Oxide-gas Interface Research Articles

New Insights into Thermal Chromia Growth on Fe18Cr(10Ni) Model Alloys at 900°C: Scaling Kinetics and Microstructures

Specimen of Fe-18Cr(-10Ni) (wt.-%) were isothermally oxidized in dry and wet high p(O2) gas at 900°C. The morphology and microstructural evolution of the chromia scales formed were analysed by SEM-EBSD. At high pO2 test gas containing O2 and H2O, the pO2 is independent of the H2O content of the test gas. However, both types of oxygen species, i.e. O2 as well as H2O contribute to the chromia scaling reaction. The effect of specimen thickness on chromia scaling allows the change of the concentration of the intrinsic defects and their diffusivities in thermally grown chromia scales independently from the test gas applied. Chromia scales are under pressure during scale growth. For thermodynamic reasons the concentration and diffusivity of the intrinsic, native defects in oxides under hydro-static pressure varies with the amount of pressure applied. Ion beam polished oxide cross-sections were analysed in terms of oxide grain growth and nucleation rates of new oxide at the oxide-gas interface. Chromia scales formed on the various substrates at high p(O2) test gas grew at different rates, and microstructures formed on Fe18Cr(-10Ni) differ in grain shape and size. The chromia growth kinetic was found to be depending on the thickness of the specimens used, the H2O content of the gas, and its Ni content. The scale growth mechanism consists of two interlinked levels. At the meta-level oxide nucleation and oxide grain growth are affected by the cation flux in the oxide scale formed and the nature of the oxygen species in the gas. This fixes the number, size and shape of the oxide grains as well as the number of oxide grain boundaries available as short-circuit diffusion paths. The lattice level denotes the atomic level. Hereby the cation flux in the single grain by lattice diffusion and cation vacancy condensation are linked. The defect structure in the oxide lattice is important. The factors affecting it are the Cr diffusion in the substrate and the presence of Ni in the alloy, local p(O2), H-defects and oxide growth stress. The experimental findings will be discussed with strong focus on the oxide growth mechanism, the chromia microstructure of the scales formed under the various conditions and the concentration and mobility of the native defects in the chromia scales and its interactions with H-containing species originating from water vapour in the test gas. Furthermore, the role of Fe as base and Ni as alloying element in the metal substrate will be discussed. This is done by considering the differences between the Ni-Cr-O and Fe-Cr-O phase diagrams and the consequence of the existence of the Fe3+ ion on the miscibility of the oxides formed by the various cations.

Effect of nanograin\u2013boundary networks generation on corrosion of carburized martensitic stainless steel

Martensitic stainless steel parts used in carbonaceous atmosphere at high temperature are subject to corrosion which results in a large amount of lost energy and high repair and maintenance costs. This work therefore proposes a model for surface development and corrosion mechanism as a solution to reduce corrosion costs. The morphology, phase, and corrosion behavior of steel are investigated using GIXRD, XANES, and EIS. The results show formation of nanograin–boundary networks in the protective layer of martensitic stainless steel. This Cr2O3–Cr7C3 nanograin mixture on the FeCr2O4 layer causes ion transport which is the main reason for the corrosion reaction during carburizing of the steel. The results reveal the rate determining steps in the corrosion mechanism during carburizing of steel. These steps are the diffusion of uncharged active gases in the stagnant–gas layer over the steel surface followed by the conversion of C into C4− and O into O2− at the gas–oxide interface simultaneously with the migration of Cr3+ from the metal-oxide interface to the gas-oxide interface. It is proposed that previous research on Al2O3 coatings may be the solution to producing effective coatings that overcome the corrosion challenges discussed in this work.

