Initial Stages Of Oxidation Research Articles

Unveiling the formation of oxide layers on Ni-16Cr and Ni-20Cr alloys by the addition of Ta

The effect of Ta on the oxidation behavior of Ni-16Cr and Ni-20Cr (wt%) alloys was investigated by isothermal oxidation tests at 1000 °C in air for 100 h. During the initial stages of oxidation, the oxide layer of the Ni-16Cr-0Ta alloy was mainly comprised of NiO, NiCr2O4, and C2O3. The addition of a small amount of Ta (0.5 and 1 wt%) effectively inhibited NiO formation and reduced the oxidation rate. However, excessive Ta failed to inhibit the formation of NiO. For the Ni-20Cr-0Ta alloy, the oxide layer was mainly comprised of NiCr2O4 and Cr2O3 in the initial stages of oxidation, and the addition of Ta has no significant effect on the composition of the oxide layer for the alloy. The formation of CrTaO4 and Ta oxides increased with the amount of Ta and Cr. The appropriate amounts of CrTaO4 and Ta oxides contributed to inhibiting the diffusion of Cr ions, while excessive amounts of CrTaO4 and Ta oxides may decrease the adhesion of the oxide layer.

A new approach to ignition time prediction in crude oil oxidation using PDSC

In-situ combustion process is recognized as being one of the promising methods in the field of enhanced oil recovery. In this study, we developed a procedure for estimating ignition time using pressurized differential scanning calorimetric technique (PDSC) and the chain reaction ignition approach. The procedure was based on the free-radical mechanism of oxidation and was successfully applied to PDSC kinetic curves processing. The initial stages of oxidation of crude oils A and B were studied in detail. It was found that the procedure for processing the oils' activity to oxidation was characterized by a heat release time exponential dependence factor (φ − factor). Even though oil A and oil B exhibit comparable kinetic parameters of oxidation, there is a marked difference in their behavior. Oil A demonstrated an induction period of 27 days and a φ value of 0.0123. Conversely, oil B did not initiate combustion within a suitable timeframe in-situ, and presented a lower φ value of 0.0067. The results of pilot tests using oil A and B were consistent with the predictions and demonstrated the potential of the proposed approach.

Oxidation reaction kinetics of HTPB-boron carbide/polytetrafluoroethylene formulations as a solid fuel

A solid-fuel ramjet engine powered by boron carbide (B4C) can generate nearly the same amount of energy as boron. However, the inherent oxide layer on the B4C surface restricts the energy released during its oxidation. In this study, we examine the effect of polytetrafluoroethylene (PTFE) on the oxidation of B4C and the removal of the oxide layer during the oxidation reaction. The B4C/PTFE binary composite powder was prepared using a ball milling process at three different concentrations (5:20, 10:20, and 15:20). Hydroxyl-terminated polybutadiene (HTPB) loaded with binary composite powder was manufactured by vacuum casting technique. The pure HTPB and HTPB/PTFE fuels were manufactured as reference formulations. The B4C/PTFE binary composite powder was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and high-resolution scanning electron microscope (HRSEM). The thermal oxidation characteristics of prepared fuel formulations were studied using the thermogravimetric technique. The kinetics of the oxidation reaction was studied using both isothermal and non-isothermal methods. The thermogravimetric curves showed that the oxidation reaction of HTPB-based fuel occurred in four steps. When B4C/PTFE binary composite powder is added to HTPB, it decomposes faster and at a higher rate. The decomposed fluorine species of PTFE significantly improved the oxidation reaction of composite powder and increased the energy release rate of B4C. The kinetic studies of oxidation suggested that the addition of B4C/PTFE binary composite powder into HTPB lowered the activation energy (Ea: S1 > S2 > S3 > S4 > S5) required to prompt the oxidation reaction. Based on thermal and kinetic results, a potential oxidation promotion mechanism of B4C/PTFE composite powder was proposed. The initial stages of B4C oxidation resulted in a glassy layer of B2O3 oxide forming at the interface of B4C and PTFE powder. As a result of the oxidation reaction between B4C/PTFE and O2, CF4, Trifluoroboron (BF3) and CO2 were generated. At high temperatures, the oxide shell ruptured, allowing the O2 to diffuse into the core of the B4C particle and releasing a large amount of heat energy.

