High-temperature Oxidation In Air Research Articles

Microstructure, oxidation resistance and tensile properties of silicide coated Nb-alloy C-103

A Fe-Cr-alloyed silicide coating has been formed on a commercially available Nb-alloy called C-103 by using a slurry-based technique followed by vacuum diffusion treatment at 1380°C. The coating microstructure reveals a three-layer structure with the outer layer consisting of NbSi2 phase and the inner interdiffusion layer consisting of lower silicides such as Nb5Si3 and Nb3Si. The intermediate layer of the coating is comprised of a Fe–Cr alloyed Nb-silicide phase and NbSi2. The coating provides good short-term protection to the substrate against high temperature oxidation in air at 1100 and 1300°C. It is found that the presence of the coating increases the tensile strength of the alloy.

Vacuum-arc chromium-based coatings for protection of zirconium alloys from the high-temperature oxidation in air

Multilayer Cr–Zr/Cr/Cr–N coatings for protection of zirconium alloys from the high-temperature oxidation in air have been obtained by the vacuum-arc evaporation technique with application of filters for plasma cleaning from macroparticles. The effect of the coatings on the corrosion resistance of zirconium alloys at test temperatures between 660 and 1100 °C for 3600 s has been investigated. The thickness, structure, phase composition, mechanical properties of the coatings and oxide layers before and after oxidation tests were examined by scanning electron microscopy, X-ray diffraction analysis and nanoindentation technique.It is shown that the hard multilayer coating effectively protects zirconium from the oxidation in air for 1 h at test temperatures. As a result of the oxidation in the coating the CrO and Cr2O3 oxides are formed which reduce the oxygen penetration through the coating. At maximum test temperature of 1100 °C the oxide layer thickness in the coating is about 5 μm. The tube shape remains unchanged independent of alloy type. It has been found that uncoated zirconium oxidizes rapidly throughout the temperature range under study. At 1100 °C a porous monoclinic ZrO2 oxide layer of ≥120 μm is formed that leads to the deformation of the samples, cracking and spalling of the oxide layer.

High-temperature crack-healing behaviour and strength recovery of (MoNb)Si2

(MoNb)Si2 materials were prepared using combustion synthesis and the vacuum hot-pressing technology. Cracks were introduced on the surface of polished materials using the Vickers indentation method. The crack-healing behaviour of (MoNb)Si2 materials were investigated with high-temperature oxidation in air. The results show that after annealing at 900°C, 1200°C and 1500°C for 1h, the crack-healing behaviour was observed in each case. The bending strength obviously decreased when the cracks were introduced, and the materials significantly recovered after the heat treatment. The crack-healed samples exhibited higher bending strengths than the original level after the 1200°C treatment, e.g. 479MPa versus 195MPa of pure MoSi2. MoSi2 micro alloyed with niobium shows a high bending strength at room temperature and excellent crack-healing properties at high temperatures.

Effect of Cr and Al Content on the Transition of Cr2O3 to Al2O3 Scale Formed on (Fe,Ni)-Cr-Al Alloys during Early Stage of High-Temperature Oxidation in Air

Effect of Cr and Al Content on the Transition of Cr₂O₃to Al₂O₃ Scale Formed on (Fe,Ni)-Cr-Al Alloys during Early Stage of High-Temperature Oxidation in Air

High Temperature Oxidation Effect on the Dissolution Rate of Hot-Dip Aluminized Steel in Aluminum Melt

Hot dip aluminizing of low carbon steel followed by high temperature oxidation in air was carried out to evaluate the dissolution rate of coated sample in molten aluminum. The microstructure of oxide and aluminide layers was investigated using scanning electron microscopy and X-ray diffraction methods. The characterization of the coating showed that Fe2Al5 has been the major phase formed on the surface of specimen before high temperature oxidation. Isothermal oxidation of the coating as resulted in the formation of Al2O3 while Fe2Al5 phase transformed into FeAl and Fe3Al phases. The coated samples were further subjected to corrosion in molten aluminum. It was proved that the oxide layer formed on the coating and developed FeAl and Fe3Al intermetallic phases play the major role in protecting the specimen from corrosion in molten aluminum.

