Parabolic Oxidation Research Articles

Oxidation kinetics of hot-pressed ZrB2–SiC ceramic matrix composites

The isothermal oxidation of ZrB2 ceramics with different content of SiC was carried out in air up to 1700°C, and the effect of chemical composition on the oxidation kinetics of ZrB2–SiC ceramics was analyzed. As indicated by the experimental results, both ceramics with 20vol% and 30vol% SiC particles followed near parabolic oxidation kinetics. And higher concentration of SiC led to better oxidation resistance performance due to the formation of more silica glass on the surface of the specimens and formation of zircon phase ZrSiO4. In addition, the oxidation of the ceramic composites is diffusion controlled and the activation energies for the diffusion of oxygen in the oxide scale of the ceramics oxidized at temperatures below 1500°C were 5134J/mol and 5960J/mol for ZS2 and ZS3, respectively. Then the oxidation of the ceramics transformed to reaction controlled under elevated temperature due to evaporation of the protective silica glass. Furthermore, the evolution process of the surface morphology and oxide scale of the ceramic during initial oxidation stage was given detailed investigation.

Effect of the sintering additive content on the protective passive oxidation behaviour of pressureless liquid-phase-sintered SiC

Abstract The long-term oxidation behaviour in air of pressureless liquid-phase-sintered SiC was investigated as a function of the sintering-additive content (a mixture of Y2O3 and Al2O3 in the 3:5 molar ratio) at oxidizing temperatures in the interval 1100–1300 °C. It is shown that oxidation under these mild conditions is always passive, and with formation of protective oxide scales. However, the oxidation kinetics cannot be described appropriately by the parabolic-rate law. Instead, due to the gradual crystallization of the oxide scales during oxidation, it is more complex, exhibiting two different stretches given respectively by the arctan- and parabolic-rate laws. Furthermore, it was found that the rate-limiting mechanism of the initial arctan oxidation is the outward diffusion of metal cations from the secondary intergranular phase into the oxide scale, with the activation energy of the oxidation being very high and decreasing from 545 to 432 kJ/mol with increasing sintering-additive content from 5 to 20 wt%. The rate-limiting mechanism of the subsequent parabolic oxidation is however the inward diffusion of oxygen through the multicomponent oxide scale, with the activation energy being lower than before and also decreasing from 345 to 205 kJ/mol as the sintering-additive content increases from 5 to 20 wt%. It is also shown that the oxidation resistance decreases with increasing sintering-additive content, but that while the decrease is moderate up to 10 wt%, it is very marked for greater contents.

Oxidation resistance of quintuple Ti-Al-Si-C-N coatings and associated mechanism

The oxidation behavior of Ti-Al-Si-C-N hard coatings with different Al contents deposited on high-speed steel and Si substrates by hybrid arc-enhanced magnetron sputtering is investigated in the temperature range of 500 °C–1000 °C. The coating hardness is maintained at around 35 GPa, and the parabolic oxidation rate constant Kp at 1000 °C decreases to 3.36 × 10−10 kg2 m−4 s−1 when the Al concentration is increased to 30 at. %, indicating that Ti-Al-Si-C-N coatings with larger Al concentrations have better oxidation resistance. X-ray diffraction, cross-sectional scanning electron microscopy, and x-ray photoelectron spectroscopy reveal a protective surface layer consisting of Al2O3, TiO2, and SiO2 that retards inward oxygen diffusion. A mechanism is proposed to elucidate the oxide formation. As a consequence of the good oxidation resistance, the Ti-Al-Si-C-N coatings have a large potential in high-speed dry cutting as well as other high temperature applications.

About the Role of Chromium and Oxygen Ion Diffusion on the Growth Mechanism of Oxidation Films of the AISI 304 Austenitic Stainless Steel

To study the role of ion diffusion on the oxidation process of the AISI 304 austenitic stainless steel, chromium diffusion coefficients were determined in oxide films grown on this steel at 750, 800 and 850 °C, in air. The isotope 54Cr was used as a chromium tracer and the diffusion profiles were established by SIMS. The bulk diffusion coefficients are five orders of magnitude smaller than the grain boundary diffusion coefficients. It was found that the values of the chromium diffusivities are lower than the corresponding oxygen diffusivities obtained in previous work. Parabolic oxidation constants calculated as a function of the chrome and oxygen diffusivities, using Wagner′s theory, are close to those determined experimentally, which shows that the growth rate of chromia is controlled by ion diffusion, but inward oxygen diffusion plays the main role on the growth kinetics of chromia formed by the oxidation of the AISI 304 steel.