Oxidation and exothermic properties of long flame coal spontaneous combustion under solid-liquid-gas coexistence and its microscopic mechanism analysis

Coal spontaneous combustion (CSC) wastes valuable resources and does great damage to the environment. To study the oxidation and exothermic properties of CSC under solid-liquid-gas coexistence conditions, a C600 microcalorimeter was used to analyze the heat released by the oxidation of raw coal (RC) and water immersion coal (WIC) under different air leakage (AL) conditions. The experimental results showed that the AL was negatively correlated with the heat release intensity (HRI) in the initial stages of coal oxidation, but as the oxidation proceeded, the AL and the HRI gradually showed positive correlations. The HRI of the WIC was lower than that of the RC under the same AL conditions. However, since water participated in the generation and transfer of free radicals in the coal oxidation reaction and promoted the development of coal pores, the HRI growth rate of the WIC was higher than that of the RC during the rapid oxidation period, and the self-heating risk was higher. The heat flow curves for the RC and WIC in the rapid oxidation exothermic stage could be fitted with quadratic functions. The experimental results provide an important theoretical basis for the prevention of CSC.

Observation of induction period and oxygenated intermediates in methane oxidation over Pt catalyst

Selective oxidation of methane is one of the most attractive routes for methane to chemicals. However, mechanistic understanding and avoiding over-oxidation have great challenges because of its very rapid reaction rate. Herein, a capillary micro-reaction system was introduced to monitor the initial stage of methane oxidation over platinum. For the first time, an induction period is observed, during which oxygenated intermediates, such as methanol, acetone, methyl methoxy acetate, etc., are detected. Induction period can be shortened by methane pretreatment at 600°C, which generates highly active species containing unsaturated bonds. Combined these findings and observations of in situ characterizations, the evolution route of methane oxidation over Pt is prosed, i.e., the reaction starts from the formation of initial species containing Pt-C bond, followed by the generation of oxygenated intermediates, and ended with the over-oxidation of the intermediates to CO/CO2.

On the surface chemisorption of oxidizing fine iron particles: Insights gained from molecular dynamics simulations

Molecular dynamics (MD) simulations are performed to investigate the thermal and mass accommodation coefficients (TAC and MAC, respectively) for the combination of iron(-oxide) and air. The obtained values of TAC and MAC are then used in a point-particle Knudsen model to investigate the effect of chemisorption and the Knudsen transition regime on the combustion behavior of (fine) iron particles. The thermal accommodation for the interactions of Fe with N2 and FexOy with O2 is investigated for different surface temperatures, while the mass accommodation coefficient for iron(-oxide) with oxygen is investigated for different initial oxidation stages ZO, which represents the molar ratio of O/(O+Fe), and different surface temperatures. The MAC decreases fast from unity to 0.03 as ZO increases from 0 to 0.5 and then diminishes as ZO further increases to 0.57. By incorporating the MD-informed accommodation coefficients into the single iron particle combustion model, the oxidation beyond ZO=0.5 (from stoichiometric FeO to Fe3O4) is modeled. A new temperature evolution for single iron particles is observed compared to results obtained with previously developed continuum models. Specifically, results of the present simulations show that the oxidation process continues after the particle reaching the peak temperature, while previous models predicting that the maximum temperature was attained when the particle is oxidized to ZO=0.5. Since the rate of oxidation slows down as the MAC decreases with an increasing oxidation stage, the rate of heat loss exceeds the rate of heat release upon reaching the maximum temperature, while the particle is not yet oxidized to ZO=0.5. Finally, the effect of transition-regime heat and mass transfer on the combustion behavior of fine iron particles is investigated and discussed.