Study of SiC/SiO2 oxidation-resistant coatings on matrix graphite for HTR fuel element

The improvement of the oxidation resistance of fuel element matrix graphite used in high temperature gas-cooled reactor (HTR) is indispensable for the further improvement of the safety of HTR. The stability of SiC coatings and SiC/SiO2 multilayer coatings in helium with little impurities is analyzed on the basis of thermodynamic calculation and it is found that SiC/SiO2 multilayered coatings are suitable oxidation resistant materials for graphite used in HTR fuel elements because they are stable both at the normal operation conditions and accidental conditions of HTR. SiC/SiO2 multilayer coatings were synthesized on the surface of fuel element matrix graphite of HTR by combination of vapor reaction diffusion, slurry method and high temperature oxidation in air; the oxidation behavior of SiC/SiO2 coated fuel element matrix graphite of HTR in different atmosphere are investigated and it is shown that SiC/SiO2 coatings can keep their stability under different experiment conditions which graphite used in HTR may encounter and greatly improve the oxidation resistance of fuel element matrix graphite of HTR.

Properties of Porous FeAl Manufactured from Ball Milled Fe/Al Elemental Powders by Two-Step Sintering

Porous FeAl was manufactured from ball milled Fe/Al elemental powders followed by medium temperature solid diffusion and high temperature sintering. Phase composition and porous structure were analyzed by XRD, SEM, Mercury Porosimeter and permeability test system. High temperature oxidation in air and high temperature sulfidation in SO2(3v%)+N2 at 600°C were carried out to investigate the behaviors of the porous FeAl, and the results were compared to 316L porous materials. The result showed that high sintering temperature hastened the transform of Fe2Al5 to FeAl intermatellic. The permeability of the porous FeAl increased and the most probable size decreased with sintering temperature. The porous FeAl had mass gains of 0.06% for air oxidation and 0.13% for sulphidation after 50 h at 600°C, compared with mass gains of 0.15% and 5.3% respectively of porous 316L stainless steel.

High-temperature oxidation of FeCrAl alloy with alumina–silica–ceria coatings deposited by sol–gel method

One-layer protective coatings made up of SiO2–Al2O3 or SiO2–Al2O3–CeO2 oxides were synthesized on a FeCrAl alloy substrate by the sol–gel method from sols containing tetraethyl orthosilicate, aluminum tri-sec-butoxide and cerium(III) 2,4-pentanedionate as the precursors. Coating solutions with the Si:Al:Ce molar ratio of 3:1:0, 3:1:0.1; 3:1:0.01, and 3:1:0.001 were used. The composition and morphology of the obtained gels were examined by TG/DSC, XRD, and SEM techniques. It was found that a small addition of cerium affected the morphology of the forming coatings and improved the FeCrAl alloy resistance to high-temperature oxidation (in air at T = 900 °C for t = 100 h). The oxidation of all the investigated samples conformed to the parabolic rate. The protection effectiveness of the one-layer coatings after 100 h of high-temperature oxidation was as high as 70 %.

High temperature air oxidation resistance of TiAlCr-Y coated Ti45Al8Nb between 750 °C – 950 °C

AbstractThis study is concerned with air oxidation of TiAlCr-Y coated Ti45Al-8Nb (at%) alloy between 750 °C–950 °C for 500 hours. It was shown in these tests that at 750 °C, 850 °C and 950 °C, the oxidation degradation increased with temperature: at 750 °C the oxide scale consisted of TiO2/Al2O3/Cr2O3 phases, whilst at 850 °C the oxide scale consisted of TiO2/Al2O3 phases with a small amount of Cr2O3, and larger number of voids. The material exposed at 950 °C showed a significant formation of TiO2 with a small amount of Al2O3 and high number of voids; the formation of Cr2O3 at 950 °C was inhibited mainly due to the formation of volatile CrO3 or CrO2(OH)2 phase.

Aqueously Dispersed Silver Nanoparticle-Decorated Boron Nitride Nanosheets for Reusable, Thermal Oxidation-Resistant Surface Enhanced Raman Spectroscopy (SERS) Devices

The impurity-free aqueous dispersions of boron nitride nanosheets (BNNS) allowed the facile preparation of silver (Ag) nanoparticle-decorated BNNS by chemical reduction of an Ag salt with hydrazine in the presence of BNNS. The resultant Ag-BNNS nanohybrids remained dispersed in water, allowing convenient subsequent solution processing. By using substrate transfer techniques, Ag-BNNS nanohybrid thin film coatings on quartz substrates were prepared and evaluated as reusable surface enhanced Raman spectroscopy (SERS) sensors that were robust against repeated solvent washing. In addition, because ofthe unique thermal oxidation-resistant properties of the BNNS, the sensor devices may be readily recycled by short-duration high temperature air oxidation to remove residual analyte molecules in repeated runs. The limiting factor associated with the thermal oxidation recycling process was the Ostwald ripening effect of Ag nanostructures.

