Initial Oxidation Period Research Articles

Fabrication and oxidation behavior of β-SiAlON powders in presence of trace Y2O3

β-SiAlON powders were fabricated using Al, Si, α-Al2O3 and trace Y2O3 powders as starting materials in flowing nitrogen atmosphere. The effect of Y2O3 on oxidation behavior of the prepared β-SiAlON powders has been investigated. The results show that the Y2O3 contributed to the generation of β-SiAlON during the fabricated process. The weight gain ratio of β-SiAlON in presence of Y2O3 is higher than that of without Y2O3 during non-isothermal oxidation up to 1723 K, which may be due to the Y2O3 dissolving into the lattice of β-SiAlON resulting in lattice deformation. At the initial oxidation period, the incorporation of Y2O3 increases the oxidation rate and decreases the oxidation activation energy (Ea) of β-SiAlON due to the lattice deformation increasing the reactivity of β-SiAlON. However, at the final oxidation period, denser glass film formed on the surface of β-SiAlON because of the incorporation of Y2O3, which reduces the opportunity for O2 contact with β-SiAlON, thus decreasing the oxidation rate and increasing the Ea of β-SiAlON.

Tailored copper oxidation in alkaline aqueous solution after helium cation implantation

Manipulating Cu oxidation is important for Cu anti-oxidation techniques and Cu oxide fabrication. In this study, Cu oxidation behavior after He+ implantation was observed after exposure to 0.1 M aqueous NaOH, and the underlying microstructural evolution and mechanism were investigated. He+ implantation and some C concomitantly introduced into the Cu surface accelerated formation of a thin oxide layer during the initial oxidation period, resulting in faster initial generation and more rapid growth of CuO during the subsequent oxidation. Furthermore, He+ implantation homogenized the distribution of CuO on the Cu substrate. Our findings will increase researchers’ understanding of the oxidation and corrosion behavior of Cu in aqueous alkaline conditions, and provide new insights into designing and growing Cu oxide nanostructures by ion implantation.

Studies on the Oxidation Behavior and Microstructural Evolution of Two Nb-Modified HR3C Austenitic Steels under Pure Water Vapor at 650 °C.

The steam oxidation behavior of three heterogeneous HR3C alloys was investigated at 650 °C comparatively. After a long-term oxidation process for 1000 h, the results demonstrated that the commercial HR3C alloy already exhibited a high oxidation resistance. However, the spallation resistance of the oxide scale was low during the initial oxidation period. The addition of a moderate amount of Nb into the alloy (1#HR3C) increased the oxidation resistance of the alloy. In addition, the improvement of the microstructural stability was substantially influenced by solid solution strengthening and fine grain strengthening. However, the addition of excessive Nb could significantly affect the growth model of the oxide scale and negatively affect the oxidation performance and microstructural evolution of the alloy (2#HR3C).

Oxidation behavior of Moss–Ti5Si3–T2 composites at intermediate and high temperatures

Three Mo–Ti–Si–B-based alloys with the compositions of 35Mo–35Ti–20Si–10B, 35Mo–35Ti–15Si–10B–5Al and 32.5Mo–32.5Ti–20Si–10B–5Cr (at.%) were designed and produced by arc-melting in argon atmosphere, and their oxidation behavior was investigated at temperature between 700 °C and 1100 °C. All the alloys were composed of three phases, i.e., Mo-rich solid-solution, Ti5Si3 and Mo5SiB2. Specific weight change against oxidation time was obtained at both intermediate and high temperatures. A rapid mass loss occurred in the initial period of oxidation, after which the oxidation rate gradually decreased and sometimes even stayed stable. The total mass loss within 24 h was less than 2.0 mg/cm2 at all testing temperatures for all compositions. All the alloys demonstrated a relatively good oxidation resistance at intermediate temperatures, which was attributed to the relatively low viscosity of the borosilicate glass in the TiO2-borosilicate duplex scale. Pest phenomenon or spalling of the oxide scale did not occur for all alloys, and Al- or Cr-addition improved the oxidation resistance at intermediate temperatures through microstructure revolution. Continuous Al- or Cr-oxide layer was not observed on the alloys even after they were oxidized at high temperature. Besides, 32.5Mo–32.5Ti–20Si–10B–5Cr (at.%) had the best oxidation resistance at 1100 °C in this study because a dense borosilicate-matrix duplex layer formed on this alloy.

