Initial Stages Of Oxidation Research Articles

Reactive molecular dynamics study of the oxidation behavior of iron‐based alloy in supercritical water

AbstractA theoretical study of the oxidation behavior of iron‐based alloy in the supercritical water (SCW) has been carried out based on ReaxFF force‐field molecular dynamics simulation. An atomic model has been proposed to simulate the initial chemisorption reactions and atoms diffusion behavior across the oxide layer. Simulation results imply that Cr addition has an important effect on the oxidation behavior of iron‐based alloy. In the initial stage of oxidation, H2O prefers to adsorb on the Cr atom, and some species in the form of Cr(OH)4 are observed on the FeCr alloy surface. Once an initial oxide layer is formed, further oxidation is controlled by the migration of vacancy. The O vacancies are formed at the oxide/FeCr alloy interface and migrate toward the steam, whereas Fe vacancies are formed at the oxide/steam interface and migrate toward the FeCr alloy. Attributed to the stronger binding energy of O–Cr bond than O–Fe bond, the Cr diffusivity in the oxide is less than Fe atoms. Thus, double oxide layers, including the inner Fe–Cr–O layer and outer Fe–O layer, are formed on the FeCr alloy, which is in good agreement with previous experimental observation.

Oxidation process of lead sulfide nanoparticle in the atmosphere or natural water and influence on toxicity toward Chlorella vulgaris

Lead sulfide nanoparticle (nano-PbS) released into environment can cause hazards to human or ecosystem. Nano-PbS potentially undergoes oxidation in the environment, but oxidation mechanism is not understood yet. Herein, oxidation kinetics and products of nano-PbS by ozone (O3), hydrogen peroxide (H2O2) and hydroxyl radical (HO·) in the atmosphere or natural water were investigated. Results show that oxidation process of nano-PbS can be divided into three stages, producing sulfate, ions and oxides of lead in sequence. O3 or HO·leads to faster release of ionic lead from nano-PbS in the initial stage than H2O2, but causes significant decrease of ionic lead by transforming divalent lead to tetravalent lead oxides in the second or third stage. Toxicity determined taking Chlorella Vulgaris as an example follows an order of PbO2 < Pb3O4 < nano-PbS < PbO < PbSO4. Toxicity of lead particles is mainly determined by sizes influencing cellular uptake and solubility product constant (Ksp) related with dissolution of lead in cells. The results indicate that the toxicity of nano-PbS increases in an initial oxidation stage and decreases in further oxidation stages. This study provides new insights into environmental behavior of nano-PbS and mechanism understandings for assessing ecological risks of nano-PbS.

Thermal behaviour during initial stages of graphene oxidation: Implications for reaction kinetics and mechanisms

Atomic level understanding of graphene oxidation behaviour is presently far from complete. During large scale preparation of graphene from graphene oxide (GO), persistent presence of up to 8% residual oxygen is an issue of great concern. Such incomplete reduction is attributed to the presence of highly stable carbonyl and ether groups. Here we present a new approach for limiting the formation and behavior of these functional groups. We report high temperature molecular dynamics simulations on the oxidation process of pristine (Pr) and mono-vacancy (MV) graphene with O2 with specific focus on the initial reaction period. An abnormal thermal behaviour was observed in the onset times of oxidation reactions; significant differences were detected in the nucleation and growth mechanisms and reaction kinetics. Overall reaction kinetics was significantly slower in the thermal region (Pr: 4350–4450 K; MV: 4300–4450 K). By identifying this region experimentally or theoretically, a narrow window of minimal carbonyl group formation and residual oxygen could be created leading to a major breakthrough in the field.