Growth mechanism of oxide layer on Ti-6Al-4 V substrate with different surface topographies during the early stage of micro-arc oxidation

The aim of this work was to explore the effect of microscopic surface topographies of the Ti-6Al-4 V alloy on the coating growth behaviour during the initial stages of micro-arc oxidation (MAO). Firstly, MAO treatments were carried out on the surfaces of Ti-6Al-4 V alloys with different surface morphologies for 5, 10 and 15 min respectively. Second, the effects of substrate topographies on the roughness, porosity, thickness, and microstructure of the coatings were studied. Then, the corrosion resistance of the MAO-treated titanium alloys was assessed by electrochemical analysis. Finally, the formation mechanisms and morphological results of the oxide layer were discussed and illustrated graphically. The results showed that the surface morphologies of the substrate affected the micro-arc discharge behaviour during the process of oxidation. Notably, rough areas on the substrate surface facilitated the breakdown of the passivation film, which was evidenced by the early appearance of plasma discharges in these areas. However, surface morphology with uneven distribution of pores observed in the oxide layer formed by the rough substrate. The coatings grown on the grooves were more porous than those grown in other areas, due to the non-uniformity of the micro-discharge on the rough substrate surface.

High temperature oxidation of additively manufactured Rene 65

This paper investigates the initial oxidation stages of additive manufactured Rene 65 Ni-based superalloy between 700 and 900 °C in air and argon for 48 h, with different building directions and heat treatments. The results show that the only factor that impact the oxidation behavior is the oxidizing atmosphere. The resulting thin oxide scales are made of NiCr2O4 and Cr2O3 at 700 °C while at 800 and 900 °C the oxide layer is composed of an external Cr2O3 layer on top of an internal layer of α-Al2O3 due to the lower partial pressure of oxygen underneath the chromia layer. Data availabilityThe raw/processed data required to reproduce these findings are available upon request from the corresponding authors.

Oxidation behavior of the CVD-Al2O3 coatings on tungsten substrate at 1000 °C

In this paper, TiN/TiCN/Al2O3 coating, as a classic coating system for cemented carbide tools, was prepared by chemical vapor deposition (CVD) on pure tungsten substrate for improved oxidation resistance. It is found that the oxidation weight gain per unit surface area of the coated sample is 7.45 mg/cm2 after 5 h oxidation at 1000 °C, significantly lower than that of 218.44 mg/cm2 in uncoated specimen. Due to the difference of thermal expansion coefficient, a network of microcracks were formed within the top Al2O3 layer during fabrication process. These microcracks provide a diffusion path for titanium ions and oxygen ions in the initial oxidation stage (0−1h), which produces TiO2 at cracking regions. As oxidation time prolongs (1–5 h), the oxidation of nitride layer beneath the microcracking region creates short-circuit oxygen diffusion paths towards the metal substrate. The generation of volcanic-like WO3 breaks the graded structure of CVD coating and expands the channel between oxygen and substrate. Nevertheless, the weight gain is still limited at this stage as the main area of Al2O3 coating is dense and protective. Finally, the oxidation kinetics accelerate at 5–10 h when coating failure occurs and severe breakaway oxidation appears in tungsten.

Peak-fitting of Cu 2p photoemission spectra in Cu0, Cu1+, and Cu2+ oxides: A method for discriminating Cu0 from Cu1+

The photoemission spectra of Cu 2p are one of the most studied. Its analysis is especially challenging because, among others, the peaks for Cu0 and Cu1+ overlap, and Cu1+ and Cu2+ coexist under many conditions. The chemical state determination of metallic copper and the initial oxidation mechanism of copper require state-of-the-art peak fitting analysis and background modeling. This work presents a detailed and quantitative analysis of the Cu 2p photoemission spectra for a metallic ultra-thin film and the initial oxidation stages. Additionally, annealed films at 200 °C for up to 5 min were analyzed to assess the full oxidation of the copper oxides and assert the validity of the proposed peak-fitting procedures. For the initial oxidation, it was possible to accurately identify Cu1+ and Cu2+ states and, alongside, quantify the corresponding O 1s species, resulting in compositions close to stoichiometric values. Using the active background, it was possible to achieve close reproduction of all the photoemission spectra; three coexisting copper species were resolved during the fitting procedure. An accurate peak-fitting analysis allowed for quantifying the relative proportion of Cu1+ to Cu0 states despite their strong overlap.

