Synthesis of Tin Powder Using Tin Oxide and Ethanol

The reduction behavior of SnO₂ powder has been investigated in the temperature range of 900–1200 K under ethanol flow. Scanning electron microscopy, x-ray diffraction, and mass measurement techniques were used to characterize the products. Full oxide reduction was attained at 1000 K, 1100 K, and 1200 K within about 20 min, 15 min, and 7.5 min, respectively. At 900 K, the extent of reduction increased with the reaction time up to 20 min, but further increases in the time (30 min and 60 min) resulted in a slight mass gain. This was attributable to the C uptake. Spherical Sn particles (diameter ~ 1 μm) were observed at 1000 K and 1100 K. At 1200 K, large beads of Sn (diameter 400–800 μm) were obtained. The spherical particle morphology was attributed to the liquid metallic phase formed during the reaction. The reduction mechanism of SnO₂ in ethanol has been discussed in the light of thermodynamic and experimental results.     

A Single-Step Process for Direct Reduction of Iron Oxide to Sponge Iron by Undiluted Methane

A single-step pyrometallurgical process for the synthesis of sponge iron is presented in this study. It aimed to investigate the reduction behavior of Fe₂O₃ in undiluted CH₄ flow to establish the process parameters for sponge Fe formation. Thermodynamic analysis predicted the reduction of Fe₂O₃ to Fe by CH₄ at 1000–1200 K. The experiments were carried out at 800–1200 K for 60 min and at 1200 K for 0–60 min. Mass measurement and x-ray diffraction (XRD), and scanning electron microscope techniques were used to characterize the products. The extent of the oxide reduction was found to increase with the temperature and time. XRD showed that single phase Fe was obtained at 1100 K for 60 min and at 1200 K within 10 min. The products synthesized at 1200 K within 15 min had spongy morphology. C deposition on the pre-reduced Fe particles resulted in the disappearance of spongy character.     

KCl-Induced Corrosion of a 304-type Austenitic Stainless Steel in O2 and in O2 + H2O Environment: The Influence of Temperature

The oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated in the temperature range 400–600 °C in 5% O₂ and 5% O₂ + 40% H₂O. Exposure time was up to 1 week. Prior to exposure, the polished samples were coated with 0.1 mg/cm² KCl. Uncoated samples were also exposed and used as references. The oxidized samples were analyzed by gravimetry and by ESEM/EDX, XRD, IC and AES. The results show that KCl is strongly corrosive. Corrosion is initiated by the reaction of KCl with the chromia-containing oxide that normally forms a protective layer on the alloy. This reaction produces potassium chromate particles, leaving a chromium-depleted oxide on the alloy surface. At 500 and 600 °C this results in rapid oxidation, resulting in the formation of a thick scale consisting of a mixture of hematite, spinel oxide ((Fe,Cr,Ni)₃O₄) and K₂CrO₄. The thick scale is poorly protective and permeable to e.g. chloride ions. The KCl-induced corrosion of alloy 304L in dry O₂ and in an O₂ + H₂O mixture increases strongly with temperature in the range 400–600 °C. The strong temperature dependence is explained partly by the temperature dependence of the chromate-formation reaction and partly by the ability of the chromium-depleted oxide to protect the alloy at low temperature. At 400 °C, the oxide was still protective after 168 h.     

Reduction of tungsten trioxide with ethanol

This study aims to investigate pyrometallurgical reduction behavior of tungsten trioxide by ethanol at the temperature range 900–1500 K. Thermodynamic analysis predicted single tungsten phase formation from tungsten trioxide and ethanol at the temperature range studied. The experiments were carried out in a flowing ethanol (7.1 vol.%)-Ar atmosphere in a tube furnace for various reaction periods of time. The extent of the reduction was followed by mass measurements. XRD and SEM were used to carry out phase and morphological studies on the products obtained at various stages of the reduction. It was found that at the temperature range 900–1200 K, oxide reduction was incomplete owing to sluggish reaction kinetics between the reactants. At higher temperatures of 1300 K, 1400 K, and 1500 K, tungsten trioxide reduction to metallic tungsten was complete within ~ 45 min, ~ 30 min and ~ 20 min, respectively. Well-facetted tungsten particles with mean sizes of 1.6 μm, 2.1 μm and 3.3 μm were observed at these conditions. At 1500 K, there was a significant tungsten loss by the flowing gas stream via the formation of gaseous WO₂(OH)₂. Possible reaction pathways leading to metallic tungsten were discussed.     