Thermal Radiation Phenomena of Surfaces of Chromium and Palladium in a High-Temperature Environment

For the purpose of thermal engineering applications, thermal radiation phenomena of surfaces of chromium and palladium in a high-temperature air-oxidation process on the basis of simultaneous measurements of spectra of normal incidence hemispherical reflectance and normal emittance in the near-ultraviolet through infrared region are investigated. As the results of the spectral measurements, clear radiation interference is observed even in the spectra of the hemispherical reflectance of a rough-finished chromium surface in the oxidation process as well as in the spectra of the normal reflectance of a specular-finished surface. The relation between the chemical reactions of oxidation and reduction of a palladium surface in the 1000 K to 1100 K range and the radiation characteristics are analyzed based on the temperature hysteresis of radiation spectra and the reaction rates of oxidation and reduction. A switching function of radiation characteristics of palladium accompanied by the oxidation and reduction reactions is suggested to be applicable for engineering applications.

Surface engineering analysis of detonation-gun sprayed Cr3C2–NiCr coating under high-temperature oxidation and oxidation–erosion environments

Detonation-gun (D-gun) spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4–5 times the speed of sound. This process provides the possibility of producing high hardness coatings with strong adherence. In the present study, this technique has been used to deposit Cr3C2–NiCr coating on T22 boiler steel. Investigations on the behaviour of this coating subjected to high-temperature oxidation in air and oxidation–erosion in actual boiler environment at 700±10°C under cyclic conditions have been carried out. The weight change technique was used to establish the kinetics of oxidation. X-ray diffraction (XRD), field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS) and EDS elemental mapping techniques were used to analyse the oxidation/oxidation–erosion products. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in both the environments. The Cr3C2–NiCr coating showed good adherence to the boiler steel during the exposures with no tendency for spallation of its oxide scale.

Combining Raman Microprobe and XPS to Study High-Temperature Oxidation of Metals

Raman microprobe spectroscopy was applied in studies of high-temperature air oxidation of a ferritic alloy (HT-9) in the absence and presence of zirconia coatings with the objective of evaluating the technique as a way to quickly screen candidate cladding materials and actinide-based mixed oxide fuel mixtures for advanced nuclear reactors. When oxidation was relatively uniform, Raman spectra collected using microscope optics with low spatial resolution were found to be similar to those collected with conventional Raman spectroscopy. These spectra could be used to identify major oxide corrosion products and follow changes in the composition of the oxides due to heating. However, when the oxidation films were comprised of multiple layers of varying composition, or with layers containing metallic phases, techniques with higher depth resolution and sensitivity to zero-valence metals were necessary. The requirements were met by combining Raman microprobe using different optical configurations and x-ray photoelectron spectroscopy.

A New Spectrophotometer System for Measuring Hemispherical Reflectance and Normal Emittance of Real Surfaces Simultaneously

A new spectrophotometer system is developed for the study of thermal radiation characteristics of real surfaces in thermal engineering environments. The system measures spectra of normal incidence hemispherical reflectance RNH and normal emittance \({\varepsilon _{\rm N}}\) in the near-ultraviolet through infrared region of wavelength of 0.30 μm to 11 μm simultaneously and repeatedly with a cycle time of 4 s. The system enables evaluation of the normal incidence absorptance AN in this wide spectral region. Transitions of spectra of specular-finished and rough-finished nickel surfaces in a high-temperature air-oxidation process are measured to demonstrate the performance of the system. Clear interference behaviors are found even in the spectra of hemispherical reflectance RNH and emittance \({\varepsilon _{\rm N}}\) of a rough-finished surface.

Analysis of components depth profile at the interface of Ti6242 alloy and TiNi coatings after high temperature oxidation in air

AbstractWe have analyzed the interfacial elemental depth profile evolution after high temperature isothermal oxidation of NiTi coatings deposited by dynamic ion mixing on a Ti6242 alloy (Ti‐6Al‐2Sn‐4Zr‐2Mo). NiTi coatings (thickness 0.4 μm) were deposited at room temperature (RT) by ion beam sputtering. High temperature isothermal oxidation tests in 1 atm flowing synthetic air (80% N2, 20% O2) have been conducted at 500 °C and 600 °C during 100 hours. We have observed a non‐monotonous depth distribution of nickel in GDOES depth profiles after oxidation of TiNi/Ti6242: nickel segregates to the surface of TiNi coating and to the interface between TiNi coating and Ti6242 alloy. We propose a kinetic model based on rate equations for analyzing the depth profile. This model includes microprocesses taking place during oxidation in air such as: adsorption of nitrogen and oxygen, diffusion of components through the film and interface, formation of chemical compounds. It is shown by modeling that non‐monotonous depth profile of nickel occurs because nickel from TiNi coating is forming a nickel oxide compound when oxygen atoms reach the film/alloy interface. XRD analysis confirms the presence of nickel oxide in the TiNi/Ti6242 interface after oxidation at both temperatures 500 °C and 600 °C (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphology and phase composition of oxide layers of steel ribbon, grade Kh23Yu5, after high-temperature processing under vacuum and in air environment