Effect of Cr/Ti/Al Elements on High Temperature Oxidation Behavior of a Ni-Based Superalloy with Thermal Exposure

High-temperature oxidation of a Ni-based superalloy was analyzed with samples taken from gas turbine blades, where the samples were heat-treated and thermally exposed. The effect of Cr/Ti/Al elements in the alloy on high temperature oxidation was investigated using an optical microscope, SEM/EDS, and TEM. A high-Cr/high-Ti oxide layer was formed on the blade surface under the heat-treated state considered to be the initial stage of high-temperature oxidation. In addition, a PFZ (γ’ precipitate free zone) accompanied by Cr carbide of Cr23C6 and high Cr-Co phase as a kind of TCP precipitation was formed under the surface layer. Pits of several µm depth containing high-Al content oxide was observed at the boundary between the oxide layer and PFZ. However, high temperature oxidation formed on the thermally exposed blade surface consisted of the following steps: ① Ti-oxide formation in the center of the oxide layer, ② Cr-oxide formation surrounding the inner oxide layer, and ③ Al-oxide formation in the pits directly under the Cr oxide layer. It is estimated that the Cr content of Ni-based superalloys improves the oxidation resistance of the alloy by forming dense oxide layer, but produced the σ or µ phase of TCP precipitation with the high-Cr component resulting in material brittleness.

Thermodynamic Study on Initial Oxidation Behavior of TiAl-Nb Alloys at High Temperature

The initial oxidation behavior of TiAl-Nb alloys was systematically investigated against the composition, temperature, and partial pressure of O2 with the CALculation of PHAse Diagrams (CALPHAD) technique. The mole fraction of each oxidation product at the initial oxidation stage of the alloys at the corresponding temperatures was predicted. The initial oxidation products of the alloys are oxides of Al, Ti, and Nb, with the oxidation order of Al, Ti, and Nb. As P(O2) increases, the Ti and Nb oxidation products move towards a high oxygen content, and the mole fractions of the Al and Ti oxides gradually decrease and increase, respectively. It was found that the temperature and partial pressure of O2 determined the types of the oxides and the oxidation order, while the concentration of Nb and Al determined the mole fraction of the oxides. The CALPHAD results are in good agreement with the experiments.

Study of the Change Laws of Free Radicals and Functional Groups during Coal Oxidation.

In order to study the change laws of free radicals and functional groups during low-temperature coal oxidation, three coal samples with different metamorphic degrees were selected for ESR and FTIR analysis. The results showed that the concentration of free radicals increased as the temperature increased; meanwhile, the types of free radicals changed constantly, and the free radical variation range decreased with the increase in coal metamorphism. The side chains of aliphatic hydrocarbons in coal with a low metamorphic degree decreased by varying amounts in the initial heating stage. The -OH content of bituminous coal and lignite increased first and then decreased, while that in anthracite decreased first and then increased. In the initial oxidation stage, -COOH first increased rapidly, then decreased rapidly, and then increased before finally decreasing. The content of -C=O in bituminous coal and lignite increased in the initial stage of oxidation. Through gray relational analysis, it was found that there was a significant relationship between free radicals and functional groups, and -OH had the strongest correlation with free radicals. This paper provides a theoretical basis for studying the mechanism of functional groups transforming into free radicals in the process of coal spontaneous combustion.