Kinetics of magnesium preparation by vacuum-assisted carbothermic reduction method

Metallic magnesium was prepared by vacuumassisted carbothermic reduction method, and its morphologies were observed and analyzed. The reduction ratios of reactions were carried out under various vacuums, reaction temperatures, and time. Reaction kinetics of carbothermic reduction process was investigated. The results reveal that the morphologies of metallic magnesium sample that crystallized in the bottom and top sections of the condensation cap appear as the shape of feather with close-packing needle structure and the shape of schistose with metal luster,compactly clumpy structure, respectively. The reduction ratio of reaction process can be facilitated through reducing vacuum, increasing temperature, lengthening time, or their combinations and can reach up to 83.7 % under the condition of 10 Pa and 1573 K with 60 min reaction time. At1423–1573 K, the reaction rate constant k of carbothermic reduction of magnesia in vacuum gets greater with the increase of temperature. The reaction activity energy is190.28, 219.71 and 451.12–528.54 k J mol-1when the procedure of carbon gasification reaction, interfacial reaction, or gaseous diffusion is the reaction rate-determining step at 1423–1573 K, respectively. The gaseous diffusion procedure has the largest activity energy value and is,therefore, the main reaction rate-determining step.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Preparation and characterization of Y3Al5O12:Ce and Y2O3:Eu phosphors powders by combustion process

A nickel hydroxide, Ni(OH)₂, was prepared by microwave-assisted heating technique from nickel nitrate aqueous solution and sodium hydroxide (assigned as PM). Then, the as-prepared PM was oxidized by liquid oxidation with sodium hypochlorite (assigned as PMO). Further, pure nanocrystalline nickel oxide (NiO) particles were obtained from the as-prepared PMO by calcination at 300, 400, 500, 600, 650 and 700 °C (labeled as C300, C400, C500, C600, C650 and C700, respectively). The as-prepared powders (PM and PMO) and the NiO nanoparticles were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), temperature-programmed reduction (TPR) and scanning electron microscope (SEM). The results indicated that the particle size of nickel oxide was controlled by the calcined temperature. The average crystal size of the NiO nanoparticles ranges from about 5 to 35 nm at 300–700 °C. Mechanism of nickel oxide nanocrystallite growth during thermal treatment was investigated.     

Corrosion and Carburization Behaviour of Ni-<Emphasis Type="Italic">x</Emphasis>Cr Binary Alloys in a High-Temperature Supercritical-Carbon Dioxide Environment

Pure Ni and Ni-xCr (x = 7, 14, 22 and 27 wt%) binary alloys were exposed to supercritical-carbon dioxide environment at 600 °C and 20 MPa for 200 h. For pure Ni, a thick NiO layer was formed on the surface. Meanwhile, for Ni-7Cr alloy, an inner oxide layer consisted of rather irregular chromia and NiO was formed below the outer NiO layer. When Cr content was greater than 14%, a continuous chromia layer was formed, resulting in much lower weight gain and oxide thickness. However, amorphous carbon layers had developed along the oxide–matrix interface when chromia was formed. The presence of the carbon layer was explained in view of the high C activity corresponding to the low equilibrium oxygen potential of chromia.     

Mechanism of Iron Oxide Scale Reduction in 5%H<Subscript>2</Subscript>–N<Subscript>2</Subscript> Gas at 650–900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H₂–N₂ gas at 650–900 °C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 °C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 °C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650–750 °C differed from that at 750–850 °C, with the former being oxygen diffusion in α-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite–steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron–wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite–iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.     

High-temperature corrosion of nickel in SO2

The reaction of nickel with SO₂ has been studied in the temperature range 650–1100°C at SO₂ pressures from 10 to 760 Torr. Reaction kinetics have been studied by thermogravimetry; the reacted specimens have been characterized by means of metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves oxidation and sulfidation except at sufficiently high temperatures and low pressures of SO₂ (e.g., 1000°C and 10 Torr SO₂) where only formation of NiO takes place. Approximately linear reaction kinetics are observed between 650 and 900°C. Reaction mechanisms are discussed, and the relative importance of oxide formation and sulfidation is interpreted in terms of the thermodynamics of the Ni-O-S system.     