Monolith block honeycomb metal substrates are characterized by superior properties for manufacturing of high-duty and heat-resistant catalytic converters of automobile exhaust gases. This article discusses the results of investigations into physicochemical properties of the oxide layers formed on the surface of these monolith substrates in the process of their manufacture from cold-rolled steel ribbon, grade Kh23Yu5, with the use of the processes of vacuum diffusion welding and high-temperature oxidation in air.

High-temperature (to 1670°C) air oxidation of AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramic materials

The kinetics and mechanism of high-temperature air oxidation of powders and compact materials with different compositions in the AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) systems are studied using nonisothermal (up to 1500°C) and isothermal (1350 and 1670°C) thermogravimetry, DTA, XRD, petrography, and EMPA. During heating, Al4O4–x N x and Si2N2O oxynitrides, α-Al2O3 and α-SiO2 (crystobalite) oxides, and mullite-based Al2O3 ∙ SiO2 solid solution of the boundary composition form during the first oxidation stage, and 3Al2O3 ∙ 2SiO2 mullite forms at higher temperatures (>1350°C) in the upper scale layer and aluminum-nickel chromite spinel in the intermediate (barrier) scale layer. The AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramics may be regarded as hightemperature composites (HTCs) owing to their high corrosion resistance.

Analysis of Component Depth Profiles at the Interface of Ti6242 Alloy and SiC, SiN Coatings after High Temperature Oxidation in Air

We have analyzed the interfacial elemental depth profile evolution after high temperature isothermal oxidation of SixCy and SixNy protective coatings deposited by dynamic ion mixing on a Ti6242 alloy (Ti-6Al-2Sn-4Zr-2Mo). Isothermal oxidation tests have been carried out at 600°C during 100 hours in air. We have observed a non-monotonic depth distribution of zirconium in GDOES and SIMS depth profiles after oxidation of SiC/Ti6242 and SiN/Ti6242 and we propose a kinetic model based on rate equations for analyzing the results. It is shown by modeling that a non-monotonic depth profile of zirconium occurs because zirconium from the Ti6242 alloy forms a zirconium oxide compound. As a result, the atomic concentration of zirconium decreases at the interface which induces a diffusion flux of zirconium from the bulk to the interface. This process leads to the increase of the total amount of zirconium at the film interface and thus formation of a non-monotonic depth profile.

High Temperature Oxidation of the Al3Mg2 Complex Metallic Alloy

High temperature air oxidation of the Al3Mg2 complex metallic alloy was investigated on powder samples and bulk polycrystals in the temperature range 350–420 °C by thermogravimetric measurements, SEM and TEM. Oxidation at 420 °C on the polycrystalline samples comprised three successive phases characterised by linear kinetics laws. The first stage corresponded to the formation of a porous non adherent nanocrystalline MgO scale having a cauliflower morphology. A first acceleration in the kinetics law was ascribed to the nucleation and growth of MgAl2O4 crystallites which form a thin (~10 nm) film at the interface between the substrate alloy and the MgO top layer. A new linear regime was observed in the oxidation process corresponding to diffusion of magnesium through the grain boundaries of the spinel film and through the porous MgO layer. Finally, fragmentation and cracking of the scale leads to a further acceleration followed by a new linear regime.

Long Term Diffusion Studies in Fe Aluminide Coatings Deposited by Slurry Application on Ferritic Steel

Diffusion iron aluminide coatings have shown excellent resistance to high temperature oxidation in air, corrosive atmospheres and steam. A study of the diffusion behaviour of slurry applied diffusion aluminide coatings deposited on ferritic steel have been carried out under a 100% flowing steam atmosphere for up to 50,000 h at 650 °C. The results have shown that initially, the coating forms by outward growth possibly including the dissolution of the steel in molten aluminium. At later stages, during exposure to steam at 650 °C, aluminium diffuses inward and moreover, Fe also diffuses outward resulting in the progressive development of Kirkendall porosity. Results have also indicated that in order to form a pure protective Al2O3 scale the Al wt.% has to be > 4. Below this content Al-Fe mixed oxides develop exhibiting a less protective behaviour.