Increasing Oxidation Time Research Articles

Preparation of AlN–SiC bonded Al2O3–C refractory and its properties evolution after oxidation

The low carbon Al2O3–C refractory was prepared using tabular alumina aggregates and fines, Al2O3 micro-powder, Al powder, and Si powder as starting materials. The physical properties, hot modulus of rupture (HMOR), and thermal shock resistance (TSR) as well as oxidation behavior of samples before and after oxidation at 1500 °C for 3–9 h were investigated. Furthermore, the relationship between properties degradation due to oxidation and microstructure evolution was discussed. The results showed that the in-situ AlN and SiC whiskers forming network structure in Al2O3–C materials conferred good thermo-mechanical properties of the obtained composites. However, the materials were seriously oxidized at 1500 °C for 3–9 h, which resulted in a significant decrease in both physical properties and HMOR. The oxidation percent reached 86% after oxidation for 9 h, and HMOR decreased from 25.8 MPa to 10.5 MPa with increasing oxidation time from 0 to 9 h. However, the oxidized samples have excellent TSR and the residual strength ratio exceeds 90%, which is attributed to the formation of microcracks, mullite, and glass phase in the oxidized layer, and the residual non-oxide whiskers in the unoxidized layer.

Clean preparation of rutile from Ti-containing mixed molten slag by CO2 oxidation

Abstract The effects of SiO2 and CO2 on the crystallization action of Ti-containing mixed molten slag (molten Ti-containing blast furnace slag and molten Ti slag) were discussed by thermodynamic calculation and specific experiments. The results of thermodynamic calculation indicated that the increase of SiO2 addition mass and CO2 oxidation time can promote the transformation of anosovite and sphene to rutile. The experiment results showed that the phase composition of modification slag was only rutile under the SiO2 addition mass of 110 g and the CO2 oxidation time of 180 s. Moreover, the formation theory of rutile was investigated. Using CO2 as an oxidizing gas can not only prepare rutile but also achieve carbon neutrality, which is a clean preparation method.

The effect of aluminide coating on the steam oxidation behavior of SS321 steel at 700 °C

Steam oxidation is considered the main attack form involved in the destruction of superheater tubes in the superheater of water-tube boiler. In this work, the effect of an aluminide coating on the way steam reacts with SS321 steel in a superheater was studied. Aluminide coating was done by powder cementation at 800 °C for 7 h and then heat treatment at 900 °C for 1 h. The coating was made with an Al-rich Fe2Al5 phase, with an inner (diffusion) layer of 5 μm and an outer layer of 25 μm. The grains were all the same size, and there were few holes. The samples were subjected to a constant stream of supersaturated water vapor at a temperature of 700 °C. The weight gain of uncoated and coated samples was measured as 1 mg cm−2 and 0.5 mg cm−2 after 20 h, and 2.5 mg cm−2 and 0.7 mg cm−2 after 350 h, respectively. The remarkable weight loss of the coated samples after 20 h and up to 350 h was attributed to the formation of stable Al2O3 oxides. This was although in the uncoated samples, the outer and inner layers of the coating were composed of Fe-rich oxides (magnetite) and Cr-rich oxides (Cr-Fe spinel oxides), respectively. Microstructural studies showed that with the increase in oxidation time, the inner layer (diffusion) increases from 5 μm to 25 μm, which is attributed to the diffusion of substrate particles towards the coating during oxidation.

Steam oxidation of Cr-coated zirconium alloy claddings at 1200 °C: Kinetics transition and failure mechanism of Cr coatings

The microscopic layered structural evolution, the diffusion behavior of O, Cr and Zr elements and oxidation kinetics of the Cr-coated zirconium alloy claddings during steam oxidation at 1200 °C were systematically studied. At the initial oxidation stage (within 1 h oxidation), the protective Cr2O3 layer formed on the Cr coating, and the interdiffusion between Cr and Zr resulted in a four-layers structure from outer to inner, Cr2O3, Cr, Cr-Zr diffusion layer and Zr substrate. At this stage, the oxidation kinetics followed parabolic law. With increasing oxidation time to 1.8 h, ZrO2 networks formed in the Cr coating due to the occurrence of the redox reaction between Zr and Cr2O3, which provided a pathway for the diffusion of oxygen toward substrate. The layered structure on the Cr coating side transformed from a four-layers to a five-layers structure with the formation of a ZrO2 layer in the substrate. Meanwhile, the oxidation kinetics became linear growth. The failure of Cr coatings resulting in the oxidation of Zr substrate accounts for the parabolic-to-linear kinetics transition.