Tantalum Oxidation in Steam Atmosphere

The oxidation behavior of tantalum in steam was investigated in the temperature range of 600–1100 °C. Thermogravimetric measurements were used to determine the oxidation behavior of the Ta metal at different steam partial pressures (10, 50 and 100 kPa). As a result of the oxidation tests, the samples show parabolic behavior for the initial period of oxidation, which is almost not visible at temperature higher than 700 °C. After the short parabolic kinetics, breakaway occurs and turns the kinetics from parabolic to linear. In this work kinetic parameters of the linear oxidation were determined. The influence of the samples´ shape was investigated using both plate and cylinder specimens. Experiments aimed at quantifying the hydrogen uptake of tantalum were conducted at temperatures between 600 and 1000 °C, and results differ from the expected ones applying the Sieverts’ law. Post-test optical microscopy was performed in order to analyze the post-oxidation appearance of the materials. The oxide scale appeared non-protective, especially at low temperature.     

Resistance of High-Nickel,Heat-Resisting Alloys to Air and to Supercritical CO<Subscript>2</Subscript> at High Temperatures

The reduction behaviour of wustite-type iron oxide scale on a low-carbon, low-silicon steel by dissolved carbon in the steel at 650–900 °C under pure nitrogen was studied. It was found that dissoved carbon in the steel examined was able to react with the wustite scale on the surface, leading to reduction of this scale. It was also found that the scale reduction rate was the most rapid within 750–800 °C, followed by that at 700 °C and then at 850 °C, whereas the rates were essentially zero at 650 and 900 °C. Decarburization occurred to the steel as a result of scale reduction, and the degree of decarburization at 750–800 °C was also the most severe. The rate of scale–carbon reaction was primarily controlled by carbon diffusion through the decarburization layer as the calculated carbon permeability, defined as the product of carbon diffusivity and the carbon concentration difference across the decarburization layer, also reached its maximum within 750–800 °C. Scale reduction led to the formation of pores at the scale–steel interface as a result of volume shrinkage when wustite was reduced to iron, but the porosity volume was smaller than calculated at 800–850 °C, which could have an inhibiting effect on the scale–carbon reaction. The calculated volume of CO + CO₂ gases generated as a result of scale–carbon reactions was about 100 times the calculated porosity volume. It was believed that the wustite scale was permeable to CO and/or CO₂, allowing the much larger volume of CO and CO₂ gases to escape through the scale layer.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Hot Corrosion Behavior of Inconel 625 Superalloy in Eutectic Molten Nitrate Salts

Corrosion resistance of Inconel 625 Ni-based superalloy was studied in a molten nitrate salt consisting of 40 KNO₃–60 NaNO₃ (wt%) at 500 and 600 °C. Open-circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy and gravimetric tests were used to evaluate the degradation mechanism and corrosion behavior of the alloy. Surface morphology and chemical analysis of corrosion products were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry. The weight-loss curves showed that with the increase in temperature, the oxidation rate and mass gain increased; the relationship between the mass gain and time was close to the parabolic oxidation law. The electrochemical corrosion results confirmed that during the exposure of Inconel 625 alloy to the molten salts, nickel dissolves as a result of non-protective NiO layer formed. The formation of a non-protective oxide layer with low barrier property was responsible for observing the weak corrosion resistance of the alloy at high temperatures (500 and 600 °C). Cyclic polarization tests showed a positive hysteresis confirming the nucleation and growth of stable pits on the surface of Inconel 625 at high anodic overpotentials. Sodium nitrite acts as an efficient pitting inhibitor for this case. In this way, the sodium nitrite with the concentration of 0.1 molal was found to have an optimum inhibition effect on pit nucleation at 600 °C.     

