Effect Of Pre-oxidation Research Articles

Effect of pre-oxidation on cyclic oxidation resistance of γ-TiAl at 900 °C

The paper presents the effect of pre-oxidation on the high temperature oxidation behavior of γ-TiAl alloy. The pre-oxidation treatments were performed at 900 °C for 2 h under various O and Ar mixtures, which resulted in the formation of very thin (200−300 nm) oxide scales and oxygen enriched subsurface layers, containing the α2-Ti3Al(O) and the Ti5Al3O2 Z-phases. It was revealed that the pre-oxidation treatment can lower the oxidation rate of the alloy up to 300 h. Detailed high resolution STEM investigations were performed to characterize the oxide scales and metal-scale interfaces.

ClO2 pre-oxidation impacts the formation and nitrogen origins of dichloroacetonitrile and dichloroacetamide during subsequent chloramination

Chlorine dioxide (ClO2) can be used as a pre-oxidant when chloramination is performed in water treatment plants. However, the effects of ClO2 pre-oxidation on the formation of nitrogenous disinfection by-products, such as dichloroacetonitrile (DCAN) and dichloroacetamide (DCAcAm), during chloramination are not well understood. In this study, the effects of ClO2 pre-oxidation on the formation of DCAN and DCAcAm during chloramination of 28 model compounds and seven real water samples were investigated. The sources of nitrogen for DCAN and DCAcAm formation were investigated using 15N-labeled monochloramine. ClO2 pre-oxidation affected DCAN and DCAcAm formation during chloramination of model compounds in different ways. ClO2 pre-oxidation increased unlabeled and 15N-labeled DCAN and DCAcAm formation during chloramination of six amino acids and peptides and five indoles and tertiary amines. ClO2 pre-oxidation decreased DCAN formation but increased DCAcAm formation during chloramination of three hydroxybenzamide compounds, but had the opposite effects for four tetracyclines. ClO2 pre-oxidation generally decreased DCAN and DCAcAm formation during chloramination of the phenolic compounds that are precursors not containing nitrogen. 2-Aminoacetophenone, formamid-trans-muconic acid, and unsaturated ketones were found to be transformation products of ClO2 oxidation of 3-methylindole, salicylamide, and resorcinol, respectively. Possible DCAN and DCAcAm formation pathways during chloramination after ClO2 oxidation were identified. For most of the water samples, ClO2 pre-oxidation decreased the amounts of DCAN and DCAcAm formed during chloramination by 36%–70% and 11%–59%, respectively. This may have been caused by ClO2 oxidation destroying phenolic precursors and macromolecular proteins rather than amino acids in the water samples.

Oxidation of Nickel-Coated AISI 430 Alloy: Effect of Pre-oxidation and Fe0.5Ni0.5 Inter-diffusion Layer

The oxidation behavior of nickel (Ni)-coated AISI 430 alloy was investigated at 800 °C in moisture-saturated (~ 3% H2O) air. Effects of pre-oxidation of AISI 430 in air and inter-diffusion layer (Fe0.5Ni0.5) of Ni-coated AISI 430, in dilute hydrogen (Ar–3%H2) at 800 °C, on the oxidation behavior were also studied. Microstructure, elemental chemistry, and compound/oxide formation across the reaction zones/oxide layer were analyzed by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction techniques. Multilayered oxides/reaction zones were found for all the samples. Ni-coated AISI 430 exhibits the lowest chromium diffusion into the oxide scale from the AISI 430/oxide scale interface. Pre-oxidation of AISI 430 and inter-diffusion of Ni-coated AISI 430 show excessive chromium diffusion into the reaction zone/oxide scale and interfacial porosity.

Characterization of Low-Pressure Nitrogen Plasma Discharge and Pre-Oxidation on the Surface Characteristics of Activated Carbons

Commercial activated carbon (CAC) was modified using low-pressure radio frequency nitrogen plasma discharge (NPD) operating at 0.3 mbar and 40 kHz. The surface chemistry of CAC was modified using HNO3 pre-oxidation to possibly influence the reactivity of NPD. The results of x-ray photoelectron spectroscopy (XPS) suggested that pre-oxidation reduces aromaticity, generates aliphatic carbons (C-C and C-H), and increases carboxylic functional groups (COOH) which probably enhances the nitrogen plasma functionalization based on the N/C ratio for CAC-O-P (4.29 %) compared to CAC-P (2.88 %). FTIR was used to confirm such effects of pre-oxidation from the functional groups present on the carbon surface. The total surface area was identified using Langmuir and Brunauer–Emmett–Teller (BET) N2 adsorption isotherms at 77 K. Both pre-oxidation and plasma treatment caused an increase in the surface area of CAC up to 150 percent. Carbon t-plot method was used to determine the micropore volume, micropore area, and external surface area. The total surface area of each activated carbon was mostly constituted of micropore area which was identified to be directly proportional to the micropore volume. Scanning electron microscope (SEM) confirms the destruction of the surface morphology for CAC-O that might have caused the increase in surface area. Development of surface threadlike structures were observed for the NPD treated CAC-O. NPD favors the development of NH2 functionalities and reduces the aromaticity of activated carbons while enhancing the surface morphology and the surface area.