Determination of Oxygen Diffusion Coefficient in Oxidation Films of the AISI 439 Ferritic Stainless Steel

In order to investigate the role of oxygen diffusion in the oxidation process of the AISI 439 ferritic stainless steel, oxygen ion diffusion coefficients were determined, for the first time, in oxide films formed by the oxidation of this steel. Steel samples were firstly oxidized from 750o C to 900o C, in synthetic air, in order to grow oxide films mainly made up of chrome oxide; the oxygen diffusion experiments were then performed using the stable isotope 18O as oxygen tracer. The introduction of the 18O in the oxide film was performed by means of the gas-solid isotopic exchange method, in the temperature range of 750-900o C, in Ar+21%18O2 atmosphere. The 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Parabolic oxidation constants calculated by means of Wagner´s theory, using the oxygen ion diffusion coefficient determined by our experimental process, are greater than oxidation constants previously determined in oxidation experiments from 850 to 950º C, in air, which indicates that the oxygen ion diffusion is large enough to assure the growth rate of the oxide film formed by the oxidation of the AISI 439 steel in these temperatures.

Effect of Phosphoric Acid Treatment on Isothermal High Temperature Oxidation Behaviour of AISI 304 Stainless Steel at 800°C

Phosphoric acid treatment is used as a way to improve the high temperature oxidation resistance of a chromia-forming AISI 304 steel. Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because of the formation of protective oxide scales. The oxide scale growth mechanisms are studied by exposing phosphoric acid-treated and untreated 304 steel samples to high temperature conditions in air. The analyses were carried out by means of thermogravimetry, and in situ X-ray diffraction (XRD). The experimental results show that the phosphoric acid treatment does not have a beneficial effect on cyclic high temperature oxidation (up to 70h of the oxidation test) of AISI 304 steel because of growth of a layer mainly formed by external cation diffusion which grows very quickly. The isothermal high temperature oxidation of this steel at 800°C in air shows a very fast initial iron oxidation towards the external interface, allowing to chromium element to be more available to the internal interface to form a continuous chromia layer, thus causing the establishment of a parabolic oxidation regime and leading to a beneficial reduction of the oxidation rate (after 70h of the oxidation test).

Kinetics and Mechanism of High Temperature Internal Oxidation of Ni-14wt%W Alloy at 1000°C

Ni-14wt%W model-alloy has been used to study the kinetics and the mechanism of high temperature internal oxidation in presence of an external oxide scale. Oxidations have been done in air at 1000°C for different oxidation times and have allowed the evaluation of the oxidation kinetic from mass gain and from oxide thickness measurements. The oxidation kinetic which follows a parabolic oxidation rate indicates a diffusive behaviour of species during the oxide formation. Cross-section observations by Scanning Electron Microscopy (SEM) and analysis by Grazing Incidence X-Rays Diffraction (GIXRD) show a multi-layered structure of the oxide. Starting from the surface towards the bulk alloy, the scale is layered as follows : (i) a pure nickel oxide (NiO) in the outer part (ii) a porous NiO matrix containing NiWO4 second phase and (iii) an internal oxidation zone of tungsten, first forming WO3 sub-micrometer oxides that progressively transform into NiWO4 oxide precipitates in the alloy.

Microstructure, Mechanical Properties and Oxidation Resistance of Nb-22Ti-14Si-2Hf-2Al-xCr Alloys

Nb-22Ti-14Si-2Hf-2Al-xCr (x=2, 6, 10, 14, 17 at%) alloys are prepared by arc-melting under argon atmosphere. Microstructural characteristics, mechanical properties and oxidation resistance of the arc-melted alloys are investigated. At 2 at% Cr content, the microstructure is composed of Nbss, Nb3Si and a small quantity of Nb5Si3, when the Cr contents increase, Nb3Si disappears. For the high Cr content (x ≥ 10 at% alloys, besides the Nbss and Nb5Si3, Cr2Nb is also detected. With the increase of Cr content, the volume fractions of Cr2Nb and Nb5Si3 increase, while that of Nbss increases firstly and then begins to degrade when the Cr content is higher than 6 at%. For the alloy with 2 at% Cr, the room temperature fracture toughness is about 14.5 MPa·m1/2, but badly decreases to about 8.5 MPa·m1/2, when the Cr contents increase. Vickers hardness of Nbss tends to increase linearly from about 400 to 500, while that of silicides is not sensitive to Cr contents, about 950. The appearance of Cr2Nb phase significantly improves the high temperature oxidation resistance of the alloys with high Cr contents. The isothermal oxidation tests show that the oxidation kinetics of the alloys with various Cr contents follows parabolic oxidation kinetics.

