Initial Stages Of Oxidation Research Articles

Oxidation of bituminous coal. 1. Expansion pressure

Experiments show that the oxidation of bituminous coal from the Zasyad’ko mine at 60°C in laboratory conditions increases its expansion pressure from 5.2 to 30.0 kPa in ~28.3 days. Oxidation occurs in stages, as already demonstrated for Ukrainian and imported coal of different metamorphic stages on oxidation in the laboratory and in industrial trials at other temperatures. The rate constant in the initial stage of oxidation is 0.4095 × 10–4 min–1 at 60°C and 3.8479 × 10–4 min–1 at 140°C. That indicates sharp increase in oxidation rate between 60 and 140°C. In oxidation, the actual coal density, the atomic ratio H/C, and the number of hydrocarbon rings per carbon atom decline, while the molar volume per g-atom of carbon increases. Saturation of the bituminous coal with oxygen on oxidation increases the viscosity of the plastic mass that forms and increases the volume of gas and vapor products. Ultimately, that results in increase in the expansion pressure as the coal is oxidized.

Photocatalytic oxidation of the organic monolayers on TiO2 surface investigated by in-situ sum frequency generation spectroscopy

In-situ vibrational sum frequency generation (SFG) spectroscopy has been employed to investigate the photocatalytic oxidation of two types of well-ordered organic monolayers, namely, an arachidic acid (AA) monolayer prepared by the Langmuir-Blodgett method and an octadecyltrichlorosilane (OTS) monolayer prepared by the self-assembling method, on a TiO2 surface under ultraviolet (UV) irradiation. The extremely high sensitivity and unique selectivity of the SFG spectroscopy enabled us to directly probe the structural changes in these monolayers during the surface photocatalytic oxidation and further elucidate their reaction mechanisms at a molecular level. It was revealed that the ordering of the monolayers during the photocatalytic reaction is strongly dependent on their interaction with the substrate; the AA monolayer maintains its ordered conformation until the final oxidation stage, while the OTS monolayer shows a large increase in disordering during the initial oxidation stage, indicating a different photocatalytic reaction mechanism of the two monolayers on the TiO2 surface.

Initial oxidation of Cr2Nb (1 1 1) surface: A density functional study

Surface properties of the clean and O adsorption Cr2Nb (111) are investigated using a density functional theory. The result shows that the Cr2Nb (111) with Cr–Nb mixed termination is more stable than the Cr-terminated surface in both Cr-rich condition and Nb-rich condition for clean (111) surface. The O atoms strongly interact with both Cr and Nb atoms on the Cr2Nb (111) with Cr–Nb mixed surface. And the O atoms strongly interact with the Cr atom on Cr-terminated surface. The binding energy increases with increasing O atom coverage due to the repulsive interaction between O atoms. It is found the Cr2Nb (111) with Cr–Nb mixed surface is more stable than the Cr-terminated Cr2Nb (111) under O-rich or O-poor conditions. And a Cr–Nb oxide layer may be produced at the initial stage of oxidation on Cr2Nb (111) surface.

Improve oxidation resistance at high temperature by nanocrystalline surface layer.

An interesting change of scale sequence occurred during oxidation of nanocrystalline surface layer by means of a surface mechanical attrition treatment. The three-layer oxide structure from the surface towards the matrix is Fe3O4, spinel FeCr2O4 and corundum (Fe,Cr)2O3, which is different from the typical two-layer scale consisted of an Fe3O4 outer layer and an FeCr2O4 inner layer in conventional P91 steel. The diffusivity of Cr, Fe and O is enhanced concurrently in the nanocrystalline surface layer, which causes the fast oxidation in the initial oxidation stage. The formation of (Fe,Cr)2O3 inner layer would inhabit fast diffusion of alloy elements in the nanocrystalline surface layer of P91 steel in the later oxidation stage, and it causes a decrease in the parabolic oxidation rate compared with conventional specimens. This study provides a novel approach to improve the oxidation resistance of heat resistant steel without changing its Cr content.