Biohazard and dynamic features of different polar compounds in vegetable oil during thermal oxidation

Total polar compounds (TPC) have been considered as one of the most objective indicators for the evaluation of deterioration of deep-frying oils. However, the cytotoxic and dynamics features of different parts of polar compound were underestimated. The present study investigated the biohazard effect of different polar compounds from heated oils. The results indicated oxidize triglycerides monomer (ox-TGM) exhibited the most serious deleterious effect on THLE-2 cell viability, followed by triglycerides polymer (TGP) and degradation products. The investigation of polar compounds variation revealed that vegetable oil with higher saturation endured longer thermal oxidation time to achieve the discarded point of 27% TPC. As the increase of thermal oxidation time, the content of polymers and oxidized products increased linearly, while the level of degradation products was altered moderately. The canonical correspondence analysis uncovered significant positive associations between oleic acid and ox-TGM at the initial stage of thermal oxidation. Taken together, our overall findings offer an evidence that ox-TGM could be acted as an accurate indicator for the evaluation of deep-frying oil quality.

Special Aspects of the Surface Scale Growth during Oxidation of V–Cr–Ta–Zr Alloy in Air

The results of a study of features of the surface scale growth on the V–Cr–Ta–Zr vanadium alloy specimens during short-term oxidation in air are presented. It is shown that in the initial oxidation stages, irrespective of the specimen microstructure, the surface scale growth rate can be an order of magnitude higher than that in the course of long-term treatment. It is found out that the scale is characterized by open porosity formed as a result of sticking of the V2O5 plate-like flakes together. It is suggested that a decrease in the oxidation rate is a consequence of an increase in the surface scale thickness, resulting in a slower supply of oxygen to the specimen surface on which the processes of scale nucleation and growth, as well as diffusion saturation of the subsurface vanadium-alloy layer with oxygen are observed.

InP and AlInP(001)(2 × 4) Surface Oxidation from Density Functional Theory.

The atomic structure and electronic properties of the InP and Al0.5In0.5P(001) surfaces at the initial stages of oxidation are investigated via density functional theory. Thereby, we focus on the mixed-dimer (2 × 4) surfaces stable for cation-rich preparation conditions. For InP, the top In–P dimer is the most favored adsorption site, while it is the second-layer Al–Al dimer for AlInP. The energetically favored adsorption sites yield group III–O bond-related states in the energy region of the bulk band gap, which may act as recombination centers. Consistently, the In p state density around the conduction edge is found to be reduced upon oxidation.

Study on the mechanism of auto-oxidation of Jatropha biodiesel and the oxidative cleavage of C[sbnd]C bond

Jatropha biodiesel was obtained according to the continuous preparation process which included vapor esterification - transesterification - methanol steam distillation. Accelerated oxidation of small Jatropha biodiesel was obtained by the Rancimat method. GC–MS and liquid phase micro-extraction were used to study and analyze the components in the oxidation process of Jatropha curcas biodiesel. The electronic effects of the related reactants and products were calculated by density functional theory, followed by the deduction of the related chemical reaction paths. Experimental investigation shows that methyl linoleate is the main factor affecting the oxidation stability of the Jatropha biodiesel. The main volatile products at the initial stages of the oxidation of methyl linoleate are hexanal, methyl octanoate, styrene, and 2-heptenal. The cis/trans-3-octyl-oxiranyl octanoic acid methyl ester (18.03% yield) is produced by the reaction of peroxy acid and methyl oleate during the oxidation of methyl oleate. The hydrogen extraction reaction is difficult to occur, and the oxidation reaction energy barrier is relatively high due to the relatively large bond energy of the CH bond in the methyl stearate molecule. In this manuscript, the auto-oxidation mechanism of the biodiesel fatty acid methyl esters at the initial stage of oxidation, the path of oxidative cleavage of the CC bond of Jatropha biodiesel and the formation process of ethylene oxide structure are obtained through DFT calculation and analysis of the oxidation products.

Diffusivities and atomic mobilities in α-Y(O) and α-Y2O3 and their applications to oxidation of pure α-Y during the initial stage