Impact of thermal oxidation parameters on micro-hardness and hot corrosion of Ti-6Al-3Mo-2Nb-2Sn-2Zr-1.5Cr alloy

Protective oxide layers on Ti-6Al-3Mo-2Nb-2Sn-2Zr-1.5Cr (TC21) alloy with equiaxed microstructure considerably influence micro-hardness and hot corrosion resistance. The present work’s thermal oxidation of TC21 alloy was performed at 600, 700, and 800 °C for 5, 20, and 50 h durations. Hot corrosion methods in NaCl and NaCl + Na2SO4 salt media were applied to raw (unoxidized) and oxidized samples at 600 and 800 °C for 50 h. Hot corrosion was conducted at 600 °C for 5 cycles with 10-h steps. The best oxide layer thickness was observed at 800 °C, which increased with increased oxidation time and temperature. The surface hardness of the oxide layer at 800 °C was 900 ± 60 HV0.05 owing to the formation of TiO2 and Al2O3 phases. Raw material hardness was 342 ± 20 HV0.05, increasing threefold due to thermal oxidation. In the case of NaCl, weight loss dominated all samples except at 800 °C for 5 h. In the case of NaCl + Na2SO4, weight gain occurred at 600 and 800 °C for 5 h. Weight loss occurred for the raw samples and those processed at 800 °C for 20 and 50 h, where the oxide layer flaked off. Surface hardness increased upon hot corrosion testing because of the formation of brittle phases, such as TiO2 and Na4Ti5O12. Samples that oxidized at 800 °C for 5 h had the highest hardness and corrosion resistance.

Impedance spectroscopy technique for estimating the oxidation of the PyC interphase layer in a SiCf/SiC ceramic matrix composite

ABSTRACT A new technique using impedance spectroscopy was developed to estimate the oxidation kinetics of the pyrolytic carbon interphase layer in SiCf/SiC ceramic matrix composites. The AC impedance Nyquist plot showed two semicircles that were curve-fitted using an equivalent circuit with two constant-phase element components. The analysis results revealed that the capacitance in the low-frequency region decreased with increasing oxidation time, whereas that in the high-frequency region remained relatively constant. This change in the capacitance in the low-frequency region was associated with the thickness of the oxidized SiO2 layer. This study might make a significant contribution to the literature, which has mostly focused on the characterization of ceramic matrix composites through measurements of mechanical properties. The limitations and potential applications of the developed method were also elucidated.

Effect of Electrochemical Pre-Oxidation for Mitigating Ultrafiltration Membrane Fouling Caused by Extracellular Organic Matter

Algal extracellular organic matter (EOM) will cause grievous membrane fouling during the filtration of algae-laden water; hence, boron-doped diamond (BDD) anodizing was selected as the pretreatment process before the ultrafiltration, and the EOM fouling mitigation mechanism and the purification efficiency were systematically investigated. The results showed that BDD oxidation could significantly alleviate the decline of membrane flux and reduce membrane fouling, and the effect was more notable with an increase in oxidation time. Less than 10% flux loss happened when oxidation duration was 100 min. The dominant fouling model was gradually replaced by standard blocking. BDD anodizing preferentially oxidizes hydrophobic organic matter and significantly reduces the DOC concentration in EOM. The effluent DOC was reduced to less than 1 mg/L when 100 min of BDD anodizing was applied. After the pre-oxidation of BDD, the zeta potential and interfacial free energy, including the cohesive and adhesive free energy, were all constantly increasing, which implied that the pollutants would agglomerate and deposit, and the repulsion between foulants and the ultrafiltration membrane was augmented with the extensive oxidation time. This further confirms the control of BDD on membrane fouling. In addition, the BDD anodizing coupled ultrafiltration process also showed excellent performance in removing disinfection by-product precursors.