Effect of Pack Cementation Temperature on Oxidation Behavior of NiCoCrAl Coated Layer

The oxidation behavior of NiCoCrAl coatings deposited on carbon steel was evaluated at 800 °C for 100 h in atmospheric air. The effect of different coating process on the oxidation of carbon steel was studied. The oxidation mechanism was discussed based on oxidation rate, formation of oxide scale, and microstructure of the coated sample. The oxidation rate significantly increased owing to the formation of metastable Al2O3 during initial oxidation stage. The oxidation rate decreased due to the transformation from d, q to a-Al2O3 from the intermediate stage up to final stage of 100 h oxidation. The effect of pack cementation temperature contributed to the thickness and diffusion of coated elements. The experimental results showed that sample developed at 800 °C exhibited better oxidation resistance than sample developed at 900 and 1000 °C.

A comparative study of Rh and Ni coated microchannel reactor for steam methane reforming using CFD with detailed chemistry

Steam methane reforming (SMR) in microchannel reactors has great potential for low-cost, compact hydrogen production due to intensified heat and mass transfer features and an evident reduction of reaction time from seconds to milliseconds. In this work simulation studies of an integrated microchannel reactor were carried out by using a detailed CFD model with elementary reaction kinetics. The model predictions were quantitatively validated by experimental data. The numerical simulations showed the comparison of catalytic performances of Rh and Ni catalysts for SMR in two operating modes, namely, inner-heating and outer-heating. The results demonstrated that Ni catalyst can exhibit excellent performance comparable to Rh under certain conditions. In the outer-heating mode, Ni showed much higher activity for water gas shift (WGS), which led to a higher H2/CO molar ratio in the products but more CO2 emission. In the inner-heating mode, during an initial stage of methane oxidation, complete oxidation of methane (COM) mainly happened on Ni, whereas partial oxidation of methane (POM) mainly occurred on Rh. The second stage was characterized by SMR and WGS, which then exhibited similar features as the outer-heating mode. Reaction path analysis showed that the surface C concentration on Rh in the inner-heating mode was lower than that in the outer-heating mode all along. The surface C concentration on Ni was very small in the initial stage of the inner-heating process, but strongly increased after O2 was consumed. Inner-heating operation is therefore preferred for reducing coking probability.

Microstructural evolution and oxidation behavior of hot-dip 55 wt.% Al–Zn–Si coated steels

The effects of temperature and heating rate on the oxidation kinetics of hot-dip 55wt.%Al–Zn–Si coated steels were investigated experimentally and theoretically. The microstructure evolution was examined by scanning electron microscope (SEM) and X-ray diffraction (XRD). The Fe2Al5Zn0.4 phases formed at the initial stage of oxidation gradually transformed into FeAl with the prolongation of time. The irregularity protuberances appeared on the surface of specimens at 550 °C were formed owing to the rapid growth of Γ phase. The main oxide, ZnO, was generated at 650 and 750 °C, but rarely observed at 550 °C. The weight gain at different temperatures was determined by thermo-gravimetric (TG) balance. The calculated results from Real Physical Picture (RPP) model suggested that the first stage of isothermal oxidation was controlled by chemical reaction and the second stage was controlled by diffusion at 650∼750 °C. The activation energies were 180.4 kJ/mol for isothermal oxidation and 171.2 kJ/mol for non-isothermal oxidation, respectively. All the calculated and predicted results agreed well with the experimental data, which confirmed the validity of RPP model.

A Screening Method to Evaluate Soybean Oil‐Based Biodiesel Oxidative Quality During Its Shelf Life

AbstractBiodiesel quality is negatively affected by oxidation, which occurs primarily during the storage and distribution steps. Therefore, the use of suitable analytical methods to assess the oxidation of this biofuel is a fundamental requirement. A direct ambient ionization mass spectrometric technique (EASI‐MS) was tested as a screening method to evaluate the oxidative quality of biodiesel during its shelf life. Using EASI‐MS, the relative abundance of the hydroperoxide ion of m/z 349 was monitored during the induction period (IP) in soybean and a blend of soybean/beef‐tallow (70/30) biodiesel samples. The peroxide value obtained by AOCS Cd 8b‐90 was used as a standard method to evaluate the EASI‐MS data by comparison. The results showed that the peroxide value and EASI‐MS data were highly correlated during the IP for both samples (r > 0.98). Relative abundances of the ion of m/z 349 below 72 % was also found to characterize biodiesel in an initial oxidation stage, whereas abundances exceeding 72 % indicated biodiesel samples nearing the end of their stability. EASI‐MS was therefore found to provide a simple and effective analytical protocol to evaluate the oxidative quality of biodiesel during the IP.