Effect of boron carbide reinforcments on the PEO process of B4C/Al matrix composite

Plasma electrolytic oxidation (PEO) is an effective way to improve the thermal control property and service lifetime of the B4C/aluminum matrix composites (B4C/AMC) used in nuclear power field. The evolution process of B4C particles during the growth of PEO film was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) morphology observation, secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) chemical composition analysis. The results show that the B4C particles keep intact status but inhibit spark discharge in the initial oxidation stage, and then a few B4C particles are partly oxidized to small size and mixed into the film. After that, plenty of B4C particles are oxidized to B2O3 and solid C which escapes from the film as carbon oxide gas under strong spark discharge. The B4C particles disrupt the continuity of the film due to lack of sparking on their surface at the early stage of oxidation, but the film compactness is improved with decreasing B4C particles in the film under high oxidation voltage.

Microstructure changes and reaction process of cotton at low-temperature oxidation stage

Low-temperature oxidation is an important stage before spontaneous combustion of cotton. It is of great significance to explore the changes of cotton microstructure during low-temperature oxidation to prevent cotton fire. The typical long-staple cotton and unginned cotton were selected for pre-oxidation treatment. The oxidation characteristics and apparent activation energy of cottons at different oxidation temperatures were studied by TG and DSC measurement. The micro characteristics of cottons were studied by SEM, X-ray diffraction and FTIR. The results showed that low-temperature oxidation stage of cotton could be divided into initial pre-heating stage (60 °C ∼ 90 °C) and rapid oxidation stage (120 °C ∼ 210 °C). The cotton was mainly in the physical adsorption stage below 90 °C, the apparent activation energy of cotton was positive, and the content of various functional groups did not change significantly. At 90 °C ∼ 210 °C, the cotton was mainly at the stage of chemical adsorption, the apparent activation energy was negative, and the content of various functional groups changed significantly. 120 °C could be regarded as the active temperature point, which the average diameter and crystallinity of long-staple cotton and unginned cotton were the largest. At 120 °C, cellulose macromolecules began to depolymerize. The glucose ring of cellulose broke to form l-glucose, which was then broken down into CO2 and water. Further, the reaction process of cotton at low-temperature oxidation stage was proposed.

Study on oxidation kinetics of low-rank coal during the spontaneous combustion latency

Spontaneous combustion latency is the initial stage of low-temperature oxidation of coal. This stage provides the original heat for coal spontaneous combustion, which has great research value. However, the oxidation process at this stage is difficult to obtain due to the low oxidation reaction rate of coal in the spontaneous combustion latency. Therefore, we designed a closed oxygen consumption experimental device. This device can realize automatic monitoring and data acquisition of the experimental gas concentration. This device was used to carry out isothermal oxidation experiments on coal at different temperatures and oxidation times. The reaction order and reaction rate constant of the kinetics of the main stage of isothermal oxidation of coal in spontaneous combustion latency are mainly studied. The key active groups that affect the reaction rate constant are discussed by the corresponding Fourier transform infrared spectroscopy experiments. The results show that the reaction order is close to 1 at 30 °C. Then, the reaction order showed an overall downward trend with increasing temperature. The reaction rate constant increases with increasing temperature. As the oxidation times increases, the reaction rate constant decreases gradually, and aliphatic groups show a decreasing trend. Aliphatic structures are key active groups that affect the reaction rate constant during the spontaneous combustion latency. This study helps to improve the understanding of the initial stage of low-temperature oxidation of coal and guide the prevention of coal spontaneous combustion.

The influence of oxygen partial pressure on the base oxide of chromia forming steels: The story prior to breakaway oxidation

The influence of pO2 on the base oxide formed on 304 L stainless steel has been investigated at 600 °C. The alloy was exposed in 5%O2-95%N2 and 10%H2-20%H2O-Ar atmospheres and the initial stages of oxidation were analysed by TEM and EDX. In both environments, the thin scale consists of a Cr-rich oxide overlaid by a FeCrMn oxide. However, the subscale formed in H2-H2O is richer in Cr compared to the 5%O2-95%N2 case (∼90 cation% and ∼70 cation% respectively). The findings are in good agreement with thermodynamic calculations and can explain breakaway oxidation of marginal chromia forming steels in H2-H2O.