Study on microstructure of rare earth Mg-9%Gd-2%Y-3%Nd -0.7%Zr alloy and properties of micro-arc oxidation films

The micro-arc oxidation protective film has been prepared on the surface of Mg-9%Gd -2%Y-3%Nd-0.7%Zr alloy by using the microarc oxidation technique of “step by step depressurization”. The microstructure of magnesium alloy substrate and micro-arc oxidation film were measured by OM, SEM and XRD, and the properties of the film were evaluated by electrochemical test of film thickness and surface morphology. The results show that the discontinuous network multiple eutectic phases are distributed in the grain boundary and there are dispersed-granular precipitates in the alloy matrix. After the micro-arc oxidation treatment, the thickness and corrosion resistance of the micro-arc oxidation film increased with the increase of oxidation time, and the surface micropore size increased first and then decreased, and the surface morphology of the micro-arc oxidation film changed from no micro-crack to micro-crack, and finally the micro-crack disappeared.

Study on Growth Mechanism and Characteristics of Zirconium Alloy Micro-Arc Oxidation Film

Ceramic coatings on R60705 zirconium alloy were prepared on the surface by the micro-arc oxidation (MAO) technique in electrolytes containing Na2SiO3, NaOH, and Na2EDTA. The growth behavior of the MAO ceramic coatings at different stages, including growth rate, microstructure, and phase composition, was investigated using the method of direct observation of the boundary area. The results showed that the growth of the MAO coatings on R60705 zirconium alloy occurred in both inward and outward directions. At an oxidation time of 5 min, the thickness of the oxidation layer increased fastest, reaching 103.43 μm, with a growth rate of 0.345 μm/s. After 5 min, the growth rate decreased and tended to level off around 15 min, with a thickness and growth rate of 162.7 μm and 0.181 μm/s, respectively. The total thickness of the coatings continuously increased throughout the process, with the outward growth thickness always higher than the inward growth thickness. The composition of the zirconium alloy micro-arc oxidation coatings mainly consisted of monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2), and a small amount of SiO2. The main elements in the coatings were Zr, O, and Si. The corrosion resistance of the zirconium alloy micro-arc oxidation coatings increased first and then decreased with increasing oxidation time, with a corrosion current density of 8.876 × 10−9 A·cm−2 at 15 min, indicating the best corrosion resistance.

Low-cost surface modification of a biomedical Zr-2.5Nb alloy fabricated by electron beam melting

• Thermal oxidation is an effective way to prepare oxide layer on a Zr-2.5Nb alloy fabricated by electron beam melting (EBM). • The morphology, phase constituents, and surface color of oxide layer change in the oxidation process. • Grain refinement significantly delays the breakaway oxidation. • A dense and black oxide layer with a hardness of 902 HV is prepared on a EBM- built Zr-2.5Nb alloy by oxidation at 600°C for 3 hours. The Zr-2.5Nb alloy with a fine microstructure consisting of α laths was successfully prepared by electron beam melting (EBM). The thermal oxidation behaviors and kinetics of the as-built, and the EBM-built and hot isostatically pressed (HIPed) Zr-2.5Nb materials in a temperature range of 450–600°C were investigated and compared with those of the alloy prepared by conventional casting and forging. It was found that the oxidation kinetics of the as-built and the forged materials followed the parabolic rate law during isothermal oxidation at 550°C, but the HIPed materials exhibited a parabolic-to-linear kinetic transition, suggesting that the larger grain sizes enhanced the oxidation. The oxide layers of all materials were composed of a large fraction of monoclinic zirconia phase ( m -ZrO 2 ) and a small fraction of tetragonal zirconia phase ( t -ZrO 2 ), and transformed from t -ZrO 2 to m -ZrO 2 with increasing oxidation time. The surface hardness of the as-built, the forged and the HIPed materials increased from 215, 204, and 188 HV before oxidation to 902, 1070, and 1137 HV after oxidation, respectively. The cross-sections of the materials showed the presence of micropores and microcracks inside the oxide layers with thicknesses ranging from 4 to 8 µm. With the oxidation temperature of 600°C and oxidation time duration of 3 h, a dense black m -ZrO 2 oxide layer with smooth surface and 902 HV hardness was obtained on the EBM as-built Zr-2.5Nb materials.

