Oxide-alloy Interface Research Articles

First-Principles Kinetic Monte Carlo Modeling of Passivation in Ni-Based Alloys

Corrosion by metal oxidation is a major problem for many structural materials, including Ni-based alloys in high-temperature applications. Passivation can be achieved through various alloying strategies, but common theoretical approaches to understanding key factors such as the critical concentration of the minor element are limited in their predictive capability. In particular, existing models often do not explicitly capture competing metal cation migration mechanisms in the oxide. In this study, we examine passivation kinetics of Ni-Cr and Ni-Al alloys by modeling the evolution of initial NiO oxide films as Cr/Al are incorporated. Surrogate models for the energies of various oxide configurations and the migration barriers of metal cations are parameterized from density functional theory calculations. These inputs are used to drive kinetic Monte Carlo simulations over long time scales. By tracking the time-dependent oxidation behavior across alloy composition, we predict critical concentrations of Cr/Al to form continuous passivation layers. Compositional and structural analyses of the simulated oxides are used to quantify fractions of different phases and provide insight into the nature of passivation in each alloy system. The effect of increasing or decreasing the exchange rate of metal atoms near the alloy-oxide interface, which can be related to alloy processing, is also explored. We discuss the applicability of our method to other systems and its incorporation into an existing open-source software package.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

The incipient oxidation behaviors of Haynes 282 at high temperatures

The purpose of this study was to investigate the oxidation behavior and the development of residual stress in the nickel-based superalloy Haynes 282 at temperatures ranging from 800 to 950 °C. Detailed examinations of the oxide scale surfaces were conducted using Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB). The residual stresses in the Cr2O3 scales were measured using X-ray diffraction (XRD). A layered chromia scale was formed as a result of the oxidation process, and the kinetics of this process were found to be parabolic, controlled by Chromium diffusion. An external Cr2O3 layer was observed in cross-sections, along with Kirkendall voids at the alloy-oxide interface. All residual stresses of chromia observed were tensile intrinsically.

Optimizing the synergy between alloy and alloy-oxide interface for CO oxidation in bimetallic catalysts.

Creating synergetic metal-oxide interfaces is a promising strategy to promote the catalytic performance of heterogeneous catalysts. However, this strategy has been mainly applied to monometallic catalysts, while scarcely applied to alloy catalysts. In this work, we present a comprehensive study on the synergetic alloy-oxide interfaces in the bimetallic Pt-Co/Al2O3 catalysts for CO oxidation. A series of Pt1Cox/Al2O3 catalysts with various Co/Pt molar ratios with x ranging from 0.5 to 3.8 was synthesized via a facile wet-chemistry strategy. Among them, the Pt1Co0.5/Al2O3 catalyst exhibits the best catalytic performance for CO oxidation, with the lowest CO complete conversion temperature of -10 °C and the highest mass specific rate of 2.61 (mol CO) h-1 (g Pt)-1. From in situ X-ray absorption fine structure and diffuse reflectance infrared Fourier-transform spectroscopy studies, the superior catalytic performance of Pt1Co0.5/Al2O3 originates from the optimal length of the three-dimensional alloy-oxide perimeter sites. We further extended this strategy to other bimetallic systems of Pt-Fe and Pt-Ni, which also show similar structural properties and remarkable promotional effects on the catalytic activity.

Initial atomic-scale oxidation pathways on a Ni\u201315Cr(100) alloy surface

To understand the atomistic phenomenon behind initial oxidation processes, we have studied the nanoscale evolution of oxide growth prior to the formation of a complete layer on a Ni–15 wt%Cr(100) alloy surface using scanning tunneling microscopy/spectroscopy (STM/STS). At the onset of oxidation, a NiO superlattice forms oxide wedges across the step edges, eventually growing across the terraces. The completion of the NiO layer is followed by nucleation of the next layer, which always commences at the groove site of the superlattice. The Cr-oxide formation initiates as disk-shaped oxide particles early in the oxidation process, which Monte Carlo simulations reveal are likely caused by Cr clustering across the alloy surface. Upon further oxidation, a Cr(100)-p(2 × 2)O reconstructed surface is observed, indicating phase separation of Cr predicates the formation of the passive Cr-oxide film. The STS results vary across the oxide–alloy interface and between each oxide, providing greater insight into the origins of electronic heterogeneity and their effect on oxide growth. Using these data, we propose an oxidation model that highlights the growth of partial oxide layers on Ni–Cr(100) alloys within the pre-Cabrera–Mott regime.