Effect of O-containing functional groups produced by preoxidation on Zhundong coal gasification

Low-temperature preoxidation is being evaluated as a preprocessing technique in gasification, but the action mechanism of preoxidation is not yet well understood. The influence mechanism of O-containing functional groups formed by preoxidation on Zhundong coal gasification is detailed in this study. The effect of preoxidation on the gasification kinetics was studied using a micro-fluidized-bed reaction analyzer. Graphite oxide was used as a model compound to simulate coal. Chemical characterization and quantum chemistry calculations were combined to analyze the mechanism of action of preoxidation in gasification. The O-containing functional groups formed during preoxidation can significantly enhance the gasification reactivity and reduce the activation energy by 32%. Experiments with coal samples with a single metal added after pickling revealed that these O-containing functional groups facilitate the efficacy of metal catalysts, particularly for the easily deactivated chloride metal salts in Zhundong coal. The results of experiments controlling the O-containing functional groups on graphite, along with density functional theory calculations, indicated that the carbonyl group was most likely responsible for facilitating catalytic gasification. The carbonyl group can combine with the metal to form more catalytic active C–O–M centers, which strengthen the link between the metal and carbon matrix, thus promoting catalytic gasification.

Effect of particle pre-oxidation on Ni and Ni20Cr splat formation during plasma spraying

During plasma spraying in air, molten metallic droplets can develop a surface oxide shell which is postulated to play a significant role on the subsequent impact and spreading on the substrate surface resulting in splat formation. In this work, oxide layers on Ni and Ni20Cr particles were formed through isothermal pre-oxidation prior to spraying and their influence on droplet spreading and splat-substrate interfacial interactions was investigated. The results showed that the presence of this oxide shell readily induced finger splashing, irrespective of the oxide shell thickness. In addition, small oxide nodules were frequently detected inside the splats. An oxygen-enriched interfacial interlayer was observed for the pre-oxidized-Ni splats. A mechanism is proposed to account for the inclusion of oxide fragments from the particle surface oxide shell, in the deposition of single plasma-sprayed Ni and Ni20Cr splats.

Improving the Bonding Strength of Nb–C/C Composite Brazed Structures by Preoxidation

Preoxidation treatment is used to modify the surface of C/C composite. An annular column gap is formed encasing the carbon fiber, which promotes the infiltration of C/C composite by liquid brazing alloy during brazing. Based on the optimized preoxidation condition of 800 °C/5 min, the effects of brazing temperature (860–940 °C) and holding time (5–20 min) on the microstructure and mechanical properties of Nb and C/C joints brazed with an AgCuTi alloy are systematically investigated. The shear strength of the brazed structure at 880 °C for 10 min reaches a maximum of 57 MPa, which is increased by 28 MPa compared with that of the structure without preoxidation. The infiltration depth, reaction layer thickness, Ti–Cu compound amount, and brazing seam width are characterized to reveal the synergistic effect of preoxidation and brazing parameters on the bonding strength of structures. The selection of an appropriate brazing parameter is conducive to maximizing the positive effects of preoxidation.

Effect of pre-oxidation on surface scale microstructure and electrical property of Cu-Fe coated steel interconnect

Pre-oxidation is introduced to optimize the performance of the Cu-Fe coated steel as interconnect for solid oxide fuel cell. Oxidation kinetics, microstructure and area specific resistance (ASR) are investigated. Pre-oxidation effectively improves the oxidation resistance of Cu-Fe coated steel, and its influence of surface scale microstructure is significant. Pre-oxidation not only inhibits the growth of the Cr2O3 layer and decreases the contents of Cr and Mn in Cu-Fe conversion oxide layer, but also stabilizes the tetragonal CuFe2O4 and CuO on scale surface. Moreover, the scale ASRs of the Cu-Fe coated steels are clearly reduced by pre-oxidation.