The atomic mobility descriptions for Y and O in both α-Y(O) and α-Y2O3 were first assessed by means of CALculation of PHAse Diagram approach based on the critically-reviewed diffusivities available in the literature. The diffusion anisotropy in α-Y(O) with hcp structure and the short-circuit diffusion in both α-Y(O) and α-Y2O3 were taken into account. Based on the assessed atomic mobilities together with the available thermodynamic descriptions, most of the experimental diffusivities can be reproduced satisfactorily. Afterwards, one-dimensional numerical simulations of oxidation of pure yttrium during the initial stage at different temperatures were performed based on a home-made code. The simulated weight gain data of α-Y(O) at different temperatures are in good agreement with the experimental ones, further validating the reliability of the presently obtained atomic mobilities in α-Y(O). It was found that the initial-stage oxidation of α-Y at higher temperatures is mainly controlled by only the bulk diffusion, while the lower temperature initial-stage oxidation is controlled by bulk and short-circuit diffusion simultaneously. Furthermore, the contribution of short-circuit diffusion at lower temperatures is dominant, and can be largely enhanced as the grain size in the specimens decreases.

Initial stages of FeMnSiCrNi shape memory stainless steels oxidation mechanism at 800 °C

FeMnSiCrNi alloys form Mn-rich oxide and Mn/Cr spinel at high temperature. However, studies on the oxidation mechanism are still incipient. Thus, in order to understand the initial oxidation stages of the alloy Fe-17Mn-5Si-10Cr-4Ni-VC at 800 °C, up to 24 h, the oxide scale has been characterized by XR-Diffraction, Raman spectroscopy and TEM-ASTAR. Ferrite expansion during cooling was detected and a new oxidation mechanism was proposed, with simultaneous Mn2O3 (externally) and Mn/Cr spinel (internally) formation since the beginning of oxidation, as also discontinuous Cr2O3 surrounded by spinel. The formation of tetragonal-(Mn,Cr)3O4 was also reinterpreted, highlighting the importance of Mn-diffusion on MnCr2O4.

Features of oxidative functionalization of multiwalled carbon nanotubes

The paper studies the oxidation laws of multiwalled carbon nanotubes (MWCNTs) of various types, which differ in geometric parameters (diameter, length, specific surface), using nitric acid. In different experiments, the duration of oxidation, the concentration of nitric acid, and the consumption of HNO3 per 1 g of MWCNTs were varied. The qualitative composition of the formed oxygen-containing functional groups and the changes occurring with the graphene layers of nanotubes were established by the methods of IR and Raman spectroscopy, and transmission electron microscopy. The total oxygen content in the samples was determined by energy dispersive analysis. The degree of MWCNTs functionalization with acidic functional groups was determined by Boehm titration. An X-ray structural analysis of the obtained samples was carried out and their specific surface area was estimated. It was determined that the most serious changes in the functionalization degree occurred during the first 1–3 hours of oxidation. During this time interval, the achieved values of the functionalization degree were determined mainly by the shape of the MWCNTs graphene layers. At 7–10 hours of treatment, regardless of the initial parameters, MWCNTs have the functionalization degree in the range of 0.5–0.7 mmoLg–1. At the initial stage of oxidation, the amorphous phase is removed. Using dilute solutions of nitric acid does not allow obtaining oxidized nanotubes with the high functionalization degree. It is expedient to reduce the consumption of 65 % HNO3 to 40 mL per 1 g of MWCNTs.

Oxidation and hot corrosion behaviors of Nb−Si based ultrahigh temperature alloys at 900 °C

Abstract Oxidation and hot corrosion behaviors at 900 °C of Nb−Si based ultrahigh temperature alloys were investigated. Both oxidation and hot corrosion kinetics curves of the alloy involve an initial parabolic stage and a later rapid linear stage. In the initial oxidation stage (1−50 h), a thin and continuous scale is formed on the alloy surface, while severe pest degradation phenomenon is observed in the linear oxidation stage. Compared with oxidation of the alloy in static air, a linear hot corrosion stage happens earlier and catastrophic scale disintegration occurs after hot corrosion for 20−100 h, demonstrating that molten salts (Na2SO4 and NaCl) could significantly accelerate the oxidation process of the alloy. STEM results indicate that the corroded scale consists mainly of TiO2, Nb2O5, TiNb2O7, amorphous silicate and NaNbO3.

