H2-H2O Atmospheres Research Articles

Oxidation kinetics and electrical properties of oxide scales formed under exposure to air and Ar–H2-H2O atmospheres on the Crofer 22 H ferritic steel for high-temperature applications such as interconnects in solid oxide cell stacks

A 100 h isothermal oxidation kinetics study for Crofer 22H was conducted in air and the Ar–H2-H2O gas mixture (p(H2)/p(H2O) = 94/6) in the range of 973–1123 K. The parabolic rate constant was independent of oxygen partial pressure in the range from 6.2 × 10−24 to 0.21 atm at 1023 and 1073 K, while at 973 and 1123 K it was higher in air than in Ar–H2-H2O. The scales consisted of Cr2O3 and manganese chromium spinel with an Mn:Cr ratio dependent on the oxidation conditions. Cross-scale resistance was evaluated with regards to the application of the steel in solid oxide fuel cells using a number of methods, including X-ray diffraction, scanning electron microscopy, confocal Raman imaging and area-specific resistance measurements.

Reduction Kinetics of Pre-Oxidized Ilmenite Pellets by H2-H2O Gas Mixtures

The reduction behavior of pelletized and pre-oxidized ilmenite is investigated in H2-H2O atmospheres containing between 0 and 7% H2O and at temperatures between 983 and 1183 K (710 and 910 °C). The reduction mechanism occurs in two stages wherein the rapid reduction of trivalent to divalent iron cations is followed by the slower metallization of iron. Both temperature and gas composition are critical to achieving high reaction rates; within the range of conditions studied, the driving force for metallization has a significant effect on the reduction rate. Based on the experimental data and thermodynamic calculations, a model is established to predict the progress of the reduction as a function of temperature, gas composition and time. The application of this model at variable temperatures permits the determination of the activation energy Ea = 51 kJ/mol for the metallization reaction.

Less Effective Hydrodynamic Escape of H2–H2O Atmospheres on Terrestrial Planets Orbiting Pre-main-sequence M Dwarfs

Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway-greenhouse condition because of a slow decline in host-star luminosity in its pre-main-sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar X-ray and extreme ultraviolet (XUV) irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows. Here we carry out 1D hydrodynamic escape simulations for an H2–H2O atmosphere on a planet with mass of 1M ⊕ considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway-greenhouse phase. We find that the atmospheric escape rate decreases with the basal H2O/H2 ratio due to the energy loss by the radiative cooling of H2O and chemical products such as OH and OH+: the escape rate of H2 becomes one order of magnitude smaller when the basal H2O/H2 = 0.1 than that of the pure hydrogen atmosphere. The timescale for H2 escape exceeds the duration of the early runaway-greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H2 and H2O could be left behind after the end of the runaway-greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs.

Impact of pre-oxidation on the reactivity and conductivity in H2–H2O atmosphere of a ferritic stainless steel for high temperature water vapour electrolysis

The as-rolled K41X (AISI 441) ferritic stainless steel presents unusual behaviour when exposed in H2–H2O atmosphere at 800 °C, with growth of non-protective iron oxides at its surface. The impact of pre-oxidation in air is investigated in this paper. The pre-oxidation induces the formation of a thin chromia layer at the steel surface. The results obtained after subsequent oxidation for 100 h in H2–H2O atmosphere evidence a change in the surface reactivity behaviour of the as-rolled alloy: iron oxides formation is avoided and high temperature behaviour (reactivity and conductivity) is improved. Air pre-oxidation seems a promising solution for large scale use of K41X as-rolled steel in H2–H2O atmosphere at high temperature.

Geometrical Construction of Auxiliary Axes in an Ellingham Diagram

To facilitate the assessment of the oxide stability in H2-H2O or CO-CO2 atmospheres, auxiliary axes are constructed in the Ellingham diagram. Based on A. Ghosh’s approach, the geometrical interpretation of the diagram is proposed for the reaction 2X + O2 = 2Y, where X and Y could be originated from H2 and H2O or CO and CO2. Two cases are considered when oxygen partial pressures are lower and higher than one bar. By a geometrical method, it is proved that with an appropriate set-up of values relating to the auxiliary axes, the axes representing the ratio between the equilibrium partial pressure of hydrogen and that of water vapour, as well as the ratio between the equilibrium partial pressure of carbon monoxide and that of carbon dioxide, can be constructed. The geometrical method on the construction of axes using thermodynamic derivation is explained in the paper.