The influence of Titanium alloying and Co3O4 coating on the oxidation behavior of Fe-20Cr ferritic stainless steels for SOFC interconnects

Ferritic steel is a potential metal candidate for interconnect solid oxide fuel cell (SOFC) because its ability to form chromia layer at the surface during high temperature operation. However, chromia based oxide scales is reluctant to the evaporation of Cr from the oxide–gas interface. In this work, a combination of titanium alloying and application of Co3O4 spinel coating using dip coating method are carried out to improve the performance of ferritic steels as SOFC interconnect. The oxidation behavior of coated and uncoated Fe-20wt.%Cr alloys with different titanium contents: 0, 0.5, and 1 wt.% are studied as a function of time in air atmosphere. The samples were isothermally oxidized at 800°C for 24, 48, and 96 h in a box furnace. Preoxidation experiment were carried out on the alloys before the application of Co3O4 coating to improve its adherence. X-ray diffraction analysis (XRD) and Scanning Electron Microscope (SEM) were used for characterization of the prepared samples. The results shows that increasing the concentration of titanium in the alloys both for coated and uncoated samples caused the increased oxidation rates. The oxidation resistance of Co3O4 coated samples indicated from the weight change measurement was larger than that of the uncoated samples. Co3O4 coating is not effective for improving the oxidation resistance of samples.

Phase-field model of oxidation: Equilibrium.

A phase-field model of an oxide relevant to corrosion resistant alloys for film thicknesses below the Debye length L_{D}, where charge neutrality in the oxide does not occur, is formulated. The phase-field model is validated in the Wagner limit using a sharp interface Gouy-Chapman model for the electrostatic double layer. The phase-field simulations show that equilibrium oxide films below the Wagner limit are charged throughout due to their inability to electrostatically screen charge over the length of the film, L. The character of the defect and charge distribution profiles in the oxide vary depending on whether reduced oxygen adatoms are present on the gas-oxide interface. The Fermi level in the oxide increases for thinner films, approaching the Fermi level of the metal in the limit L/L_{D}→0, which increases the driving force for adsorbed oxygen reduction at the gas-oxide interface.

Scaling Kinetics and Scale Microstructure of Chromia Scales Formed on Ni-25%Cr Model Alloy during Oxidation in H2o-Containing High and Low pO2 Test Gas at 1000°C

The isothermal oxidation behaviour of a Ni25Cr model alloy was studied employing the JuSie-methodology, i.e. using specimens of different thickness for each specific test gas composition at 1000°C in dry and wet N2-1%O2 as well as in Ar-7%H2O. The oxidation mechanisms were evaluated using thermo-gravimetry and SEM/EBSD analyses of oxide scale cross sections. The ion beam polished oxide cross-sections were analysed in terms of oxide grain growth and nucleation rates of new oxide at the oxide-gas interface. The findings can be explained by considering the effect of H-defects and in-scale stress state on point defect concentrations in the chromium sub-lattice.

Oxygen activation on oxide surfaces: A perspective at the atomic level

Thin oxide films have been used as model supports to unravel the influence of the oxide–metal and oxide–gas interface. We discuss the influence of defects in the oxide lattice and the oxide surface on properties of adsorbed species, in particular the formation of oxide particles from a deposited transition metal onto ceria and of adsorbed oxygen from the gas phase. Here we correlate the structure of the particles, as revealed by a combination of STM and spectroscopic as well as theoretical calculations, with their reactive properties. The nature of the involved defects is characterized by adsorption of Au as a way to correlate the influence of various defects of different structure on the Au charge state. In a second case study, we demonstrate the influence of dopants within the supporting oxide on the adsorption of oxygen on a defect free surface. This is shown to be potentially relevant for the activation of methane in oxidative methane coupling reactions, as recently demonstrated.

A model for the initiation of reaction sites during the uranium–hydrogen reaction assuming enhanced hydrogen transport through linear oxide discontinuities

A model for the initiation of hydride sites on uranium metal is described for hydride attack in the region of linear discontinuities in the surface oxide film. The model considers the effect of variations in hydrogen permeation through such discontinuities due to intrinsic oxide and metal parameters and operational variables. Expressions are derived for the time dependence of the hydrogen concentration in the metal and the maximum attained hydrogen concentration in terms of these parameters. The derived expressions therefore relate hydride precipitation time in the metal underlying any oxide discontinuity to the physical width of the discontinuity, its length, the hydrogen diffusion coefficient down the oxide discontinuity, the hydrogen diffusion coefficient in the underlying metal lattice and the hydrogen concentration in the oxide discontinuity at the gas-oxide interface. The model can therefore account for how changes in operational conditions such as hydrogen pressure may change hydride attack times in the region of such oxide discontinuities. The model can also explain why, after any given time, hydride attack may occur at some but not all of the oxide discontinuities surrounding any individual metal grain. The model also considers how hydride attack times may, or may not, be modified by the intersection of linear discontinuities in the surface oxide film. Finally, the model is able to explain the reported experimental observation that oxide discontinuities having only a short rather than a long length are less preferred sites of hydride attack.