Concomitant oxidation-diffusion-creep processes for stress generation and suppression of oxide-alloy interfacial instabilities

Metallic materials under extreme thermo-mechanical-environmental service conditions oftentimes rely on the formation of dense and protective oxide layers, e.g., typically alumina or chromia. Their failures, however, usually emerge from the morphological instability of such layers at the oxide-alloy interface. This work suggests that a tensile, biaxial stress arises in the substrate alloy due to the concomitant oxidation-diffusion-creep processes, which, in contrast to the growth stress in the oxide layer, provides a stabilizing factor that suppresses the rumpling or compositional variation in chromia or alumina interlayers. For rumpling analysis, the substrate tensile stress compensates the growth stress in the oxide layer. The compositional variation is modeled by the stress-domain analysis, which suggests that the substrate tensile stress prefers the growth of high-growth-stress oxide phase at the expense of low-growth-stress one. Our findings have been successfully employed to rationalize recent experiments in superalloys under cyclic oxidation conditions and in carburized or irradiated stainless steels under isothermal oxidation conditions.

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.

Mysterious failure in load-free superalloys under repeated thermal shocks

The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

Engineering a homogeneous alloy-oxide interface derived from metal-organic frameworks for selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

The selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is an important biomass conversion reaction. However, the multiple intermediates of the reaction make the catalyst design challenging. We engineered an active and selective catalyst with an active interface between Au-Pd alloy nanoparticles (NPs) and cobalt oxide supports via calcination of a composite of NPs encapsulated in metal-organic frameworks (MOFs). The catalyst shows an effective HMF-to-FDCA oxidation with total conversion and 95 % yield by 10 % hydrogen peroxide solution at 90 °C in one hour under atmospheric pressure. The mechanistic study shows that the engineered interface promotes the formation of hydroperoxyl radicals and dioxygen molecules, which accelerate the oxidation of reactive intermediates to FDCA. This work demonstrates the advantage of using MOF composites as a precursor to generate complex but active interfaces with a strong interaction between the metal and metal oxides.

Mysterious Failure in Load-Free Superalloys Under Repeated Thermal Shocks

Abstract The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

A comparison of the high-temperature oxidation behaviour of conventional wrought and laser beam melted Inconel 625

The microstructure and oxidation resistance of Laser Beam Melted (LBM) and Conventionally Manufactured (CM) Inconel 625 alloys were studied at 900 °C and 1050 °C. The microstructure of the LBM samples was cellular, with Nb and Mo segregations located at the cell walls. At 900 °C, the oxidation rate was similar for both materials but was clearly higher for the LBM material at 1050 °C. This high oxidation rate induced poor oxide scale compactness, void formation in the subsurface region and the formation of a high amount of Nb1.5Cr0.5O4 at the alloy-oxide interface.

Oxidation study of porous metal substrates for metal supported proton ceramic electrolyzer cells

Isothermal corrosion of coated and uncoated porous, ferritic stainless steel (Höganäs), with 20.6 wt% Cr and 30–35 % porosity, has been studied at temperatures from 600 to 800 °C in air and 2.5 %H2O/5 %H2/Ar. Uncoated specimens form protective Cr2O3 scales following parabolic kinetics at 600 and 700 °C in air and at 800 °C in 2.5 %H2O/5 %H2/Ar. Non protective scales were formed in 2.5 %H2O/5 %H2/Ar at 600 and 700 °C and at 800 C in air. The coating overall improves the corrosion behavior. The scaling behavior was explained from Fe-Cr-O2 stability diagrams and ratios between consumption and feed of Cr at the alloy-oxide interface.

Effect of CO2 gas pressure on composition, growth rate, and structure of duplex oxide formed on 9Cr steel at 550 °C

The oxidation behaviour of F91 in 0.1, 5, and 10 MPa CO2 was studied at 550 °C for 500 h. Previous results on the effect of CO2 pressure were summarised and compared with the results in this study. The oxidation kinetics decreased from 0.1 to 5 MPa and thereafter showed a lower dependence on the pressure (5–10 MPa). The carbon concentration at the oxide–alloy interface increased with time, but with different rates depending on the CO2 pressure. The amorphous carbon layers, and M23C6 carbides were identified in the inner oxide layer in the case of 5 MPa CO2.