Effect of powder pre-oxidation on the microstructure and ablation resistance of ZrB2–SiC coating

Abstract To further improve the densification and properties of ZrB2–SiC coating, the ZrB2–SiC powder was pre-oxidized at different temperatures before plasma spraying. And the ablation property of the obtained coatings were assessed using an oxyacetylene torch flame. A core-shell structure powder was obtained after the pre-oxidation treatment at 800 °C. And the oxidized shell was dense and integral with an uniform thickness. Compared with the IPS-coating, the porosity percent of the corresponding pre-oxidized coating showing a classical lamellar structure decreased from 12.7% to 8.8%. The pre-oxidized coating showed a dense ZrO2 surface with a honeycomb-structure after ablation at 1800 °C for 60s.

Effects of Pre-Oxidation on Haloacetonitrile and Trichloronitromethane Formation during Subsequent Chlorination of Nitrogenous Organic Compounds.

The reaction between organic matter and disinfectants leads to the formation of disinfection byproducts (DBPs) in drinking water. With the improvement of detection technology and in-depth research, more than 1000 kinds of DBPs have been detected in drinking water. Nitrogenous DBPs (N-DBPs) are more genotoxic and cytotoxic than the regulated DBPs. The main methods are enhanced coagulation, pretreatment, and depth technologies which based are on conventional technology. Amino acids (AAs) are widely found in surface waters and play an important role by providing precursors from which toxic nitrogenous disinfection by-products (N-DBPs) are generated in chlorinated drinking water. The formation of N-DBPs, including dichloroacetonitrile, trichloroacetonitrile, and trichloronitromethane (TCNM), was investigated by analyzing chlorinated water using ozone (OZ), permanganate (PM), and ferrate (Fe(VI)) pre-oxidation processes. This paper has considered the control of pre-oxidation over N-DBPs formation of AAs, OZ, PM, and Fe(VI) pre-oxidation reduced the haloacetonitrile formation in the downstream chlorination. PM pre-oxidation decreased the TCNM formation during the subsequent chlorination, while Fe(VI) pre-oxidation had no significant influence on the TCNM formation, and OZ pre-oxidation increased the formation. OZ pre-oxidation formed the lowest degree of bromine substitution during subsequent chlorination of aspartic acid in the presence of bromide. Among the three oxidants, PM pre-oxidation was expected to be the best choice for reducing the estimated genotoxicity and cytotoxicity of the sum of the measured haloacetonitriles (HANs) and TCNM without bromide. Fe(VI) pre-oxidation had the best performance in the presence of bromide.

Passivation mechanisms and pre-oxidation effects on model surfaces of FeCrNi austenitic stainless steel

Passivation mechanisms were investigated on (100)-oriented Fe-18Cr-13Ni surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and scanning tunneling microscopy and electrochemical characterization. Starting from oxide-free surfaces, pre-oxidation at saturation under ultra-low pressure (ULP) oxygen markedly promotes the oxide film Cr(III) enrichment and hinders/delays subsequent iron oxidation in water-containing environment. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species. Anodic passivation forces oxide film re-growth, Cr(III) dehydroxylation and further enrichment. ULP pre-oxidation promotes Cr(III) hydroxide formation at open circuit potential, compactness of the nanogranular oxide film and corrosion protection.

Pre-oxidation effects on properties of bismuth telluride thermoelectric composites compacted by spark plasma sintering

ABSTRACTResearch on harvesting alternative energy sources is of interest to meet human demands for energy while reducing environmental pollution caused by the extensive use of fossil fuels. Thermoelectric materials are a promising technology for converting heat into electricity. Among thermoelectric materials, the binary bismuth telluride system (Bi-Te) is widely used. To produce high-quality Bi-Te systems with low materials consumption, spark plasma sintering (SPS) is commonly applied. Because SPS is a fast low-temperature process, controlling the ratio and crystallization of Bi-Te is critical for effective energy conversion. Here, we investigated the quality of Bi-Te systems formed by SPS compaction of raw powders with an in-depth examination of the oxidation effects on their thermoelectric performance. With increasing measurement temperature (300→420 K), the mechanically mixed sample and a commercial Bi2Te3 alloy (sintered at 533 K) showed differences in Seebeck coefficients (0.245→0.267 and 0.223→0.246 mVK−1, respectively). The alloy sample showed a decreased figure of merit (0.863→0.331) while that of the mechanically mixed sample (0.543→1.671) increased with temperature. This was related to the degree of oxygen impurity in each Bi-Te process based on XPS analysis. This study proposes that the integration of oxygen species to Bi-Te can be considered to maximize the thermoelectric efficiency at the specified temperature.