Oxidation behaviour of electron beam physical vapour deposition β-NiAlHf coatings at 1100 °C in dry and humid atmospheres

NiAl intermetallic compounds attract increasing attention for high-temperature applications due to their combination of excellent properties, especially as metallic protective coatings on superalloys or as bond coats in thermal barrier coatings (TBCs). Oxidation behaviour of the β-NiAlHf coatings prepared by electron beam physical vapour deposition (EB-PVD) was investigated at 1100 °C for short time and 100 h in 20 % O2 + Ar and 15 % H2O + Ar, respectively. The results of mass changes reveal that the addition of minor reactive element Hf significantly improves the cyclic oxidation resistance of the β-NiAl coatings in both dry and humid atmospheres. During the initial oxidation stage, water vapour retards the phase transformation from θ-Al2O3 to α-Al2O3. Moreover, compared with the case in dry atmosphere, water vapour significantly reduces the surface roughness of the oxide scales formed on EB-PVD β-NiAlHf coatings after 100 h cyclic oxidation, which corresponds with the difference of surface morphologies of the oxide scales.

The Role of Grain Boundaries during the Initial Oxidation Stages in Cu-Added Austenitic Stainless Steel at 700 °C Studied at the Atomic Scale

The early stages of the oxidation behavior (≤ 12 h, in air with 20% water vapor at 700°C) of Cu-added austenitic stainless steels with various grain sizes was investigated using atom probe tomography (APT), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD). A thin Cr-rich oxide layer is formed over the entire surface of a small-grained (8 µm) sample, which is similar to that formed near grain boundaries of medium-grained (17 µm) and large-grained (27 µm) samples. Oxidation of all samples proceeds via the lattice and grain boundary diffusion of Cr, leading to the formation of a protective Cr-rich oxide first at the grain boundaries and then via lateral growth toward the grain interiors. APT and TEM studies clearly reveal that within 4 μm of the grain boundaries, the oxide layer exhibits a duplex-layer structure consisting of a thin Fe-rich (Fe,Cr)3O4 oxide (~55 nm) above and a protective Cr2O3 oxide (~40 nm) below as a diffusion barrier. In contrast, further away from the grain boundaries, a non-protective Fe-rich (Fe,Cr)3O4 oxide (~160 nm) and a Cr-rich (Fe,Cr)3O4 oxide (~40 nm) are formed as the outer and inner layers, respectively. The oxidation kinetics was studied in terms of mass gain measurements. The critical grain size (< 8 µm) to prevent the formation of fast-growing, non-protective Fe-rich oxides is discussed based on the experimental findings.

Effect of microstructure of carbon steel on magnetite formation in simulated Hot Conditioning environment of nuclear reactors

The objective of present investigation is to establish the role of starting microstructure of carbon steel on the magnetite formation behaviour in Hot Conditioning simulated environment. Two grades of carbon steel (low and high carbon) were subjected to selective heat-treatments to generate different microstructures: martensite, tempered martensite and modified ferrite–pearlite. Oxidation was carried out in lithiated water of pH 10–10.2 in a static autoclave at 270°C. The results of the investigation clearly establish that: (a) high carbon steel (0.63% C) showed a relatively higher rate of oxidation over the low carbon (0.08% C) grade at all the test durations and (b) the oxidation rates for both the grades were sensitive to microstructural differences at initial stages of oxidation while the differences narrowed down after 72h of exposure. The oxide formed was established to be magnetite on all the specimens.

Effect of temperature on the initial stages of oxidation of γ-Al4Cu9(110)

Combined X-ray photoemission spectroscopy and low energy electron diffraction investigations have been performed to study the influence of temperature on the first stages of oxidation of the (110) surface of γ-Al4Cu9 complex metallic alloy in the pressure range 2–6×10−7mbar. The oxidation mechanism is driven by aluminium segregation. At 115K, a pure aluminium oxide film is formed, which does not present any long range order. CuO bonds are never observed. The lower oxidation kinetics compared to that previously observed at room temperature is due to blocking of the oxygen dissociation and/or a decrease in the aluminium segregation. However, aluminium segregation is still observed, even for this very low temperature, due to the low energy value of the activation energy for aluminium diffusion in γ-Al4Cu9 (0.65±0.12eV) which is related to the presence of two vacancies in the crystal structure. At 925K, fast aluminium segregation and possible oxygen diffusion into the bulk leads to the formation of domains or clusters of γ-like alumina either on top of or, more likely, below the surface.