Oxide film prepared by selective oxidation of stainless steel and anti-coking behavior during n-hexane thermal cracking

In order to reduce coke formation on the substrate surface, selective oxidation pre-treatments were systematically assessed for 316L stainless steel under H2-H2O atmospheres at 650 °C–950 °C and 0.1 MPa for 10 h. The phase constitution, elemental composition and surface morphology of oxide films were characterized by a combination of SEM, XRD, and EDS. The results showed that the oxidation temperature had a great influence on the morphology and microstructure of oxide films. The Mn and Cr content on the metal surface gradually increased with the increasing exposure temperature. The element composition was mainly associated with MnCr2O4 and Cr2O3. Thickness of oxide films was evaluated in various conditions. The anti-coking performance was evaluated by the n-hexane thermal cracking. It was found that the amount of the coke deposited on the oxidized tube was much smaller than that on the untreated tube, with an average anti-coking ratio of 83.2%. Moreover, the morphologies of carbon deposits were mainly filamentous on the inner surface of untreated tube, but granular for the case of the oxidized tubes.

Influence of Different Annealing Atmospheres on the Mechanical Properties of Freestanding MCrAlY Bond Coats Investigated by Micro-Tensile Creep Tests

The mechanical properties of low-pressure plasma sprayed (LPPS) MCrAlY (M = Ni, Co) bond coats, Amdry 386, Amdry 9954 and oxide dispersion strengthened (ODS) Amdry 9954 (named Amdry 9954 + ODS) were investigated after annealing in three atmospheres: Ar–O2, Ar–H2O, and Ar–H2–H2O. Freestanding bond coats were investigated to avoid any influence from the substrate. Miniaturized cylindrical tensile specimens were produced by a special grinding process and then tested in a thermomechanical analyzer (TMA) within a temperature range of 900–950 °C. Grain size and phase fraction of all bond coats were investigated by EBSD before testing and no difference in microstructure was revealed due to annealing in various atmospheres. The influence of annealing in different atmospheres on the creep strength was not very pronounced for the Co-based bond coats Amdry 9954 and Amdry 9954 + ODS in the tested conditions. The ODS bond coats revealed significantly higher creep strength but a lower strain to failure than the ODS-free Amdry 9954. The Ni-based bond coat Amdry 386 showed higher creep strength than Amdry 9954 due to the higher fraction of the β-NiAl phase. Additionally, its creep properties at 900 °C were much more affected by annealing in different atmospheres. The bond coat Amdry 386 annealed in an Ar–H2O atmosphere showed a significantly lower creep rate than the bond coat annealed in Ar–O2 and Ar–H2–H2O atmospheres.

A comprehensive study of the TiN/Si interface by X-ray photoelectron spectroscopy

In this paper, a comprehensive X-ray photoelectron spectroscopy (XPS) study of the first atomic layers of TiN nanofilms grown by ion beam assisted deposition on crystalline silicon is reported. This deposition technique allows a fine control of the ion species and energy arriving at the substrate. The substrates are prepared by ion beam cleaning involving Xe+ ion bombardment in different partial pressures of molecular H2. The expected hydrogen passivation effect by the Si–H formation bond limiting the Si–O bonds was quantitatively evaluated and correlated with some retention of H and O at the substrate surface. The effects of molecular H2 and residual H2O atmosphere present during the process on the chemical bonding on both the naked Si substrate and afterward on the interface are reported. A detailed XPS analysis performed in an attached UHV chamber to the preparation chamber of the TiN/Si interface shows that the bombarding cleaning procedure plays an important role in the bond formation at the interface since minute amounts the oxygen jeopardize the bulk properties.

High temperature oxidation of HP40 alloy under H2–H2O and air atmospheres: a surface study

In the present work, high temperature oxidation of HP40 alloy was carried out at 1050 °C under H2–H2O and air atmospheres; the influence of atmosphere on surface morphology and composition was studied. Octahedral crystals with considerable spalled regions are present on the surface of alloy oxidized under air, the oxide scale composes of MnCr2O4, Cr2O3 and (Fe, Ni)Cr2O4 and spalled regions exhibit base alloy and SiO2‐rich regions. The surface of alloy oxidized under H2–H2O is fully covered by small granular crystals and blade‐type structures without spallation, and the oxide scale composes of MnCr2O4 and Cr2O3. Moreover, X‐ray photoelectron spectroscopy analysis shows considerable difference in chemical valence states of Mn, Cr and O elements on both alloy surfaces, and hydroxyl compounds exist on the alloy oxidized under H2–H2O atmosphere. Copyright © 2017 John Wiley & Sons, Ltd.