Intensification of humic acid extraction from lignites

One of the methods of initial properties of solid fuels changing by their organic mass oxidation is considered in the paper. Creation of innovative ways of intensification of existing solid fuels processing and their adoption by industry has always been and remains an actual task. Thus the chemical process is the most important stage of raw materials processing into target products. Chemical exposure allows to optimize the technological mode, expanding the scope of control of technological process parameters and, to a certain extent, modify the properties of the resulting products. The article presents the results of experimental research of influence of preliminary oxidation on the lignite organic mass from Kharanorsk and Kangalas deposits by different chemical reagents to determine a degree of this process impact on the efficiency of their processing into humic substances. Inorganic and organic oxidizers of different concentrations were used as reagents. The greatest effect for increasing the humic acids yield was observed when using 6-10 % hydrogen peroxide for oxidation, and 10 % hydrochloric acid. The results of experiments on the coal preoxidation effect can be used as a basis for the development of a new method of lignites processing into humic substances.

Oxidation behavior of 9Cr-4.5Al ODS steel in 600 °C supercritical water and the effect of pre-oxidation

The oxidation behavior of a 9Cr-4.5Al ODS steel in 600℃ supercritical water with 200 ppb dissolved oxygen was investigated with and without a pre-oxidation heat treatment. In general, results suggest that all samples follow the “available space model” oxidation mechanism as exposure time is extended. Based on the microstructural analysis, it is found that a thin alumina layer formed by pre-oxidation treatment is initially protective up to approximately 100 h. However, this passive film becomes ineffective due to the existence of M23C6 (M = Cr, Fe) and the insufficient content of Al or/and Cr in the steel.

Evaluating the effects of the preoxidation of H2O2, NaClO, and KMnO4 and reflocculation on the dewaterability of sewage sludge

The preoxidation effects of H2O2, NaClO, and KMnO4 on the dewaterability of sewage sludge were compared by analyzing the changes in specific resistance to filtration (SRF), filter cake moisture content (FCMC), extracellular polymeric substance (EPS) fractions and components, and floc properties. The three oxidants varied in oxidation efficiency and exhibited distinctive mechanisms. NaClO not only destroyed sludge flocs and EPSs but also effectively caused cell lysis, resulting in release of a considerable amount of organic matters and subsequently significant deterioration of dewatering performance. The oxidation of H2O2 and KMnO4 was relatively mild and occurred mainly on the outer layers of the sludge flocs and cell-bound EPSs. By contrast, the SRF and FCMC of the sludge conditioned with a low dose of KMnO4 were slightly improved, and a fraction of soluble EPS was compressed because of the coagulation effect of the oxidation product MnO2. The pH of the sludge conditioned with H2O2 and KMnO4 showed no considerable change. Meanwhile, NaClO evidently increased the alkalinity of the sludge because of the hydrolysis effect. After the pH of the NaClO-treated sludge was readjusted to 7.0, the partial protonation efficiency slightly alleviated the deterioration of sludge dewatering performance. The preoxidized sludge was then subjected to reflocculation treatment using FeCl3, polyacrylamide, and a cationic starch-based flocculant, respectively. The combined treatment of preoxidation and reflocculation showed a high dewatering efficiency owing to their synergistic effect.

The effect of pre-oxidation on microstructure, mechanical properties and high-temperature tribological behaviors of HVOF-sprayed NiCoCrAlYTa coating

Abstract The HVOF-sprayed NiCoCrAlYTa coating would generate a dense oxidative layer under high temperature so that the microstructure, chemical component, mechanical properties and high-temperature tribological behaviors of the coatings surface were changed significantly. The as-sprayed NiCoCrAlYTa coating is mainly composed of the phases of γ-Ni and β-NiAl as well as γ′-Ni3Al. The Al-rich phase of β-NiAl in the post-oxidized coating has disappeared, replaced by the emergence of α-Al2O3. The formation of the oxidative layer with α-Al2O3 columnar crystal could greatly improve the mechanical properties of the NiCoCrAlYTa coating. Along with the elevated temperature, the friction coefficient and wear rate of as-sprayed coating gradually decreased, and the primary mechanism changed from abrasive wear to adhesive wear. Although the friction coefficient of the post-oxidized coating is slightly higher than that of as-sprayed coating, the wear rate decreases, obviously. This is because the oxidative layer of post-oxidized coating exerts a great influence on the wear resistance, especially under the high temperature.