Formation and phase-mineralogical composition of converter slags

Vanadium slag samples, which are taken at various stages of the converter devanadation of lowsilicon (0.05–0.15% Si) cast iron and after cooling in converter pans, are studied by electron-probe microanalysis and X-ray diffraction. The evolution of the compositions of spinelide, silicate, and metallic phases is investigated. The experimental results are compared with the well-known formation mechanism and the phase composition of the slags of the devanadation of silicon cast irons during oxygen or air blowing. It is shown that, despite the differences in the compositions and the grain sizes of a spinelide phase, the mechanism of its formation at the initial stages of oxidation of cast iron elements with different silicon contents is the same. The spinelide composition and the element distribution over a grain depend on the cast iron composition and the quantity and composition of the silicate phases that form during devanadation in a converter and upon cooling of a slag ingot

Promotion of α-Al2O3 formation on an Ni–Al alloy using a Ni–Fe2O3 nano-composite seeding layer

Thin seeding layers of Ni and Ni–Fe2O3 composites were successfully formed on an Ni50Al alloy using the pulse electrodeposition method (PED) from a Watts bath in order to promote α-Al2O3 formation for improving the high temperature oxidation resistance of the Ni50Al alloy. The conditions for the PED and the effects of these seeding layers on Al2O3 scale formation on the Ni50Al alloy were investigated. The coated samples showed thicker oxide scales than bare Ni50Al samples in oxidation tests at 1000°C in air. At the initial stage of oxidation (1h), a single layer scale composed of blade-like crystals of metastable θ-Al2O3 was formed. The oxide scales formed on the PED coated samples had a multilayered structure consisting of an inner layer of either a mixture of θ-Al2O3 and α-Al2O3 or of single-phase α-Al2O3, intermediate NiAl2O4 layer and an outer NiO layer. The Ni-seeding layer was found to delay the phase transformation of θ-Al2O3 to α-Al2O3 to 100h of oxidation. Addition of 3.1% Fe2O3 to the Ni-seeding layer accelerated the transformation of θ-Al2O3 to α-Al2O3 after 9h of oxidation. Further addition of Fe2O3 nano-powder (5.2%–7.4%) to the Ni-seeding layer successfully suppressed the θ-Al2O3 formation even at 1h of oxidation. The Al2O3 phase structure and thickness of the oxide scales were found to be considerably affected by these seeding layers via two main factors: (i) formation of NiAl2O4 and (ii) the effect of Fe2O3 to accelerate the transformation of metastable Al2O3 to stable α-Al2O3 or to suppress the formation of metastable Al2O3.

Nucleation and growth of oxide islands during the initial-stage oxidation of (100)Cu-Pt alloys

The initial-stage oxidation of (100) Cu-Pt alloys has been examined by in situ environmental transmission electron microscopy and ex situ atomic force microscopy (AFM). It is shown that the oxidation proceeds via the nucleation and growth of Cu2O islands that show dependence on the alloy composition and oxidation temperature. The kinetic measurements on the oxide nucleation reveal that both the nucleation density and surface coverage of Cu2O islands can be promoted by alloying more Pt in the Cu-Pt alloys. Increasing the oxidation temperature above 700 °C results in the growth of large Cu2O islands that transits to a dendritic growth morphology. The ex situ AFM studies reveal that the nucleation of oxide islands can occur on surface terraces and the subsequent oxide growth depletes local terrace Cu atoms that results in the formation of surface pits.