The study of heat energy transfer through the porous structure of the intermetallide catalyst

Abstract. Purpose. The purpose of the work is to find verified dependences for the calculation of heat and mass exchange processes in the porous structures of intermetallide catalysts, as well as to establish interdependences between the structure of the catalyst and its thermal characteristics. Methodology. The object of study was selected intermetallide catalysts of the Ni−Al system with a various granule filling scheme: in-line and staggered arrangement. To simulate the processes of heat and mass exchange, computer simulation with grid methods was used. To obtain experimental data, tests of a known composition of the catalyst obtained under thermochemical pressing were carried out. The catalytic activity and specific surface area of the catalyst obtained were evaluated. Findings. It was established that the staggered arrangement of the catalyst granules exhibits a large catalytic activity already in the initial period of time, its activity gradually increases with increasing temperature, reaching a maximum value in the investigated temperature range at T = 300 °C. The conversion of carbon monoxide at the staggered arrangement in the initial oxidation period exceeds NiAl 3 in-line arrangement by 1.3…1.5 times. However, the use of a staggered intermetallide catalyst backfill as a catalyst in the oxidation of carbon monoxide will be subjected to coking to much greater extent than a in-line backfill (high density of backfill), as a result of which it will be deactivated much faster. Originality. Research largely eliminates the gap formed between the practice of formation of thermophysical properties and information about changes in the structure and properties of intermetallide catalysts. Practical value. Research allows us to expand the scope of high-performance methods for producing porous intermetallide alloys in the technology of catalysts and to improve their quality.

Medium pressure UV-activated peroxymonosulfate for ciprofloxacin degradation: Kinetics, mechanism, and genotoxicity

Sulfate radical (SO4rad−)-based advanced oxidation processes (SR-AOPs) are promising technologies for organic micropollutant removal. In this study, medium pressure (MP) UV was used to activate peroxymonosulfate (PMS) as an emerging SR-AOP for the degradation of ciprofloxacin (CIP), which is a typical fluoroquinolone. It was found that ∼80% of CIP was removed at a UV dose of 200 mJ/cm2 in the MPUV/PMS system (3.02 μM CIP, 0.2 mM PMS, pH = 3.75). Scavenging experiments indicated that both hydroxyl radical (radOH) and SO4rad− contributed to CIP removal in the MPUV/PMS system. In acidic conditions, the predominant reactive radical was SO4rad−, while the role of radOH in CIP degradation was enhanced under neutral and alkaline conditions. A high PMS dose, high pH, low CIP concentration and the presence of Cl−, CO32− and NO3− promoted CIP removal. Transformation products (TPs) of CIP were detected in the MPUV-only and MPUV/PMS experiments, and the initial transformation pathways were proposed. Defluorination and transformation of the piperazine ring were found to be the main pathways for degrading CIP during MPUV-only and the MPUV/PMS processes, respectively. An umu test indicated that MPUV/PMS was effective at reducing the genotoxicity of CIP solutions, but the MPUV-only was not. However, some TPs formed during the initial oxidation period in the MPUV/PMS system had equivalent and even higher toxicity than CIP. The results of this study contribute to our understanding of CIP degradation by MPUV/PMS and provide some basic information for use in future studies and applications.

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.