A model for the initiation of reaction sites during the uranium–hydrogen reaction assuming enhanced hydrogen transport through thin areas of surface oxide

A model for the initiation of hydride sites on uranium metal is described for conditions of constant hydrogen pressure. The model considers variations in hydrogen permeation through the surface oxide film due to intrinsic variations in the oxide thickness. It is proposed that thin areas of surface oxide favour enhanced hydrogen permeation through the oxide and lead to the more rapid initiation of hydride sites. The time and spatial dependence of the hydrogen concentration field in the metal underlying thin areas of oxide is calculated in terms of the local oxide film thickness, the hydrogen diffusion coefficients in the oxide and metal and the hydrogen concentration in the oxide at the gas–oxide interface. The time to precipitate hydride at any location is calculated by assuming that precipitation occurs once the hydrogen concentration in the metal attains the terminal solubility limit of the metal at the prevalent temperature. The model is compatible with the reported temperature and pressure dependence of the hydride induction time. The model can also explain observations such as precipitation of hydride at or beneath the oxide–metal interface and the arrested growth of hydride sites. Finally, an expression is derived for the number of hydride sites initiated on an entire sample surface in any given time by assuming a Gaussian oxide film thickness distribution over the entire sample surface.

Considerations on ultra-high frequency electric field effects on oxygen vacancy concentration in oxide thin films

Atomistic simulations employing dynamic charge transfer between atoms are used to investigate ultra-thin oxide growth on Al(100) metal substrates in the presence of an ac electric field. In the range of 1-10 GHz frequencies, the enhancement in oxidation kinetics by ∼12% over natural oxidation can be explained by the Cabrera-Mott mechanism. At field frequencies approaching 0.1-1 THz, however, we observe a dramatic lowering of the kinetics of oxygen incorporation by ∼35% compared to the maximum oxidation achieved, which results in oxygen non-stoichiometry near the oxide-gas interface (O/Al ≈ 1.0). This is attributed to oxygen desorption from the oxide surface. These results suggest a general strategy to tune oxygen concentration at oxide surfaces using ac electric fields that could be of interest in diverse fields related to surface chemistry and applications such as tunnel barriers, thin dielectrics and oxide interfaces.

Effect of Humidity on the Corrosion Kinetics of Ferritic Stainless Steels Subjected to Synthetic Biogas

Ferritic stainless steels have nowadays been used as materials for interconnectors in solid oxide fuel cells (SOFCs) at intermediate temperatures (800°C). Their degradation in contact with dry synthetic biogas used instead of other fuel gas has already been studied. In such biogas atmosphere, humidity may play an important role. The objective of this study is therefore to understand the effect of H2O on the corrosion kinetics of the ferritic stainless steels type AISI441 (18CrTiNb) under synthetic biogas (70%CH4and 30%CO2) mixed with 3%H2O. The thermodynamic analysis by FactSage was used to determine the partial pressure of oxygen and the activity of carbon in the humid biogas. The results showed that the partial pressure of oxygen is in the range 10–24.8to 10–21.2bar for temperatures between 600-800°C and that the formation of solid carbon can occur in these conditions. This was not different compared with the conditions in dry biogas. These conditions lead to the stability of some important oxides such as Cr2O3and Cr-Mn spinel and to carbon deposition and/or carbide formation. The surface morphology of 441 subjected to humid biogas showed oxide scale composed mainly of Cr2O3topped with Cr-Mn spinel. Some carbide such as Cr7C3was found in chromia scale. Kinetic experiments under both dry and humid biogas at temperatures between 600 and 800°C showed linear weight changes. Arrhenius law was followed and the rate-determining steps were identified as parallel oxidation and carburization limited by oxide-gas interface reactions.

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol−1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO2–MnCr2O4 spinels, inner layer of Cr2O3, and the inner most layer composed of Ni3Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO2 at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO2) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese–chromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.