Study into initial stages of plasma electrolytic oxidation of magnesium alloy

In the present work, a coating growth during plasma electrolytic oxidation on magnesium alloy was investigated. We observed the initial stages: electrochemical anodizing accompanied by anodic dissolution; oxidation with low power spark discharges; oxidation with microarc discharges. The study discusses the evolution of morphology, porosity, elemental composition in comparison with the PEO electrical characteristics and optical emission of microdischarges. The kinetic model, which is consistent with the observed mechanism of coating growth, is constructed.

Oxidation behavior and interdiffusion of Ta[sbnd]Al multilayer films and Inconel 617 alloy

In this study, TaAl multilayer films were cosputtered on Inconel 617 through cyclical gradient concentration deposition. The oxidation behavior of the TaAl films and the interdiffusion between the TaAl films and Inconel 617 were evaluated through thermogravimetric analysis, X-ray diffraction, transmission electron microscopy, and Auger electron spectroscopy after the films were annealed in air at 400–800 °C. The results indicated that the Ta0.79Al0.21 multilayer films had a low O content even though O diffused inward through the entire film thickness after 8 h of annealing at 800 °C, which was attributed to the formation of an Al oxide scale due to the outward diffusion of Al in the initial oxidation stage. This under-stoichiometric Al2O3/Ta2O5 structure caused the produced films to exhibit a high hardness of 23.2 GPa and a high Young's modulus of 316 GPa. Moreover, the outward diffusion of Inconel 617 elements, namely Ni and Cr, was restricted by the oxidized films. However, the high Ta content in the TaAl films deteriorated the film adherence to the substrates due to a high volume ratio of Ta2O5/Ta. Furthermore, laminated Ta0.79Al0.21/Ta0.34Al0.66/Ti films were fabricated to improve the film–substrate adhesion. These films exhibited a high hardness of 17.1 GPa after annealing at 800 °C for 24 h.

Formation of Protective Alumina on Ni3Al By Oxygen Partial Pressure Control Using Oxygen Pump Sensor

Introduction The oxidation of Ni aluminide with a low Al concentration produces NiAl2O4 and Al2O3 on the surface. It has been reported that the formation of NiO during the early stage of oxidation leads to the formation of NiAl2O4, resulting in poor-oxidation resistance. In this study, an oxygen pump-sensor was installed in front of the oxidation furnace to accurately control the oxygen partial pressure in the atmosphere. During the initial stage of oxidation, the partial pressure was reduced to a level at which NiO was not generated, and the partial pressure of oxygen was rapidly increased to the temperature at which α-Al2O3 was formed. Oxygen Partial Pressure Control and Oxidation Kinetics Assessment Fig.1 shows the oxidation test equipment used in this study. An oxygen sensor and oxygen pump sensor were installed at the latter stage of the electric furnace, and the oxygen partial pressure and hydrogen oxidation current (hydrogen concentration) of the discharged gas atmosphere were measured. Yttria-stabilized zirconia tubes (8 mol%Y2O3-ZrO2) were used for the oxygen sensor and oxygen pump sensor.For the oxygen sensor, the electromotive force was measured and substituted into the Nernst equation (Eq. 1) to obtain the oxygen partial pressure. E=(RT/4F)ln(P O2(ref)/P O2(mea)) (1)where, R: gas constant, T: temperature, F: Faraday constant, E: measured electromotive force, : oxygen partial pressure in the reference gas (atmosphere 0.21 atm) and : measured oxygen partial pressure.The oxygen pump sensor consists of a sensor part and a pump part. The electromotive force is measured by the sensor, and the electromotive force is set so that the initial oxygen partial pressure is obtained. Since it is an oxide ion conductor, when a current is applied to the pump section, oxygen is supplied from the inside wall of the tube in the pump section via reactions (2) (inside wall) and(3) (outside wall).2O2-→O2+4e- (2)O2+4e-→2O2- (3)As a result, the oxygen partial pressure of the pump section is maintained at the initial oxygen partial pressure.Furthermore, by applying a negative current at the pump section, oxygen in the tube can be discharged by reaction (2) on the outside and reaction (3) on the inside. The experiment was performed by controlling the oxygen partial pressure of the atmosphere in this manner.The calculation of the oxidation rate using the oxygen pump sensor was performed according to Faraday's law (Eq. 4). J=I/4F (4)where, F: Faraday constant, I: applied current, and J: amount of oxygen supplied to the measurement system by the oxygen pump. The amount of hydrogen was calculated from the supplied amount of oxygen J, and the oxidation rate was calculated. Results Fig.2 shows the results of a oxidation test in which the oxygen partial pressure P O2 of the Ni-25at.%Al alloy was changed from 10-19 atm to 10-6 atm at 1050 °C. In the figure, change in oxygen partial pressure, the oxidation current, and the oxidation rate are shown in (a), (b), (c), respectively. The P O2 change (a) shows that the initial P O2 is 10-19 atm. Thereafter, when the temperature reached 1050 ° C, the P O2 was increased to 10-6 atm. It can be seen from the change in the current value (b) that the current abruptly increased when the P O2 was changed. In other words, it can be seen that oxidation rapidly progressed when the P O2 was changed, which agrees with the oxidation rate result(c).The oxidation rate rapidly increased when P O2 was changed. then showed a low oxidation rate at 1250 °C.Fig.3 shows the cross-sectional microstructure and the corresponding element map by EPMA after the oxidation experiment when the P O2 was changed from 10-19 atm to 10-6 atm at 1050 °C. During the initial stage of the oxidation, the P O2 was lowered, and a thin Al2O3 film was formed on the surface by increasing the P O2 at 1050 °C. When the P O2 was changed, only Al2O3 was formed. Therefore, at the beginning of oxidation, the P O2 is reduced to avoid formation of Ni oxide. It was clarified that when the P O2 was increased at 1050 ° C at which the a-Al2O3 was formed, only Al2O3 with a high oxidation resistance was formed. The oxide formed in this manner is expected to have high oxidation resistance. Conclusion The accurate control of oxygen partial pressure in the furnace is successfully conducted by applying the oxygen sensor and oxygen pump sensor. Using the furnace, we the direct formation of Al2O3 on Ni3Al was confirmed.AcknowledgementsPart of this study was supported by the NIMS Joint Research Hub Program. Figure 1