Dielectric and microwave absorption properties of Ti3SiC2/cordierite composite ceramics oxidized at high temperature

The oxidation behavior of the bulk Ti3SiC2/cordierite composite ceramics containing 15wt.% Ti3SiC2 are investigated in air. The results demonstrate the ceramic displays excellent oxidation resistance at 800–1000°C, and the oxidation kinetics basically obey a parabolic rate law. As revealed by XRD and SEM/EDS, the oxidation scales mainly consist of rutile TiO2 and amorphous SiO2. The dielectric and microwave absorption properties of the ceramics oxidized at high temperature are also studied. Both the real and imaginary part of the complex permittivity are decreased as the oxidation temperature increases from 800 to 1000°C, which is attributed to the oxidation of Ti3SiC2 fillers close to the surface. At a given temperature, the complex permittivity decreases at the initial oxidation stage while it keeps stable as the oxidation time increases. Although the complex permittivity of the ceramics are decreased after oxidation at high temperature, the microwave absorption performances are still favorable. The results indicate the Ti3SiC2/cordierite ceramics are potential microwave absorption materials for high temperature application.

Effect of oxidation on the damping behavior of 2D C/SiC composites

A carbon fiber preform was chemical vapor infiltrated with a pyrocarbon (PyC) interphase and a SiC matrix, and then coated with a SiC outer layer by chemical vapor deposition to prepare 2D C/SiC composites with a density of 2.1 g/cm3. The composites were oxidized at 700, 1000, 1300 °C for 2, 5 and 10 h, respectively. The damping behavior of the oxidized composites was measured by a dynamical mechanical analyzer and the microstructural damage produced by the oxidation was investigated by scanning electron microscopy. Results show that the damping of the composites oxidized at 700 °C and 1000 °C increases initially and then decreases with increasing oxidation time while that of the composites oxidized at 1300 °C is independent of the oxidation time.The damping capacity of the C/SiC composites is determined by the carbon fibers, PyC interphase, SiC matrix and their interaction. The oxidation of the composites increases the damping by weakening the interfacial bonding due to the oxidation of PyC during the initial stage of oxidation, and decreases the damping by the oxidation loss of carbon fibers and excessive damage of the PyC interphase during the latter stages of oxidation. SiO2 formed at 1300 °C by the oxidation of SiC fills the voids produced by carbon oxidation, which increases the damping and compensates for the decrease of damping produced by carbon loss and excess damage of the PyC interphase. [New Carbon Materials 2014, 29(6): 522–528]

Preparation and characterization of α-Al2O3 film by low temperature thermal oxidation of Al8Cr5 coating

In this paper, α-Al2O3 film was prepared by low temperature thermal oxidation of Al8Cr5 coating. The Al8Cr5 alloy coating was prepared on SUS430 stainless steel through a two-step approach including electrodepositing Cr/Al composite coating and subsequent heat treatment at 740°C for 16h. After mechanical polishing removal of voids on the surface, the Al8Cr5 coating was thermal oxidized at 720°C in argon for 100h. The samples were characterized by SEM, EDX, XRD, XPS and TEM. XPS detection on the surface of oxidized Al8Cr5 coating showed that the oxide film mainly consisted of Al2O3. TEM characterization of the oxide film showed that it was α-Al2O3 films ca. 110nm. The formation of α-Al2O3 films at low temperature can be attributed to the formation of Cr2O3 nuclei at the initial stage of oxidation which lowers the nucleation energy barrier of α-Al2O3.

The formation of a surface layer during microarc oxidation of aluminum alloys

The formation of a surface layer during microarc oxidation has been investigated. The results of experimental studies aimed at determining the structure and composition of oxide coatings on aluminum alloys are presented. The studies found out that, in contrast to existing ideas about the initial stages of micro-arc oxidation, a "barrier layer" grows unevenly over the sample area and there is no clear boundary between a pre-spark and a spark mode of the process. Uneven growth of a " barrier layer " is connected with different distribution of electric field on the surface of a sample and different concentration of electrolyte components. Depending on the electrolytes used the inside coating layers consist mainly of aluminium oxide, and the outer layers contain components formed as a result of thermochemical transformation of the electrolyte.

Inside Cover: High‐Resolution Imaging of the Initial Stages of Oxidation of Cu(111) with Scanning Electrochemical Potential Microscopy (ChemElectroChem 1/2015)

Inside Cover: High‐Resolution Imaging of the Initial Stages of Oxidation of Cu(111) with Scanning Electrochemical Potential Microscopy (ChemElectroChem 1/2015)