STEM-EELS identification of TiOXNY, TiN, Ti2N and O, N dissolution in the Ti2642S alloy oxidized in synthetic air at 650 °C

Oxide-alloy interface of Ti6242S Ti-based alloy was investigated by STEM-EELS technique, after oxidation in both N2-free and synthetic air atmospheres at 650 °C for 1000 h. The chemical shift of Ti-L2,3 edge and its specific fine structure along the oxide-alloy interface were used as fingerprint to distinguish the different compositions of titanium nitrides and oxynitrides. TiN, Ti2N and TiNxOy were identified in the sample oxidized in synthetic air at the oxide-alloy interface. Moreover, a decreasing in the oxidation state of Ti oxides was found along with Sn segregation at the oxide-alloy interface for the sample oxidized in N2-free atmosphere.

Influence of Pre-oxidation on Filamentary Carbon Deposition on 20Cr25Ni Stainless Steel

The results of a pre-oxidation heat treatment at 930 °C in Ar/H2/H2O environments on a Si-bearing Nb-stabilised 20Cr25Ni austenitic steel are presented. The heat treatment was conducted under low pO2, achieved by the introduction of controlled amounts of moisture into the gas. The atmosphere promoted the formation of a continuous, dense, adherent, protective surface scale composed of Cr2O3 and MnCr2O4 with a thin Si-rich oxide at the oxide alloy interface. Samples with different oxide layer thicknesses were produced and further exposed at 700 °C, to a gas of nominal composition CO2/1%CO/1000 vpm C2H4 for 4 h. This gas mixture has a carbon activity greater than unity and readily forms filamentary carbon on the non-pre-oxidised alloy. This is catalysed by nickel particles formed intrinsically from the alloy during the early stages of oxidation of the unprotected surface. The oxide layers produced, as a result of the pre-oxidation process, could suppress carbon deposition onto the alloy; a significant reduction in carbon deposit was noted with an oxide of 125 nm thickness, and no deposit was found on the sample with an oxide thickness of 380 nm. The depth of depletion of chromium from the alloy correlated with the thickness of the oxide formed during the pre-oxidation heat treatment, but the chromium concentration at the oxide/metal interface remained at ~ 15–16 wt% and considered to be sufficient to reform a protective layer in the event of mechanical damage to the original. No additional chromium depletion of the alloy occurred during the 4-h deposition stage.

From Alloy to Oxide: Capturing the Early Stages of Oxidation on Ni-Cr(100) Alloys.

The interaction of oxygen with Ni-Cr(100) alloy surfaces is studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to observe the initial steps of oxidation and formation of the alloy-oxide interface. The progression of oxidation was observed for Ni(100) and Ni-Cr(100) thin films including Ni-8 wt % Cr(100) and Ni-12 wt % Cr(100), which were grown on MgO(100) in situ. These surfaces were exposed to between 1 and 150 L O2 at 500 °C, and additional annealing steps were performed at 500 and 600 °C. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and band gap distributions. Initial NiO nucleation and growth begins along the step edges of the Ni-Cr alloy accompanied by the formation of small oxide particles on the terraces. The incubation period known in oxidation of Ni(100) is absent on Ni-Cr alloy surfaces illustrating the significant changes in surface chemistry triggered by Cr-alloying. Step edge faceting is initiated by oxide decoration along the step edges and is expressed as moiré patterns in the STM images. The surface oxide can be characterized by NiONi(6 × 7) and NiO-Ni(7 × 8) coincidence lattices, which have a cube-on-cube epitaxial relationship. Small patches of NiO are susceptible to reduction during annealing; however, additional oxide coverage stabilizes the NiO. NiO regions are interspersed with areas covered predominantly with a novel cross-type reconstruction, which is interpreted tentatively as a Cr-rich, phase-separated region. Statistical analysis of the geometric features of the surface oxide including step edge heights, and NiO wedge angles illustrates the layer-by-layer growth mode of NiO in this pre-Cabrera-Mott regime, and the restructuring of the alloy-oxide interface during the oxidation process. This experimental approach has offered greater insight into the progression of oxide growth in Ni-Cr thin films and underscores the dramatic impact of alloying on oxidation process in the pre-Cabrera-Mott regime.