Effect of the ZrB2 content on the oxygen blocking ability of ZrB2-SiC coating at 1973K

ZrB2-SiC coatings were prepared by spark plasma sintering method. The effect of ZrB2 content ranging from 20wt.% to 80 wt.% on the oxidation behavior of the ZrB2-SiC coatings was investigated at 1973K in air. In the initial oxidation stage (<30 min), although the increased ZrB2 content with high oxidation activity aggravated the oxidation of coating, the slackened SiC consumption enhanced the oxygen blocking ability of the coating itself. With increasing oxidation time, owing to sufficient dispersion of Zr-oxides nanocrystal particles in the self-generated glass, the glassy layer on the 60ZrB2-40SiC coating exhibited the best oxygen blocking ability. The dispersed Zr-oxides nanocrystal particles conduced to broaden crystalline areas and increased the viscosity of the glassy layer, restraining the oxygen permeability and weakening the damage of oxygen to the glassy layer. With increasing oxidation time, the dispersion degree of Zr-oxides nanocrystal particles would strengthen the oxidation protection efficiency.

Experimental investigation on spontaneous combustion of coal affected by exothermic reaction of polyurethane in underground coal mines

Organic elastomers such as polyurethane are widely used as a filling material in underground coal mines. However, the risk of fire hazard in coal seams is increased due to the flammability of polyurethane. Laboratory tests illustrate that in a thermostatic reactor with initial temperature of 25 °C, the temperature during the production of polyurethane increases up to > 140 °C. Adiabatic oxidation experiments and TG–DTG analysis showed that the heating rate of the mixed samples of polyurethane and coal was higher than that of any single component in the initial oxidation stage. From the initial temperature to 100 °C, increasing the proportion of polyurethane was conducive to the acceleration of temperature rise. From 0.180 to 0.850 mm, the self-heating rate was inversely proportional to the particle size. At the initial stage, the low ventilation rate promoted the spontaneous combustion of mixed samples, but when the temperature is > 70 °C, an opposite trend was seen. Kinetic parameters suggested that the activation energy decreases with the introduction of polyurethane. According to the mathematical model, the variation of the spontaneous combustion period of coal and polyurethane was predicted.