The growth and evolution of thin oxide films on δ-plutonium surfaces

The common oxides of plutonium are the dioxide (PuO2) and the sesquioxide (Pu2O3). The nature of an oxide on plutonium metal under air at room temperature is typically described as a thick PuO2 film at the gas-oxide interface with a thinner Pu2O3 film near the oxide-metal substrate interface. In a reducing environment, such as ultra high vacuum, the dioxide (Pu4+; O/Pu=2.0) readily converts to the sesquioxide (Pu3+; O/Pu=1.5) with time. In this work, the growth and evolution of thin plutonium oxide films is studied with x-ray photoelectron spectroscopy (XPS) under varying conditions. The results indicate that, like the dioxide, the sesquioxide is not stable on a clean metal substrate under reducing conditions, resulting in substoichiometric films (Pu2O3-y). The Pu2O3-y films prepared exhibit a variety of stoichiometries (y~0.2-1) as a function of preparation conditions, highlighting the fact that caution must be exercised when studying plutonium oxide surfaces under these conditions and interpreting resulting data.

Effect of minor elements and a Ce surface treatment on the oxidation behavior of an Fe–22Cr–0.5Mn (Crofer 22 APU) ferritic stainless steel

This paper details the oxidation behavior of coupons from three different Crofer 22 APU sheets. Oxidation tests were conducted at 800 ∘ C in moist air for 2300 h. The major features of the scales were similar in all three coupons, with a continuous Cr–Mn spinel layer forming at the gas–oxide interface and a Cr 2O 3 layer forming at the oxide–metal interface. Also, at the Cr 2O 3–metal interface, large Cr–Mn oxide particles formed in the base metal. Metal extrusions in the oxide scale were observed in the vicinity of these Cr–Mn oxide particles. All three alloys had internal oxide particles forming in the base metal beneath the oxide scale, and the extent of this internal oxidation was related to the Al and Si-content in the coupons (as also reported by Piron-Abellan J, Huczkowski P, Ertl S, Shemet V, Singheiser L, Quadakkers W. In: Fuel cell seminar, 2004. San Antionio, TX, November 1–5, 2004). Of note, discontinuous Ti-oxide and Mn-oxide particles formed above the outer spinel layer on the coupons with relatively high Al and Si contents. The effectiveness of rare-earth surface modification on improving oxidation resistance of this alloy was demonstrated. Pack cementation-like processes were used to treat the surface of the coupons with Ce. A continuous Cr–Mn outer oxide layer and a continuous inner Cr 2O 3 layer formed on the surface of the treated coupons during oxidation. However, the thickness of the scales and amount of internal oxidation were significantly reduced with the treatment, leading to the decrease in oxidation rate. The amount of internal oxidation was also reduced with the treatment. The treatment appeared to suppress the formation of SiO 2 at the scale–metal interface in high Si containing alloys. ASR measurements indicate that the treatment decreases the electrical resistance, which is a benefit for SOFC interconnect application.

Influence of gas transport on the oxidation rate of aluminum arsenide

The effect of gas transport on the lateral oxidation kinetics of aluminum arsenide has been studied by limiting the gas transport to the gas–oxide interface using closely spaced mesas. At 440 °C the gas transport factor is found to be 8.5±1.3 times the reaction rate coefficient, resulting in a measurable decrease in oxidation rate for mesas closer than 200 nm. This confirms the usual assumption that the initial oxidation rate is limited by the reaction kinetics at the oxidizing interface and not the gas transport for normal oxidation conditions

High temperature oxidation of TiCrN coatings deposited on a steel substrate by ion plating

The oxidation behavior of TiCrN coatings having compositions of Ti 36Cr 26N 38, Ti 31Cr 35N 34 and Ti 14Cr 52N 34 was studied between 700 and 1000°C in atmospheric air. The oxidation resistance of TiCrN coatings which were composed of TiN and CrN phases increased in the order of Ti 36Cr 26N 38, Ti 31Cr 35N 34 and Ti 14Cr 52N 34, implying that chromium within the coating played a decisive role in providing the oxidation protection. The oxides formed always consisted of TiO 2 and Cr 2O 3. During oxidation, all the involved elements diffused either outwardly or inwardly, depending on the concentration gradients. The substrate elements diffused outwardly toward the oxide-gas interface, oxygen from the atmosphere diffused inwardly and the coating elements diffused predominantly outward.