Properties of oxide films grown on 25Cr20Ni alloy in air-H2O and H2-H2O atmospheres

The oxidation kinetics, surface morphology and phase structure of oxide films grown on 25Cr20Ni alloy in air-H2O and H2-H2O atmospheres at 900 °C for 20 h were investigated. The anti-coking performance and resistance to carburization of the two oxide films were compared using 25Cr20Ni alloy tubes with an inner diameter of 10 mm and a length of 850 mm in a bench scale naphtha steam pyrolysis unit. The oxidation kinetics followed a parabolic law in an air-H2O atmosphere and a logarithm law in a H2-H20 atmosphere in the steady-state stage. The oxide film grown in the air-H2O atmosphere had cracks where the elements Fe and Ni were enriched and the un-cracked area was covered with octahedral-shaped MnCr2O4 spinels and Cr1,3Fe0.7O3 oxide clusters, while the oxide film grown in the H2-H2O atmosphere was intact and completely covered with dense standing blade MnCr2O4 spinels. In the pyrolysis tests, the anti-coking performance and resistance to carburization of the oxide film grown in the H2-H2O atmosphere were far better than that in the air-H20 atmosphere. The mass of coke formed in the oxide film grown in the H2-H2O atmosphere was less than 10% of that in the air-H2O atmosphere. The Cr1.3Fe0.7O3 oxide clusters converted into Cr23C6 carbides and the cracks were filled with carbon in the oxide film grown in the air-H2O atmosphere after repeated coking and decoking tests, while the dense standing blade MnCr2O4 spinels remained unchanged in the oxide film grown in the H2-H2O atmosphere. The ethylene, propylene and butadiene yields in the pyrolysis tests were almost the same for the two oxide films.

Re-oxidation Kinetics of Flash-Reduced Iron Particles in H2-H2O(g) Atmosphere Relevant to a Novel Flash Ironmaking Process

A novel flash ironmaking process based on hydrogen-containing reduction gases is under development at the University of Utah. The goal of this work was to study the possibility of the re-oxidation of iron particles in a H2-H2O gas mixture in the lower part of the flash reactor from the kinetic point of view. The last stage of hydrogen reduction of iron oxide, i.e., the reduction of wustite, is limited by equilibrium. As the reaction mixture cools down, the re-oxidation of iron could take place because of the decreasing equilibrium constant and the high reactivity of the freshly reduced fine iron particles. The effects of temperature and H2O partial pressure on the re-oxidation rate were examined in the temperature range of 823 K to 973 K (550 °C to 700 °C) and H2O contents of 40 to 100 pct. The nucleation and growth kinetics model was shown to best describe the re-oxidation kinetics. The partial pressure dependence with respect to water vapor was determined to be of first order, and the activation energy of re-oxidation reaction was 146 kJ/mol. A complete rate equation that adequately represents the experimental data was developed.

Effect of H2/H2O ratio on thermally grown chromia scales formed on Ni25Cr alloy in Ar–H2–H2O atmospheres at 1000°C

The oxidation behaviour of a NiCr-base model alloy with a 25% chromium content is investigated at 1000°C in different low p(O2) environments (Ar –H2–H2O) with the emphasis of how hydrogen andyor water vapour affects the oxidation behaviour of chromia forming alloys \\in low p(O2) environments containing H2 and H2O. After equilibrium conditions are achieved, chromia growth on Ni25Cr in low p(O2) type test gases containing H2 and H2O is ruled by the concentration of Cr interstitials available for Cr outward transport. The chromia microstructure, therefore, consists largely of columnar shaped oxide grains. The chromia scales are growing predominantly from oxygen coming from the water vapour in the test gas. H-species may facilitate oxide growth in pores at the metal – oxide interface. H-defects affect the transport properties of chromia scales formed on Ni25Cr. At low p(O2) the H-defects in the chromia scale are compensated by a lower concentration of Cr interstitials. Depending on the nature of the test gas, i.e. p(O2), p(H2) and p(H2O), as well as on the Cr activity in the metal substrate, the growth of chromia scales on NiCr alloys may be featured and shaped differently.

