtained using first-principles calculations and phonon spectrum calculations. The calculated SOFs results show that there is an obvious site preference for stoichiometry binary Ni₃Al, and its site configuration changes from (Al)_1a(Ni)_3c at room temperature to (Al_0.9984Ni_0.0015)_1a (Al₀Ni_0.9994)_3c at 1273 K. For the γ′ phase with the composition 78Ni-26Al-4M_i (atom ratio and x_Ni/x_Al = 3:1), Mo atoms always preferred to occupy the 1a sublattice (Al site), Co, Mn, and Ti atoms always prefer the 3c sublattice (Ni site) in the whole temperature range, while the site preference of Cr, Cu, Fe, Re, Ta, V, or W atom is affected by temperature. For example, when the heat treatment temperature is lower than 700 K, Cr, Cu, Fe, Ta, V, and W atoms occupy the 1a and 3c sublattice randomly, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr, Cu, and W atoms prefer 3c sublattice, Fe and Ta atoms prefer to 1a sublattice, while all Re atoms occupy the 3c sublattice exclusively, and all V atoms occupy the 1a sublattice exclusively, respectively. Likewise, the site preference of the quaternary system with selective compositions 78Ni-26Al-2 M₁-2 M₂ was also predicted. Based on calculated SOFs results, the mechanical and thermodynamic properties were studied at the ground state. It has been revealed that Cr, Re, and V doping can improve the microhardness of Ni₃Al alloys; in particular, the effect of Cr is extraordinary; and all elements, except Mn, Mo, and Ti, would enhance the bulk modulus of Ni₃Al-based γ′ phase, in which Mn have the greatest influence on reducing the bulk and shear modulus, respectively. Furthermore, all the B/G ratios of the computed Ni₃Al-based γ′ phase are >1.75, showing inherent ductility. Only Cr doping significantly enhances the Debye temperature of the Ni₃Al-based γ′ phase.     

Dissolution of oxygen in polycrystalline palladium at $${P_{{O_2}}}$$= 100 Pa and temperatures of 500 to 950 K

The dissolution of oxygen in polycrystalline palladium Pd(poly) at an O₂ pressure of 100 Pa and temperatures of 500–950 K has been investigated by temperature-programmed desorption. At 500 K, the process yields a surface palladium film that includes an oxide-like reconstructed structure on a rarefied metal surface layer. At this temperature, palladium sorbs ~2 monolayers (ML) of oxygen. At 600–800 K, palladium dissolves up to ~140 ML of oxygen as a result of O₂ chemisorption on the surface of the oxide film, penetration of O_ads atoms under the oxide film, and their diffusion into the metal bulk. The dependence of the amount of oxygen sorbed by Pd(poly) (n) on the time of exposure to an O₂ atmosphere is described by a nearparabolic function, n = at^b, indicating that oxygen atoms diffuse in the metal lattice. The activation energy of this diffusion, Е _dif, is ~83.5 kJ/mol. At high temperatures (800–950 K), palladium sorbs much less oxygen (≤10 ML). This is due to the complete decomposition of the surface oxide film, a process that markedly hampers the insertion of Oads atoms under the surface layer of the metal.     

Planting of bis(1,5‐cyclooctadiene) nickel upon silica to harvest NiO (<5 nm) nanoparticles in a silica matrix

Bis(1,5‐cyclooctadiene) nickel [Ni(COD)₂] was employed as a nickel precursor to prepare nickel oxide nanoparticles upon high‐surface‐area mesoporous silica. Under protection of argon, Ni(COD)₂ was dissolved in tetrahydrofuran (THF) to react with surface silanols of mesoporous silica SBA‐15, which formed a black powder after completion of the surface reaction. Calcination of the powder produced ultrafine NiO inside the mesoporous silica matrix, which was evidenced by X‐ray diffraction, N₂ adsorption–desorption, transmission electron microscopy and thermogravimetric analysis. The thermogravimetric analysis suggests that NiO formation is a result of surface nickel species calcination, whereas structural characterization clearly show that NiO nanoparticles of <5 nm are evenly distributed inside the silica SBA‐15 matrix and mesoporosity is well preserved upon calcinations and NiO formation. The surface reaction between Ni(COD)₂ and surface silanols was found for the first time, and the method used here may be extended conveniently to prepare other metal oxide nanoparticles upon high‐surface‐area supports as well. Copyright © 2005 John Wiley & Sons, Ltd.     

Photocatalytic activity of TiO2-MO x composites in the reaction of hydrogen generation from aqueous isopropanol solution

The photocatalytic activity of the composites TiO₂-MO _x (M = Ni, Cu, Zn, Fe, Cr) was studied in the reaction of hydrogen generation from aqueous alcoholic suspensions under UV light. The samples modified by the oxides of the metals capable of being reduced from oxides under photocatalytic conditions showed a high catalytic activity. The studied modifiers were divided in three groups in terms of their effect of the photocatalytic activity of TiO₂: activating (NiO, CuO), inhibiting (Fe₂O₃), and indifferent (ZnO, Cr₂O₃).     

High‐temperature oxidation behavior of Ni‐based superalloys with Nb and Y and the interface characteristics of oxidation scales

Ni‐based superalloys with niobium (Nb) or/and yttrium (Y) were prepared by vacuum melting. The oxidation kinetics of these alloys was studied by thermogravimetry at 800 °C for 100 h in static air. Morphology of oxides was studied using SEM, and the composition was analyzed by X‐ray diffraction. Energy‐dispersive X‐ray spectrometer was employed to examine the linear element distribution of the cross section of the oxidation films. Results showed that the oxidation kinetics all followed a parabolic law at different stages. The oxide films were mainly comprised of Cr₂O₃, NiCr₂O₄, Al₂O₃ and TiO₂. All the oxide films exhibited layered structure owing to different oxidation stages. With the addition of Nb or Y, the high‐temperature oxidation resistance of the superalloy was improved significantly and the surface morphology of the oxidation film was ameliorated. The comprehensive effect of Nb and Y was remarkable in improving the high‐temperature oxidation resistance of Ni‐based alloys. Copyright © 2014 John Wiley & Sons, Ltd.