Oxidation–Nitridation Mechanism in Eutectic Cr-Silicide Alloy and Its Mitigation by Germanium Alloying

The oxidation and nitridation of the eutectic Cr–Cr3Si alloys during exposures in air was investigated with focus on the detailed mechanism of the attack. Exposures in synthetic air (N2–21%O2), and nitrogen (N2–5%H2) atmospheres revealed inward diffusion of nitrogen as the key mechanism for the nitridation process which was markedly higher in nitrogen compared to that in air. Post-exposure investigations proved selective nitridation of the Cr-solid solution phase in both synthetic air and N2–5%H2 atmosphere. During oxidation in air, a continuous A15 silicide layer (Cr3Si) formed at the oxide–alloy interface inhibiting the internal nitridation. This layer formed exclusively in oxidizing atmosphere as a result of the primary oxidation and outward diffusion of chromium, and dissolution of the Cr-solid solution phase. The barrier character of the A15 phase against nitridation was evident based on microstructural and chemical investigations as well as on thermodynamic considerations. Long-term oxidation results indicated that overtime the protective A15 layer was damaged by the formation porosities and internal oxidation during exposure which led to internal selective nitridation. Additions of germanium (Ge) was found beneficial by supporting the formation of the self-protecting A15 layer with long-term protective behavior against internal nitridation for more than 1000 h.

Effect of thermal cycling on protective properties of alumina scale grown on thin Haynes 214 foil

The isothermal and cyclic oxidation of a thin alloy 214 foil (130μm thick) has been studied in air at 1200°C for up to 720h. Haynes 214 forms a duplex oxide scale consisting of pure, columnar alumina overlaid by mixed Ni(Al,Cr)2O4 spinel with minor inclusions of (Al,Cr)2O3. A transition from external alumina to chromia scaling (abbreviated as TACS) occurs at the oxide-alloy interface after the Al reservoir of the foil is exhausted due to oxidation. The tensile cracking of the oxide scale upon thermal cycling is shown to promote the early transition from protective to accelerated breakaway oxidation.

Modelling compositional changes in nickel base alloy 602 CA during high temperature oxidation

The high chromium nickel base alloy 602 CA is used as construction material for high temperature components in aggressive environments such as gas burners or heat exchangers. The high temperature oxidation behaviour of this alloy is characterised by the formation of an external Cr2O3 scale and internal precipitation of alumina. The main goal of the present investigation was to elucidate the effect of aluminum on the subscale chromium depletion phenomena. The chromium concentration profiles were found to be flat and sometimes even increasing towards the oxide-alloy interface. The experimental observations were interpreted in terms of alloy thermodynamics, i.e. effect of oxidation induced compositional changes on chemical potentials of the alloy constituents. The concentration profiles in alloy 602 CA observed during high temperature exposure were modelled using the CALPHAD approach for thermodynamic and kinetic calculations. Thermodynamic calculations revealed that the Al depletion in the alloy subsurface due to internal alumina precipitation leads to enhanced Cr diffusion towards the alloy surface.

First-principles investigations of oxygen adsorption at TiNi surface and the TiO2/TiO–TiNi interface

Ab-intio study of the interaction of atomic and molecular oxygen with the TiNi(110) surface was performed using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. The oxygen adsorption energies were calculated and the preferential adsorption sites of oxygen atom on the surface were determined. Our results confirmed the formation of a Ni-rich layer at the alloy–oxide interface. Atomic and electronic structure of both TiO/TiNi(110) and TiO2/TiNi(110) interfaces were analyzed. The formation energies (Ef) of point defects at the interfacial layers as well as in bulk TiNi, monoclinic TiO, and rutile TiO2 were estimated. It was shown that Ef of Ti–Ni swap defect has a lower energy than that for the Ni antisites at the TiO2(100)/TiNi(110) interface. In the case of TiO(100)/TiNi(110) interface, the formation energies of Ti–Ni swap and Ni-antisites defects are close to each other. Our results demonstrated that Ef of Ni-defect in TiO is twice less than that in TiO2. The increase in the formation energies of defects in TiO2 reveal the increase in diffusion barriers of Ni atoms in comparison with those in oxides with a lower oxygen content, which hampers Ni segregation.

Predicting the depletion of chromium in two high temperature Ni alloys

The oxidation resistance of the high temperature Ni-Cr alloys alloy 601 and alloy 602 CA was studied. Based on microanalysis data from cyclic oxidation experiments performed in a burner test rig, the total loss of chromium in the alloy was determined. A numerical analysis using the general diffusion equation was performed to model the mass transport for the observed chromium loss. Chromium depletion profiles were calculated with an effective diffusion coefficient derived from the data of the shortest exposure time. Good agreement was found between the measured and the computed chromium concentration at the alloy-oxide interface for longer exposure.