Oxidation behavior of SiC ceramics synthesized from processed cellulosic bio-precursor

Oxidation behavior of Si/SiC ceramic composite synthesized from processed cellulosic bio-precursor was studied in dry air over the temperature range 1200–1350°C. The material was synthesized from processed bio-precursors (bleached bamboo kraft pulp in the form of flat board of bulk density 0.58gcm−3) and had a bulk density of 2.66gcm−3, porosity of 0.6vol% and contents of Si and SiC phases of 39.1% and 60.3% (v/v) respectively. The process of oxidation could be described closely by a parabolic oxidation equation. An activation energy of 141.4kJ/mol was obtained. Both the SiC and Si phases oxidized and the oxidation was mainly controlled by the transport of molecular oxygen through the growing oxide layer. Pre-oxidation at 1300°C for 24h in ambient air increased the strength of Si/SiC ceramics by around 46% because of the healing of the surface defects created during surface preparation by the oxide layer.

A new approach to interpreting the parabolic and non-parabolic oxidation behaviour of hot-pressed β-SiAlON ceramics

The oxidation behaviour of hot-pressed β-SiAlON ceramics with different densities has been investigated up to 1853K in air using thermogravimetry (TG). The microstructures of the oxidised samples were determined by scanning electron microscopy (SEM), and the phases after oxidation were confirmed by X-ray diffraction (XRD). The oxidation kinetics of the higher density sample was diffusion controlled. For the lower density sample, it was controlled by both an interface reaction and diffusion in the early stage but diffusion controlled after a certain time. The oxidation curves can be well described by Chou’s model with the “characteristic oxidation time” index introduced.

Comparison of oxidation behaviours among three Fe–Cr based alloys for solid oxide fuel cell interconnect

Two metallic alloys, containing comparable amounts of Cr, underwent oxidation in hot air simulating (the solid oxide fuel cell cathode atmosphere) for various periods. The results demonstrated that the oxidation kinetics of Crofer22 APU and equivalent ZMG232 followed the parabolic rate law and oxidation rates increased with temperature. Typical oxidation rates of Crofer22 APU and ZMG232 upon annealing treatment are approximately 0·21 orders of magnitude lower than that of ZMG232. An oxide scale electron probe microanalyser, a scanning electron microscope and X-ray diffractometer were adopted to verify the applicability of Fe–Cr based alloys in the solid oxide fuel cell interconnect component. Two alloys contain comparable amount of Cr, Mn and Fe, and their surface oxides as analysed are indicated to be Cr2O3 and (Mn,Fe)Cr2O4 spinel compound. In summary, Crofer22 APU had the best oxidation resistance of any of the alloys of interest.

A comparison of the oxidation behavior of CrN films deposited using continuous dc, pulsed dc and modulated pulsed power magnetron sputtering

The study is aimed at comparing the oxidation behavior of the stoichiometric CrN films deposited by continuous dc magnetron sputtering (dcMS), mid-frequency pulsed dc magnetron sputtering (PMS), and modulated pulsed power (MPP) magnetron sputtering techniques in a closed field unbalanced magnetron sputtering system. These as-deposited CrN films exhibited a cubic structure and similar stoichiometric compositions, but with different microstructures and residual stresses. After annealing in the ambient air from 600 to 1000°C, the changes in the crystal phase, microstructure, and hardness of the films were characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy line scan, and nanoindentation. The oxidation activation energies of the films were calculated using Arrhenius equation. It was found that the MPP CrN film exhibited superior oxidation resistance than dcMS and PMS CrN films. After annealing at 900°C, the MPP CrN film exhibited an extremely dense structure and the cubic phase was well maintained. On the other hand, the dcMS and PMS CrN films were severely oxidized into a porous structure with the development of β-Cr2N and Cr2O3 phases and a rapid degradation of the cubic phase after 700 and 800°C, respectively. The results indicate that all films showed a parabolic oxidation rate below 900°C. The oxidation activation energies for the dcMS, PMS and MPP CrN films are 116kJ/mol, 141kJ/mol, and 195kJ/mol, respectively. The better oxidation resistance of the MPP CrN film is attributed to its dense microstructure and low residual stress.