Thermal and Thermooxidative Degradation of Blends Based on Polylactide and Polyethylene

The thermal stability of the blends based on polylactide and low-density polyethylene of various compositions is studied. A decrease in the degradation temperature is observed with an increase in the polylactide content in samples (from 414°C for 100% polyethylene to 364°C for 100% polylactide). The influence of aged polyethylene on the thermooxidative degradation of the blends containing 30 wt % polylactide is studied. The third component (aged polyethylene) is determined to accelerate the initial oxidation stage.

Rapid assessment of structural and compositional changes during early stages of zirconium alloy oxidation

A multimodal chemical imaging approach has been developed and applied to detail the dynamic, atomic-scale changes associated with oxidation of a zirconium alloy (Zircaloy-4). Scanning transmission electron microscopy, a gas-phase reactor chamber attached to an atom probe tomography instrument, and synchrotron-based X-ray absorption near-edge spectroscopy were employed to reveal morphology, composition, crystal, and electronic structure changes that occur during initial stages of oxidation at 300 °C. Oxidation was carried out in 10 mbar O2 gas for short exposure times of 1 and 5 min. A multilayered oxide film with a cubic ZrO adjacent to the oxide/metal interface, a nanoscopic transition region with a graded composition of ZrO2−x (where 0 < x < 1), and tetragonal ZrO2 in the outermost oxide were formed. Partitioning of the major alloying element (tin) to the oxide/metal interface and heterogeneously within the oxide accompanied the development of the layered oxide. Our work provides a rapid, high-throughput approach for detailed characterisation of initial stages of zirconium alloy oxidation at an accelerated time scale, with implications for several other alloy systems.

Exploring the initial oxidation of Pt, Pt3Ni, Pt3Au (111) surfaces: a genetic algorithm based global optimization with density functional theory

Noble metal alloys are one of the most commonly used heterogeneous catalysts. During many reactions, the surface composition and oxidation states of the noble metal alloy particles have been reported to be dynamic. This paper describes a density functional theory study to explore the initial oxidation stages of the Pt-based surfaces, which are widely-used catalysts in various clean energy conversion processes. By applying a genetic algorithm based global optimization, we identified new surface phases at relatively high O coverages, 1 ML and 3/2 ML, on Pt and Pt alloy (111) surfaces. The existence of O transforms the metallic surfaces, creating oxide skins with different morphology and composition. Metals with higher reducibility are pulled out to the outmost surface, to bind with O atoms. The lattice constant affects the binding strength of O atoms over certain oxide skins. Moreover, the strain effect plays a crucial role in the formation of oxide overlayers.

Effect of thermal oxidation on surface chemistry and elemental segregation of Al-Cu-Li alloy

The thermal stability of Al-Cu-Li alloys is critical for the extreme working conditions in the aircraft industry. The initial stages of thermal oxidation of an industrial AA 2050-T8 Al-Cu-Li alloy were investigated by in situ XPS and ToF-SIMS.Annealing the sample at 200 °C leads to a surface enrichment in metallic Li and Ag. When exposed to oxygen at low pressure (PO2 < 10−6 mbar) a mixed Li-Al oxide layer is formed. Photoemission intensities (recorded at 45° and 90° relative to the surface) are consistent with the formation of a continuous oxide layer for oxygen exposure from 0 to 5000 langmuirs (L). Assuming a homogeneous composition of the oxide layer, its thickness was 9 ± 1 Å at 5000 L. The relative content of Li and Al in the oxide layer evolves from an initial value of 50% to stabilize at 70% of Li above 100 L of oxygen exposure. Simultaneously, the content of Li at the alloy substrate surface is reduced from 10% to 0%. Moreover, specific segregation sites are evidenced by ToF-SIMS three-dimensional chemical mapping. For Li, a homogenous surface distribution while for Mg the accumulation at the grain boundaries were observed. The segregation of Ag at the oxide/alloy interface was confirmed by ToF-SIMS ion depth profiles.