The oxidation of uranium hydride during its instantaneous or gradual exposure to oxygen

The oxidation behaviour of uranium hydride when exposed to oxygen gas either gradually or instantaneously at an ambient temperature of approximately 37 °C has been investigated. The proportion of the sample converted to oxide and the mean hydride mound temperature rise accompanying the oxidation process were recorded. Pressure changes in the reaction cell were used to deduce the nature of the oxide generated during the oxidation process and the nature of the gaseous reaction products. For the gradual oxidation of the hydride, typically only about 15% of the hydride mass was converted to oxide during the initial period of oxidation defined by an observed temperature excursion lasting a finite time; during this oxidation period it is concluded that, despite the only moderate recoded mean temperature rises, the generated oxide is U3O8 rather than UO2. Typically also for such gradual exposure experiments, hydrogen was the principal gaseous reaction product with negligible water being produced. For the instantaneous exposure of the hydride to oxygen, much higher hydride mound mean temperature rises were recorded but the proportion of hydride converted to oxide was quite variable and apparently dependent on several experimental variables; for these experiments, although hydrogen was a major reaction product typically some water was also produced. The boundary between pyrophoric and non-pyrophoric oxidation of uranium hydride for given oxidising conditions is discussed in terms of a calculated particle “thermal runaway temperature”.

Oxidative degradation of synthetic ester and its influence on tribological behavior

The oxidative degradation process and oxidation mechanism of di-2-ethylhexyl sebacate (DEHS) were studied in present work. GC–MS results revealed that 2-ethyl-1-hexanol and mono (2-ethylhexyl) sebacate were major degradation products in the liquid phase. Further investigation of degradation mechanism of DEHS showed that oxidation reactions and hydrolysis were main factors degradation of DEHS. The four-ball test results showed that chemical composition changes of DEHS during oxidation could significantly influence its tribological behavior. The formation of hydroperoxides in the initial oxidation period could seriously deteriorate lubrication properties of DEHS. However, as the oxidation proceeded, high polar products (acids, alcohols and monoesters) formed could significantly improve the anti-wear and friction-reducing properties of DEHS.

Selective Oxidation of Chromium by O2 Impurities in CO2 During Initial Stages of Oxidation

This study shows that the corrosion behaviour of 12 wt% Cr steel in CO2 at 550 °C is determined in the first stage of oxidation by reaction with O2 impurities. Depending on the amount of theses impurities and the thermal ramp rate, selective oxidation of chromium could lead to the formation of a protective chromium-rich oxide. An oxidation model describing qualitatively the nature of the oxide layer formed in the initial period of oxidation is presented. From these observations, surface engineering processes for protecting 9–12 wt% chromium steels from fast corrosion rate have emerged.

Effects of sulphide oxidation on selected soil properties

This study covered soils containing sulphides (Potential Acid Sulphate Soils - PASS), located on Karsiborska Kepa Island (NW Poland). The aim was to analyse changes in some soil properties caused by the oxidation of sulphides to sulphates for developing a new methodology. Soil samples taken from one representative pedon were oxidized under laboratory conditions for 8 weeks (incubation method). During this period the concentration of sulphates and pH in the soil extract was measured at multi-day intervals. Soil organic horizons characterized by a relatively high content of total sulphur (> 0.75%) showed no decrease in pH below 4 due to the oxidation. The opposite was observed in the case of sandy horizons, poor in sulphur. The rate of pH decline during incubation was the highest in the initial period of oxidation, which justifies the need to perform some soil analyses in samples of fresh /wet soil within 48 hours of sampling. The results enabled us to propose a procedure for soils containing sulphides, which included both field and laboratory tests, with special care taken to preserve as much as possible the natural reducing environment.