DFT investigation of the dissolution trends of NiTi alloys with the B2 and B19' phases during the initial oxidation stage.

The selective corrosion of NiTi alloys was studied using density functional theory calculations, and the dissolution trends of the NiTi-B2 and NiTi-B19' phases in the initial oxidation stage were compared to predict their corrosion difference. The dissolution process of Ni and Ti was simulated by creating Ni or Ti vacancies on the unoxidized and oxidized NiTi alloy surfaces. The results show that the surface vacancy formation energy of Ti vacancies is higher than that of Ni vacancies, indicating that Ti is more difficult to dissolve than Ni. Furthermore, oxidation promotes and impedes the dissolution of Ni and Ti, respectively. This study improves the fundamental understanding of the corrosion mechanism of NiTi alloys.

Initial stage of titanium oxidation in Ti/CuO thermites: a molecular dynamics study using ReaxFF forcefields.

The paper elucidates the main driving mechanisms at play during the early stage of the Ti/CuO thermite reaction using reactive forcefields in the frame of molecular dynamics calculations. Results show that TiO preferentially forms in immediate contact to pure Ti at temperatures as low as 200 K rather than TiO2. Increasing the temperature to 700 K, the 2 nm TiO2 in contact to Ti is found to be homogeneously depleted from half of its oxygen atoms. Also, the first signs of CuO decomposition are observed at 600 K, in correlation with the impoverishment in oxygen atom reaching the titanium oxide layer immediately in contact to CuO. Further quantification of the oxygen and titanium mass transport at temperatures above 700 K suggests that mostly oxygen atoms migrate from and across the titanium oxide interfacial layer to further react with the metallic titanium fuel reservoir. This scenario is opposed to the one of the Al/CuO system, for which the mass transport is dominated by the Al fuel diffusion across alumina. Further comparison of both thermites sheds light on the enhanced reactivity of the Ti-based thermite, for which CuO decomposition is promoted at lower temperature, and offers a novel understanding of thermite initiation at large.

The role of microstructure on high‐temperature oxidation behavior of hafnium carbide

AbstractMicrostructure development of the products formed upon oxidation of hafnium carbide (HfCx, x = 0.65, 0.81, or 0.94) at 1300°C and 0.8 mbar oxygen pressure was investigated using Raman spectroscopy, X‐ray diffraction, electron microscopy, and electron energy‐loss spectroscopy. For all specimens a multilayered oxide scale was observed featuring an outermost porous hafnia layer and an interlayer adjacent to the parent carbide containing hafnia interspersed with carbon. The outermost hafnia features coarse pores presumably formed during initial stages of oxidation to allow rapidly evolving gaseous products to escape from the oxidation front. As the oxidation scale thickens, diffusional resistance results in slower oxidation rates and smaller quantities of gaseous products that are removed via networks of increasingly fine pores until the local oxygen partial pressure is sufficiently low to selectively oxidize the parent carbide. Electron microscopy studies suggest that the oxidation sequence at this stage begins with the transformation of parent carbide to an amorphous material having empirical formula HfO2Cx that subsequently phase separates into hafnia and carbon domains. Hafnia polymorphs in the phase‐separated region vary from cubic to monoclinic as grains coarsen from ca. 2–20 nm, respectively. Immediately adjacent to the phase‐separated region is carbon‐free mesoporous hafnia whose pore morphology is inherited from that of prior carbon domains. The average pore size and pore volume fraction observed in mesoporous hafnia are consistent with predictions from kinetic models that ascribe gaseous diffusion through a pore network as the rate determining step in oxidation behavior of hafnium carbide. These observations imply that high‐temperature oxidation behavior of hafnium carbide under the employed test conditions is linked to microstructure development via phase separation and coarsening behaviors of an initially formed amorphous HfO2Cx product.