Self-repairing metal oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O2 gas near 20 mbar. Information on the position of oxide growth has been gained from the 18O/SIMS technique. Rates of O2 dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O2. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O2. An increased rate of O2 dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.

On the use of the micromarker technique for studying the growth mechanism of thin oxide films

The high temperature behavior of microscopic markers, deposited on to pure and ceria-coated nickel, was tested. The markers, having a form of 20-nm-thick palladium bumps with a fractional surface coverage of 0.05 and 30-nm thick gold bumps with a surface coverage of 0.25, apparently interfered with the oxide growth, increasing the oxidation rate on their surface at 973 K over one order of magnitude. As detected by a combination of Auger electron spectroscopy and Rutherford backscattering spectrometry, for ceria modified nickel, the markers’ depth-position coincided with the ceria particles dispersed in the vicinity of the oxide–gas interface. For pure nickel oxide films, the markers were easily incorporated into the growing oxide during very early stages of oxidation and were difficult to detect at the oxide–metal interface. It is proposed that the catalytic behavior of the markers was caused by their fragmentation into a large number of islands, leading to the refinement of the oxide grains, which in turn accelerated the substrate oxidation rate. Some precautions necessary in conducting the marker experiments are defined.

High temperature oxidation of an oxide-dispersion strengthened NiAl

The oxidation behavior of an oxide-dispersion strengthened (ODS) NiAl has been studied between 900 and 1100°C in air. The dispersoids of mostly Al 2O 3 in fine-grained β-NiAl were incorporated by mechanical alloying (MA) in an argon atmosphere and hot pressing. It was found that excessive amounts of dispersoids and voids within the matrix had serious negative effects on the oxidation resistance of β-NiAl, by allowing for a more rapid formation of oxide scales and by providing fast diffusion paths for oxygen. Below the thin surface oxide scales consisted of α-Al 2O 3, NiAl 2O 4 and Ni 2O 3, an internal oxidation zone was formed deep into the matrix. No metastable transient aluminas were formed during oxidation. The oxide ridge structure began to evolve after oxidation at 1100°C at the oxide–gas interface.

Initial oxidation stage of yttrium-implanted pure iron at 700 °C by in situ high temperature X-ray diffraction

Oxidation study, at 700 °C and under controlled atmosphere (oxygen partial pressure: 0.04 Pa) for 24 h, of unimplanted or yttrium implanted pure iron specimens was carried out using in situ X-ray diffraction to observe the yttrium implantation effect. In situ X-ray diffraction analyses performed on unimplanted pure iron clearly show the formation of FeO (wustite) and Fe3O4 (magnetite) compounds at the beginning of the oxidation test whereas the formation of Fe2O3 (hematite) is only detected after 1h30 of oxidation. In situ X-ray diffraction analyses performed with the same experimental conditions on yttrium implanted pure iron show the presence of two Mayn yttrium-iron mixed oxides (i.e. FeYO3 and Fe2YO4) in combination with wustite, magnetite and hematite structures. The presence of the Fe2YO4 compound is important because several authors have reported its anticorrosion properties at high temperature. X-ray diffraction results also show that the formation of FeYO3 and Fe2YO4 is Maynly reinforced at the gas-oxide interface which suggests an outward yttrium diffusion through the oxide layer from implanted pure iron.

Oxidation behavior of AgCuTi brazing alloys

Abstract Two kinds of brazing alloys, Ag56.9Cu37.6Ti5.5 and Ag54.7Cu36.3Ti9.0, were brazed on Si3N4 substrate and their oxidation behavior was investigated at 400, 500 and 600°C in air. Under the experimental conditions, Ag was not oxidized, whereas Cu and Ti were oxidized parabolically. The oxide scale formed on the brazing alloy consisted of a mixture of Cu2O and CuO, which grew by the outward diffusion of cations. Beneath this external scale, an Ag-enriched zone was formed due to the selective oxidation of Cu at the surface. Some isolated and small Ag particles were present at the oxide-gas interface. Also, the inward diffusion of oxygen led to the formation of internal oxides of TiO2 and Ti2O3.