Interaction mechanisms between slurry coatings and solid oxide fuel cell interconnect alloys during high temperature oxidation

Six different coatings consisting of fluorite-, corundum-, spinel- or perovskite-type oxides were deposited on a Fe22Cr alloy (Crofer 22APU) and oxidized at 900°C in moisturized air.Five of the coatings prevented break-away oxidation otherwise observed for the uncoated alloy, and the parabolic oxidation rate constant was reduced with 50–90% of that for uncoated alloy. One coating consisting of MnCo2O4 did not significantly affect the oxidation rate of the alloy, and just as for uncoated samples break-away oxidation occurred for MnCo2O4 coated samples. The interaction mechanisms between the growing oxide scales and applied coatings can be classified according to three types.

Oxidation kinetics of nitrogen doped TiO2−δ thin films

The oxidation kinetics of nitrogen doped, oxygen deficient titanium dioxide thin films has been studied in atmospheres of pure oxygen or nitrogen at 500 °C, 550 °C, and 600 °C, respectively, by means of in situ optical spectroscopy. The thin films show high electronic absorbance in the visible and NIR region, accompanied by a red shift of the absorption edge of about 0.4 eV, e.g., from about 2.9 to 2.5 eV at 600 °C. The time dependent decrease of absorbance due to oxidation is found to follow a parabolic rate law. An activation energy of about 1.96 eV can be obtained from the temperature dependence of the parabolic oxidation rate constant. In the framework of a microscopic oxidation model, this energy barrier is attributed to the diffusion of titanium interstitials in the re-oxidized part of the thin films as a rate-determining process. In addition, an attempt is made to evaluate the kinetics of nitrogen release from the time dependent blue shift of the absorption edge during re-oxidation.

Discussions on some Properties of Alumina Scales and their Protectiveness

This small review deals mainly with three issues regarding the nature and protectiveness of alumina scales grown during high-temperature oxidation: (1) sequences of phase transportation of alumina scales formed on Fe-Cr-Al and NiAl alloys, and a few aluminides, (2) combined additions of reactive element (RE) and (3) convolution of α-Al2O3 scales. Though the general phase transformation sequence of alumina scales is γ to θ to α phases at intermediate temperatures, variations have been reported. Directional growth of transient aluminas such as γ-Al2O3 and θ-Al2O3 is discussed with a particular emphasis on its driving force. Parabolic rate constants for the growth of α-Al2O3scales are smaller when the period of transient alumina is longer because of larger α-Al2O3grains. The effect of RE in slowing the parabolic oxidation saturates at a certain concentration, however combined addition further decreases the oxidation rate. The α-Al2O3 scales on Fe-Cr-Al alloys without RE are highly convoluted, however those on NiAl and other aluminides are not so convoluted. The α-Al2O3 layer beneath the outer NiO layer or NiAl2O4 layer is flat in the oxidation of Ni3Al. Directions for future work are proposed.

Investigation of MoSi 2 melt infiltrated RSiC and its oxidation behavior

Melt infiltration process was employed to fill molten MoSi 2 into the pores of recrystallized silicon carbide (RSiC) to improve the oxidation resistance of RSiC, and an almost fully dense RSiC–MoSi 2 composite with the microstructure of two-phase interpenetrated network was obtained. The phase compositions of the composites are mainly SiC and MoSi 2, including a small amount of Mo 4.8Si 3C 0.6 and Mo 5Si 3. The flexural strength of the composites at room temperature was 11–32% higher than that of as-received RSiC. The cyclic oxidation tests at 1500 °C indicated that both the RSiC–MoSi 2 composites and RSiC exhibited a parabolic oxidation behavior, and the corresponding parabolic rate constants of the composites were almost an order of magnitude lower than those of RSiC, resulting in a great improvement of oxidation resistance of RSiC–MoSi 2.