Oxidation Kinetics of Chromium Carbide Coating Produced on AISI 1040 Steel by Thermo-Reactive Deposition Method during High Temperature in Air

Oxidation of chromium carbide coating formed on AISI 1040 steel deposited by thermo-reactive deposition method (TRD) has been realized by two stepped reactions. In the initial part of the reactions in the oxidation process, carbon atoms combined with chromium on the outer part of the coating layer react with the oxygen in air, effectively up to 120 min. After that, the chromium atoms react with oxygen in the air and produce Cr2O3 phase on the coating layer. The higher the temperature and the longer the treatment time, the more the Cr2O3 phases became. The kinetic study was realized for the reactions of carbon and chromium with oxygen, individually. The kinetic study of oxidation was calculated by weight changing of the coated samples at the temperatures of 973 K, 1073 K and 1273 K up to 720 min. We established that the chromium carbide coated steel are characterized by an insignificant increase in the mass in the oxidation period up to 3.5 h, after which the degree of oxidation increases somewhat. The nature of oxidation kinetics for chromium carbide coated steel varies from some mass degrease in the initial period ( 2 h) in connection with the formation of CO and CO2 to later mass increase with in connection with the formation of Cr2O3 layer. The oxidation resistance of chromium carbide coated steel decrease with an increase in oxidation temperature. The growth rate constant of oxidation of chromium carbide coated steel ranged from 5.13x10-13 to-9.617x10-11 g4.cm-2s-1 in the initial period of oxidation (up to 120 min), while it ranged from 3,163x10-13 to 2.188 x10-10 g4.cm-2s-1 in the second period of oxidation test (over 120 min). The activation energies of oxidation of the chromium carbide coated steel are 185 kJ/mol for the initial period and 215 kJ/mol for the second period.

Formation of Ni-aluminide Coating Containing Hf by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

The formation of a coating layer consisting of Ni-aluminide containing Hf on a Ni–10at.%Cr–8at.%Al alloy substrate was attempted by the electrodeposition of Hf, Ni, and Al. The cyclic-oxidation resistance for the alloy covered with this coating was then evaluated in air at 1,423 K. Ni was deposited by aqueous solution electrolysis. Hf and Al were deposited by molten-salt electrolysis. For the sample first treated with the Hf-deposition, subsequently treated with the Ni-deposition, followed by Al-deposition, a coating consisting of Ni2Al3 of about 40-μm thickness was uniformly formed on the alloy. At the center region of this coating, a Hf-concentration layer was formed. The cyclic-oxidation test showed that, for the untreated sample and the sample with only Ni and Al depositions, a mass reduction due to spallation of a scale took place during the initial oxidation period. On the contrary, for the sample with Hf, Ni, and Al depositions, a mass reduction due to spallation of a scale scarcely took place. The cross-sectional observation using SEM showed that, for the sample with Hf, Ni, and Al depositions after the oxidation test, an adhesive scale having a spiked shape was formed. This scale mainly consisted of α-Al2O3, and contained HfO2 particles. It was postulated that, for this sample, the Hf in the Hf-concentration layer diffused into the surface region of the Ni-aluminide layer, contributing to improvement in the exfoliation resistance of the scale.

Effect of preozonation on the characteristic transformation of fulvic acid and its subsequent trichloromethane formation potential: Presence or absence of bicarbonate

The effects of bicarbonate on the characteristic transformation of fulvic acid (FA) and its subsequent trichloromethane formation potential (TCMFP) were investigated in the process of preozonation. Dissolved organic carbon (DOC) removal rate and the residual aqueous ozone concentration during preozonation were measured with different bicarbonate concentration. The presence of bicarbonate inhibited DOC removal and decreased TCMFP yields in the initial oxidation period. In order to explain these phenomena, the molecular weight (MW) distribution (<5, 5–10, 10–30, and >30 kDa) and corresponding TCMFP were investigated for FA and its subsequent oxidation products. Furthermore, transformation of molecular structure, based on MW distribution, was also characterized with Fourier transform infrared (FTIR) spectrum. Bicarbonate showed different inhibiting effects on TCMFP of organic species with different MW, and more significant TCMFP decrement was observed for the high MW fraction (>30 kDa) than for the low MW fractions. Preozonation led to obvious reduction on DOC and UV 254 in most of MW fractions wherever bicarbonate was present or not, demonstrating that ozone contributed to both organics mineralization and structure variation, synchronously. As being indicated from the results of FTIR and gas chromatography–mass spectrometry, the functional groups such as alcohols, epoxides and phenols, the formation of which was promoted with hydroxyl radicals ( OH) and would be remarkably inhibited by bicarbonate, were responsible for the increment of TCM precursor’s concentration during ozonation. Results of these studies confirmed low dosage bicarbonate affecting the ozonation pathways, influencing the intermediate species formation and impacting its subsequent TCMFP yields through inhibiting the OH radicals reactions mainly occurred in high MW fractions.