Oxidation behavior of alloy HK40 in H2 H2 O atmosphere

In the present paper, the isothermal oxidation behavior of alloy HK40 in H2H2O atmosphere in the temperature range 850–950 °C was systematically investigated by means of TGA, XRD, SEM, and EDS. The results demonstrated that the oxidation of alloy HK40 under low oxygen partial pressure obeyed a cubic instead of parabolic rate law. The values of the oxidation rate constant k at 850, 900, and 950 °C were 0.0079, 0.01886 and 0.04031 mg3/(cm6 h) at 850, 900, and 950 °C, respectively. The scales were composed of MnCr2O4 and Cr2O3. MnCr2O4 mainly existed in the form of blades in the outer part of the scale and its amount gradually increased with increasing temperature. In addition, there were silica particles along the scale/metal interface and indications were found that they contribute to the improvement of scale spallation resistance at high temperature as found by scratch tests.

Effect of H2/H2O ratio on thermally grown chromia scales formed on Ni25Cr alloy in Ar–H2–H2O atmospheres at 1000°C

Effect of H₂/H₂O ratio on thermally grown chromia scales formed on Ni25Cr alloy in Ar–H₂–H₂O atmospheres at 1000°C

Influence of Hafnium Additions and Preoxidation Treatment on the Metal Dusting of Ni–Al Alloys

Alloys of γ-Ni(Al), γ–γ′ Ni(Al), γ′-Ni3Al and β-NiAl, each containing 0.5 wt% hafnium, were exposed in 1 h cycles to a carbon supersaturated CO–H2–H2O atmosphere (aC = 36.7, \( p_{{{\text{O}}_{2} }}\) = 2.83 × 10−26 atm) at 650 °C and an overall pressure of 1 atm. The alloys accumulated surface coke deposits and, to varying degrees, underwent metal dusting. While morphologies of the reacted samples were found to be very similar to those of equivalent hafnium-free alloys, the weight uptake kinetics were faster for alloys containing 0.5 wt% Hf. The hafnium-containing alloys were preoxidised in a H2–H2O mixture at 1,100 °C to form α-Al2O3 prior to metal dusting exposure. This treatment led to external oxide formation on all alloys except the low Al γ-Ni(Al) alloy, which formed internal oxides. Cyclic metal dusting resistance of all alloys was improved significantly by the preoxidation treatment, as alumina forms a barrier to inward carbon diffusion, and hafnium improves the quality of the barrier oxide.

Electrochemical Oxidation at SOFC Anodes: Comparison of Patterned Nickel Anodes and Nickel/8YSZ Cermet Anodes

Patterned Ni anodes are a common method to determine elementary kinetic reaction mechanism at the anode of Solid Oxide Fuel Cells. Their well defined Triple-Phase-Boundary (TPB) of electrode, electrolyte and gas phase, where the electrochemical oxidation takes place, is easily accessible. However, the direct transfer of knowledge gained with patterned Ni anodes to technical Ni/8YSZ cermet anodes is doubtful. For the first time a comparison of the electrochemical performance of both anode concepts is performed by means of (i) transmission line model calculations and (ii) performance simulations using a 3D FEM microstructure model. The good agreement observed between the performance of patterned Ni anodes and Ni/8YSZ cermet anodes indicates similar reaction mechanisms at the TPB of both anode concepts. Especially in H2-H2O atmosphere, the agreement of (i) absolute ASR values and (ii) parameter dependencies is very good and shows the applicability of patterned Ni anodes for the investigation of the electrochemistry of technical Ni/8YSZ cermet anodes.

Unusual decrease in conductivity upon hydration in acceptor doped, microcrystalline ceria

The impact of hydration on the transport properties of microcrystalline Sm(0.15)Ce(0.85)O(1.925) has been examined. Dense, polycrystalline samples were obtained by conventional ceramic processing and the grain boundary regions were found, by high resolution transmission electron microscopy, to be free of impurity phases. Impedance spectroscopy measurements were performed over the temperature range 250 to 650 °C under dry, H(2)O-saturated, and D(2)O-saturated synthetic air; and over the temperature range 575 to 650 °C under H(2)-H(2)O atmospheres. Under oxidizing conditions humidification by either H(2)O or D(2)O caused a substantial increase in the grain boundary resistivity, while leaving the bulk (or grain interior) properties unchanged. This unusual behavior, which was found to be both reversible and reproducible, is interpreted in terms of the space-charge model, which adequately explains all the features of the measured data. It is found that the space-charge potential increases by 5-7 mV under humidification, in turn, exacerbating oxygen vacancy depletion in the space-charge regions and leading to the observed reduction in grain boundary conductivity. It is proposed that the heightened space-charge potential reflects a change in the relative energetics of vacancy creation in the bulk and at the grain boundary interfaces as a result of water uptake into the grain boundary core. Negligible bulk water uptake is detected under both oxidizing and reducing conditions.