TG–mass spectrometry studies in coating design for supercritical steam turbines

Ferritic steels in steam turbines for the power industry operate without coatings in the temperature range of 590–600 °C. For higher operation temperatures the substrate has to be replaced or coated, otherwise the ferritic substrate at a temperature of 650 °C develops thick oxide scales that promote sudden turbine blade failure. The advantage of the use of coatings is that coated ferritic steels are much less expensive than austenitic stainless steels or nickel base superalloys. In order to go forward to coatings design, the Thermo‐Calc code was used as a base for the mass spectrometry (MS)‐data. Thermogravimetry (TG)–MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. From those results the role of the different coating element could be established and optimized for the coating durability. An oxidation mechanism based on the TG–MS results is given.     

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H₂O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.     

High-Temperature Oxidation of Zircaloy-4 in Oxygen–Nitrogen Mixtures

Isothermal oxidation experiments with cladding tube segments of Zircaloy-4 (Zr-1.3%Sn) in oxygen–nitrogen model mixtures were performed at 800, 1000, and 1200 °C for 6, 1 h, and 15 min, respectively. The gas compositions varied between 0 and 100 vol% nitrogen including 1 and 99 vol%. A strong accelerating effect of nitrogen on the oxidation kinetics was seen for a wide range of boundary conditions. At 800 °C, oxidation in all mixtures with 1–99 % nitrogen resulted in higher reaction rates compared to the pure gases, especially after transition from protective to non-protective oxide scales. At 1000 and 1200 °C, only starvation of oxygen in mixtures with low oxygen contents resulted in lower rates compared to pure oxygen. The oxide scales formed in the mixtures were very porous due to the formation of zirconium nitride at the metal-oxide interface and its oxidation during continuing reaction. The extension of the oxide-nitride zone increased with temperature and with nitrogen content in the gas mixture. Nitrogen seems also to affect the pre-transition reaction kinetics. The mechanism of the faster oxidation kinetics of zirconium alloys in atmospheres containing nitrogen will be discussed in this paper.     

Oxidation Characteristics and Oxide Layer Evolution of Alloy 617 and Haynes 230 at 900 °C and 1100 °C

To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900 °C and 1100 °C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900 °C, for Alloy 617, a mixture of the extensive NiO–Cr₂O₃ double layer and isolated NiO–NiCr₂O₄–Cr₂O₃ triple layer were observed at a steady-state condition. For Haynes 230, a MnCr₂O₄ layer was formed on top of the Cr₂O₃ layer, resulting in a lower oxidation rate. At 1100 °C, both alloys showed a double layer consisting of an inner Cr₂O₃ and outer MnCr₂O₄ or TiO₂. The spallation of outer layer and subsequent volatilization of the Cr₂O₃ layer produced a rugged surface and interface as well as internal oxidation.     

Effect of Silicon on the Steam Oxidation Resistance of a 9%Cr Heat Resistant Steel

The steam oxidation of 9Cr–0.5Mo–1.8W steels containing 0.06 to 0.49%Si was investigated at 500°, 550°, 600°, 650° and 700°C. The steam oxidation rate of the steel decreased with increasing silicon content. The effect of silicon was most remarkable at 700°C. At 500°, 550° and 600°C, the effect was almost the same, and was smaller than that at 700°C. At 700°C, the formation of a protective amorphous-SiO₂ film reduced the oxidation rate considerably. On the other hand, at 600°C or less, silicon dissolved in the Fe–Cr spinel lattice with no evidence of SiO₂. At 650°C, although amorphous SiO₂ was observed, as at 700°C, at the scale–metal interface, the effect of silicon was the least within the test-temperature range. Thus, 650°C was a peculiar temperature for the effect of silicon on the steam oxidation of 9%Cr steels. The relatively small effect of silicon at 650°C is attributed to the formation of metastable FeO.     

Oxidation Behavior of PAN-based Carbon Fibers and the Effect on Mechanical Properties

PAN-based carbon fibers were oxidized both in dry air and wet air in the temperature range of 400–600 °C. Kinetic laws are established that follow an Arrhenius-type temperature dependence. Oxidised fiber surfaces were investigated by SEM and AFM. Oxidation leads to the modification of surface morphology with the disappearance of the axial striations initially present. Then, residual properties were evaluated by failure tests in tension on single-filaments. Oxidation has a dramatic effect even for a low level of weight loss. The tensile failure stresses are reduced by 25–40 % for a mass loss of 2.5–5 %. This excessive embrittlement is more related to the creation of new defects by oxidation than a significant reduction in fiber cross-section area.