Thermal characterization of commercially pure titanium for dental applications

Thermal characterization of commercially pure titanium was carried out in dry air to investigate the oxidation kinetics, the oxide structures and their properties. Oxidation kinetics were performed by thermogravimetry in isothermal conditions between 300 and 750 °C for 48 hours and the oxide structures were studied by differential thermal analyses and X ray diffraction between room temperature and 1000 °C. The oxidation kinetic increases with temperature and is very fast in the initial period of oxidation, decreasing rapidly with time, especially up to 600 °C. Kinetic laws varied between the inverse logarithmic for the lower temperatures (300 and 400 °C) and the parabolic for the higher temperatures (650, 700 and 750 °C). Evidences from X ray diffraction and differential thermal analyses showed that crystallization of the passive oxide film, formed at room temperature, into anatase occurs at about 276 °C. The crystallized oxide structure is composed of anatase between 276 and 457 °C, anatase and rutile sublayers between 457 and 718 °C, and a pure layer of rutile after 718 °C. Rockwell-C adhesion tests reveled that the oxide films formed up to 600 °C have a good adhesion. Vickers indentations on the oxidized surfaces showed that the hardness of the oxide film, measured at 600 and 650 °C, is approximately 9500 MPa. At these temperatures the surface roughness varied between 0.90 and 1.30 mm.

電析法による Zr を含む Ni アルミナイド膜のコーティングとその耐サイクル酸化性

The formation of a coating layer consisting of Ni aluminide containing Zr on a Ni-10 at%Cr-8 at%Al alloy substrate was attempted by the electrodeposition of Zr, Ni and Al. The cyclic oxidation resistance for the alloys covered with this coating was then evaluated. Ni was deposited by aqueous solution electrolysis. Zr and Al were deposited by molten salt electrolysis. For the sample treated with the Ni deposition, followed by the Al deposition, a layer consisting of Ni2Al3 was uniformly formed. On the other hand, for the sample treated with the Zr deposition, followed by the Ni and Al depositions, a Ni-Zr alloy layer and a Ni2Al3 layer were formed in that order from the substrate alloy. The cyclic oxidation test showed that for the untreated sample and the sample only with Ni and Al depositions, a mass reduction due to exfoliation of a scale took place during the initial oxidation period. For the sample treated with the Zr deposition for 0.03 and 0.24 ks, followed by the Ni and Al depositions, a mass reduction was observed during the course of the oxidation test. However, for the sample treated with the Zr deposition for 0.18 ks, no mass reduction was observed. The result of a cross-sectional observation for this sample treated with the Zr deposition for 0.18 ks after the 5-cycle oxidation test showed that the Ni-Zr alloy layer disappeared and a wedge-shaped α-Al2O3 scale was formed on the Ni aluminide layer. Consequently, it was considered that for this sample, the Zr in the Ni-Zr alloy layer diffused into the Ni aluminide layer, contributing to an improvement in the exfoliation resistance of the Al2O3 scale.