ChemInform Abstract: Corrosion and Strength of SiC-Whisker-Reinforced Alumina Exposed at High Temperatures to H2-H2O Atmospheres

The flexural strengths and corrosion behavior of a SiC-whisker-reinforced alumina composite material (Al2O3—SiCw) were studied as functions of time, temperature, and PH2O in H2—H2O environments. Experimental conditions included temperatures of 1300° and 1400°C, exposure times up to 30 h, and water vapor levels ranging from very dry (PH2O= 4.6 × 10−7 MPa) to relatively wet (PH2O= 2.2 × 10−3 MPa). The study showed that the weight, strength, composition, and microstructure were strongly dependent on the PH2O level in the environment. When the PH2O in the H2 atmosphere was low, active oxidation of the SiC whiskers in the composite occurred, causing severe reductions in the weight and strength of the samples. At 1400°C, the most severe degradation was observed following exposure to an atmosphere with PH2O= 1.3 × 10−5 MPa. Exposure for 10 h to such an environment reduced the room-temperature flexural strength of the material to less than 50% of that of the unexposed samples. At higher water vapor levels, the reductions in weight and strength became less severe because of the formation of aluminosilicate glass and mullite at the sample surface. Finally, when the composite was exposed at 1400°C to H2 atmospheres containing water vapor pressures of 5 × 10−4 MPa and above, weight gains were observed; and the strength of the material was significantly higher than that of the unexposed material. The increase in strength was attributed to crack healing or blunting as a result of the formation of aluminosilicate glass and mullite on the sample surface. Similar dependencies of strength and weight on the level of oxidant (H2O) in the atmosphere were noted following the exposures at 1300°C; however, the effects were smaller in magnitude.

Atmosphere dependence of oxidation kinetics of unstabilized and Nb-stabilized AISI 430 ferritic stainless steels in the temperature range 850–950°C

The effect of oxygen partial pressure and temperature on the oxidation behaviour of unstabilized and Nb-stabilized AISI 430 ferritic stainless steels were investigated over the temperature range 850–950°C in air, Ar+1 vpm O2 or Ar–H2–H2O atmospheres. Isothermal thermogravimetric analyses were performed for 50 h. The microstructure of the oxide films grown on the steels were characterized by SEM and chemical analyses were performed by EDS, grazing X-ray diffraction and secondary ion mass spectrometry (SIMS). Nb-stabilized AISI 430 steel is more resistant to oxidation than unstabilized AISI 430 under all tested conditions, except above 900°C in Ar–H2–H2O. While the oxidation behaviour of unstabilized AISI 430 is strongly affected by the atmosphere composition, Nb-stabilized AISI 430 oxidation rates do not depend strongly on the atmosphere. For both steels, the chemical analyses show Cr2O3 as the main phase in the oxide scales, but solid solutions such as FeFe2−xCrxO4 and MnCr2O4 formed in almost all scales for atmospheres of Ar+1 vpm O2 or in Ar–H2–H2O, and Fe2O3 and Mn1.5Cr1.5O4, are also observed for oxidation in air.

The Kinetics of Reduction of Dense Synthetic Nickel Oxide in H2-N2 and H2-H2O Atmospheres

An investigation of the kinetics of reduction of dense synthetic nickel oxide has been carried out in H2-N2 and H2-H2O mixtures between 500 °C and 1000 °C. The progress of the reduction was followed metallographically by the measurement of the advance of the nickel product layer. The influences of hydrogen partial pressure, hydrogen-steam ratio, and temperature were systematically investigated in both sets of the mixtures. Increasing hydrogen partial pressure under all conditions investigated results in an increase in the reduction rate. In H2-N2 mixtures and H2-H2O mixtures with low steam content, the initial reduction rate was found to be first order with respect to hydrogen partial pressure. In both sets of mixtures, it was found that the progress of Ni thickness was not a monotonic function of temperature. A minimum rate of advancement of Ni product was observed between 600 °C and 800 °C, depending on the hydrogen partial pressures and reduction time. The change in reduction behavior is shown to be directly linked to changes in Ni product microstructure.