Reforming of coal volatiles over ilmenite ore

Chemical-looping combustion is one of the most promising technologies for generating electricity from coal with high CO2 capture. This process burns the fuel with the oxygen from metal oxide particles in the fuel reactor while the depleted oxygen in the metal oxides is compensated for in the air reactor.This study investigated the performance of ilmenite as an oxygen carrier during the steam reforming when actual coal volatiles are used as fuel. Steam reforming experiments on coal volatiles with ilmenite were carried out in a two-stage fixed-bed reactor under various steam conditions. The effect of pre-oxidation and of steam ratios on the conversion reactivity of ilmenite was investigated at 1173 K. The conversion reactivity of ilmenite was greatly enhanced after pre-oxidation, and there was no tar formation when pre-oxidized ilmenite was used. The internal and external porosity of ilmenite also increased after steam reforming of coal volatiles.Ilmenite showed good performance under the various steam conditions. Carbon formation decreased considerably as steam ratios increased. Moreover, the reduced phase FeTiO3 of ilmenite was maintained with the addition of steam, and its reactivity was relatively stable compared with a synthetic iron-based carrier, Fe2O3/Al2O3.

Comparison of ferrate and ozone pre-oxidation on disinfection byproduct formation from chlorination and chloramination

This study investigated the effects of ferrate and ozone pre-oxidation on disinfection byproduct (DBP) formation from subsequent chlorination or chloramination. Two natural waters were treated at bench-scale under various scenarios (chlorine, chloramine, each with ferrate pre-oxidation, and each with pre-ozonation). The formation of brominated and iodinated DBPs in fortified natural waters was assessed. Results indicated ferrate and ozone pre-oxidation were comparable at molar equivalent doses for most DBPs. A net decrease in trihalomethanes (including iodinated forms), haloacetic acids (HAAs), dihaloacetonitrile, total organic chlorine, and total organic iodine was found with both pre-oxidants as compared to chlorination only. An increase in chloropicrin and minor changes in total organic bromine yield were caused by both pre-oxidants compared to chlorination only. However, ozone led to higher haloketone and chloropicrin formation potentials than ferrate. The relative performance of ferrate versus ozone for DBP precursor removal was affected by water quality (e.g., nature of organic matter and bromide concentration) and oxidant dose, and varied by DBP species. Ferrate and ozone pre-oxidation also decreased DBP formation from chloramination under most conditions. However, some increases in THM and dihaloacetonitrile formation potentials were observed at elevated bromide levels.

Pre-oxidation as a Means to Increase Corrosion Resistance of Commercial Superheater Steels

This study addresses the effect of pre-oxidation on the corrosion resistance of the following three commercial steels: a low-alloyed ferritic 10CrMo9-10 steel; a Nb-stabilized austenitic AISI 347 steel; and a high-alloyed austenitic Sanicro 28 steel. Samples of these three steels were pre-oxidized under various conditions and then exposed to potassium chloride (KCl) for 168 h at 550 °C under dry conditions. In the case of the ferritic 10CrMo9-10 and the austenitic AISI 347 steels, despite differences in oxide scale composition and structure, their corrosion resistance could not be improved by any of the studied pre-oxidation procedures. In contrast, pre-oxidation conditions resulting in chromium-enriched oxides improved the corrosion resistance of the high-alloyed austenitic Sanicro 28 steel in terms of oxide scale growth during exposure to KCl.

Effect of preoxidation on the reduction of ilmenite concentrate powder by hydrogen

In this study, hydrogen reduction of raw and preoxidized ilmenite concentrates powder, with particle size less than 74 μm, is investigated using a thermal analyzer at 1123, 1173, and 1223 K in a 30% H2 + 70% Ar atmosphere. The reduction rate by hydrogen was found to be accelerated due to preoxidation treatment of the raw ilmenite concentrate. The reduction of both raw and preoxidized ilmenite concentrates can be divided into two stages: Fe3+→Fe2+ and Fe2+→Fe. The Fe3O4→FeO stage overlaps with the Fe2O3→Fe3O4 stage during reduction process of the preoxidized ilmenite concentrate. Moreover, the preoxidation treatment can effectively decrease the reduction activation energy. The scanning electron microscopy (SEM) and the X-ray powder diffraction (XRD) techniques were used to characterize the micromorphology and phase of the products. The promotion mechanism of reduction of ilmenite by hydrogen through preoxidation treatments is also discussed.