Modeling of Oxygen Diffusion Along Grain Boundaries in a Nickel-Based Superalloy

Finite element analyses of oxygen diffusion at the grain level have been carried out for a polycrystalline nickel-based superalloy, aiming to quantify the oxidation damage under surface oxidation conditions at high temperature. Grain microstructures were considered explicitly in the finite element model where the grain boundary was taken as the primary path for oxygen diffusion. The model has been used to simulate natural diffusion of oxygen at temperatures between 650∘C and 800∘C, which are controlled by the parabolic oxidation rate and oxygen diffusivity. To study the effects of mechanical stress on oxygen diffusion, a sequentially coupled deformation-diffusion analysis was carried out for a generic specimen geometry under creep loading condition using a submodeling technique. The material constitutive behavior was described by a crystal plasticity model at the grain level and a unified viscoplasticity model at the global level, respectively. The stress-assisted oxygen diffusion was driven by the gradient of hydrostatic stress in terms of pressure factor. Heterogeneous deformation presented at the grain level imposes a great influence on oxygen diffusion at 750∘C and above, leading to further penetration of oxygen into the bulk material. Increased load level and temperature enhance oxygen concentration and penetration within the material. At 700∘C and below, mechanical loading seems to have negligible influence on the oxygen penetration because of the extremely low values of oxygen diffusivity and pressure factor. In the case of an existing surface microcrack, oxygen tends to accumulate around the crack tip due to the high stress level presented near the crack tip, leading to localized material embrittlement and promotion of rapid crack propagation.

Hi‐NicalonTM‐SSiC Fiber Oxidation and Scale Crystallization Kinetics

The oxidation and scale crystallization kinetics of Hi‐NicalonTM‐S SiC fibers were measured after oxidation in dry air between 700° and 1400°C. Scale thickness, composition, and crystallization were characterized by TEM with EDS, supplemented by SEM and optical microscopy. TEM was used to distinguish oxidation kinetics of amorphous and crystalline scales. Oxidation initially produces an amorphous silica scale that incorporates some carbon. Growth kinetics of the amorphous scale was analyzed using the flat‐plate Deal‐Grove model. The activation energy for parabolic oxidation was 248 kJ/mol. The scales crystallized to tridymite and cristobalite, starting at 1000°C in under 100 h and 1300°C in under 1 h. Crystallization kinetics had activation energy of 514 kJ/mol with a time growth exponent of 1.5. Crystalline silica nucleated at the scale surface, with more rapid growth parallel to the surface. Crystalline scales cracked from thermal residual stress and phase transformations during cool‐down, and during oxidation from tensile hoop growth stress. High growth shear stress was inferred to cause intense dislocation plasticity near the crystalline SiO2–SiC interphase. Crystalline scales were thinner than amorphous scales, except where growth cracks allowed much more rapid oxidation.

Long term corrosion on T91 and AISI1 316L steel in flowing lead alloy and corrosion protection barrier development: Experiments and models

Considering the status of knowledge on corrosion and corrosion protection and especially the need for long term compatibility data of structural materials in HLM a set of experiments to generate reliable long term data was defined and performed. The long term corrosion behaviour of the two structural materials foreseen in ADS, 316L and T91, was investigated in the design relevant temperature field, i.e. from 300 to 550 °C. The operational window of the two steels in this temperature range was identified and all oxidation data were used to develop and validate the models of oxide scale growth in PbBi. A mechanistic model capable to predict the oxidation rate applying some experimentally fitted parameters has been developed. This model assumes parabolic oxidation and might be used for design and safety relevant investigations in future. Studies on corrosion barrier development allowed to define the required Al content for the formation of thin alumina scales in LBE. These results as well as future steps and required improvements are discussed. Variation of experimental conditions clearly showed that specific care has to be taken with respect to local flow conditions and oxygen concentrations.

Air oxidation of Zircaloy-4, M5® and ZIRLO™ cladding alloys at high temperatures

The paper presents the results of isothermal and transient oxidation experiments of the advanced cladding alloys M5® and ZIRLO™ in comparison to Zircaloy-4 in air at temperatures from 973 to 1853 K. Generally, oxidation in air leads to a strong degradation of the cladding material. The main mechanism of this process is the formation of zirconium nitride and its re-oxidation. From the point of view of safety, the barrier effect of the fuel cladding is lost much earlier than during accident transients with a steam atmosphere only. Comparison of the three alloys investigated reveals a qualitatively similar, but quantitatively varying oxidation behavior in air. The mainly parabolic oxidation kinetics, where applicable, is comparable for the three alloys. Strong differences of up to 500% in oxidation rates were observed after transition to linear kinetics at temperatures below 1300 K. The paper presents kinetic rate constants as well as critical times and oxide scale thicknesses at the point of transition from parabolic to linear kinetics.