Growth Strains and Stress Relaxation in Alumina Scales during High Temperature Oxidation

A novel X-ray technique was used, exploiting synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory, to investigate the growth stresses in {alpha}-Al{sub 2}O{sub 3}. In-situ measurements of Debye-Scherrer diffraction patterns from the scale were recorded during oxidation and cooling, and the elliptical distortion of the diffraction rings was analyzed to yield the in-plane strain. Fe-28Al, Fe-40Al, Fe-40Al-0.2Hf, Fe-20Cr-10Al and Ni-50Al (at. %) were studied. Data were acquired in air at temperatures between 950-1100 C and during cool down. In all cases, the steady stage growth strain was relatively low (<0.1%) and was either tensile or compressive depending on the alloy. A higher tensile strain often existed during the initial oxidation period when transition alumina was present. Thermal stresses imposed on NiAl by reducing the sample temperature to 950 C for a period of time showed noticeable stress relaxation by creep. Different degrees of relaxation were also found during cooling depending on alloy composition and scale microstructure. On all Fe-based alloys, the first formed {alpha}-Al{sub 2}O{sub 3} was highly textured with the degree of texture decreasing with further oxidation. The relationships between stress development, scale wrinkling, oxide phase changes, and the effect of reactive element addition on growthmore » stresses are discussed. Results are compared with other reports of growth stresses in Al{sub 2}O{sub 3} scales.« less

Effect of P, B4C, and (Ti, Cr)C Additives on the Kinetics of High-Temperature Oxidation of Chromium Carbide Alloys at 1000°C

We have used the thermogravimetric method to study the effect of B4C, (Ti, Cr)C, and phosphorus additives on the kinetics of oxidation of chromium carbide alloys of the type 85% Cr3C2 ― 15% Ni (KKhN15) and 85% Cr3C2 ― 15% (Ni ― P) (KKhNF15), in air under isothermal heating conditions at 1000°C from 0 to 5 h. We have established that the alloys are characterized by insignificant increase in mass in the initial oxidation period (≈3 h), after which the degree of oxidation increases somewhat. The nature of the oxidation kinetics for KKhNF15 alloys, containing 0.016-0.64% B4C, varies from some mass increase in the initial period (≈20 min) to later mass loss in connection with formation of CO, CO2 and rapid evaporation of boron anhydride B2O3. The maximum resistance to oxidation is exhibited by alloys of the type KKhN15 and KKhNF15, the mass increase of which is 0.128-0.256 mg/cm2 after 5 h of heating at 1000°C.

The formation of diffusion coatings on some low-alloy steels and their high temperature oxidation behaviour: Part 2. Oxidation studies

The oxidation of aluminised coatings on FeNi, FeSi and FeCr alloys and of chromised coatings on FeNi and FeSi alloys were studied at temperatures in the range 700 to 1000°C and at oxygen pressures from 10 −8 to 1 atm. and related to appropriate phase diagrams. Up to 800°C Al 2O 3 or Cr 2O 3 scales were formed leading to good oxidation resistance. At higher temperatures (∼1000°C) the oxidation resistance was frequently lost due to oxide-scale failure and formation of iron oxides. Only aluminised Fe-19.2 wt%Cr alloy remained fully protective up to 1000°C in pure oxygen. Parabolic rate constants for the initial period of oxidation were a linear function of oxygen pressure. Activation energy data for the oxidation of chromised steels gave values in reasonable agreement with those obtained from the literature for the oxidation of pure Cr and FeCr alloys. The effect of substrate structure on the oxide-scale failure mechanisms was studied in detail. The cracking of the scales has been shown to be associated with substrate grain boundaries and the formation of nodular oxides has been shown to be associated with grain boundaries and in particular with the triple points of grain boundaries.

Adsorption of atomic oxygen on GaAs(001)-(2×4) and the resulting surface structures

The naturally occurring oxide of GaAs has a high density of defects that pin the Fermi level at the GaAs surface. The principle electronic defect causing the Fermi level pinning is widely believed to be an arsenic antisite double donor. We have used scanning tunneling microscopy to show that the arsenic antisite defects are formed during the initial period of oxidation of GaAs(001) by atomic oxygen. Atomic oxygen displaces a single arsenic atom in the top layer leaving behind an undimerized arsenic atom with a half-filled dangling bond. The displaced arsenic atoms bond to remaining arsenic dimers to form arsenic antisites. Cluster calculations confirm the assignment of the oxygen binding sites and the role of undimerized single arsenic atoms as acceptor sites.