Growth mechanism of 2024 aluminum alloy micro-arc oxide layer in cobalt-containing electrolyte

The coatings were prepared on the surface of 2024 aluminum alloy by micro-arc oxidation. The effect of the addition of cobalt salt on the growth mechanism of the coating was investigated. The addition of cobalt salt prolonged the spark discharge phase of microarc oxidation and achieved the co-film formation of the solution component and the substrate. XRD, SEM, EDS, and XPS were used to investigate changes in the physical composition, surface morphology, and chemical composition of coatings prepared at different times. The hardness and corrosion resistance of the coatings were investigated as oxidation time increased. The surface of the coatings can be divided into two regions: the loose “cobalt-rich zone” and the dense “aluminum-rich zone”, and with the increase of oxidation time, the surface of the coating becomes more dense and flat, and the pores are mostly closed. The time-voltage curve shows that the growth of the micro-arc oxide coating in cobalt-containing electrolyte can be divided into three stages. The coating grows rapidly in the first 15 min, and a black coating is obtained, with the color-revealing substance being CoO; the coating turns from black to blue after 15 min, as CoO transforms to blue CoAl2O4. XPS and XRD analyses showed that the addition of cobalt salt significantly promoted the formation of alumina in the coating, and the conversion of CoO to CoAl2O4 and γ-Al2O3 to α-Al2O3 occurred at the later stage of micro-arc oxidation, when the hardness of the coating would obtain a large enhancement. The corrosion resistance test results showed that the coating prepared by treatment in the cobalt-containing electrolyte for 60 min showed the best corrosion resistance with a corrosion potential 100.3 mV higher than that of the substrate and a corrosion current reduced by 4 orders of magnitude to 6.20 × 10−9 A·cm−2.

Study the effect oxidation time on physical properties of cdo thin films prepared by thermal evaporated

thin films were prepared by thermal evaporated method of cadmium metal on a glass substrate, then conventional oxides in air at 500ᵒC and different oxidation times (30, 60 and 90) mint. The characterization of samples were carried out by atomic force microscopy and UV-VIS spectroscopy. Detailed optical and morphological properties of the prepared films are presented at different oxidation times. A UV-VIS transmition spectrum of produced films shows high transmission in visible, IR region and low transmission in UV region. Average grain size in the range of nanometer and decreases with increasing oxidation time.

Growth characteristics of scanning micro-arc oxidation coating on Ti6Al4V alloy

ABSTRACT In this study, the ceramic coating was prepared on Ti6Al4V alloy by scanning micro-arc oxidation (SMAO). The growth features of SMAO coating were investigated by combining the voltage and discharge variation. The result shows that a higher working voltage is obtained during SMAO treatment owing to additional resistance from the electrolyte column. As the SMAO goes on, the discharge region moves to the electrolyte column edge, exhibiting an annular shape. The current density with Gaussian distribution is observed between the stainless-steel tube (cathode) and titanium substrate (anode), which causes the SMAO coating to show a higher thickness in the middle and lower on both sides. Similar to traditional MAO, the SMAO coating has an enhancement in the thickness, pore size and Si content with increasing oxidation time, but coating degradation occurs in the later stage of SMAO. The SMAO coating mainly consists of amorphous SiO2, rutile and anatase phases.

Multi-scale structure characterization of ozone oxidized waxy rice starch

The elucidation of multi-scale structural variation and oxidation reaction mechanism of ozone oxidized waxy rice starch molecules remains a big challenge, limiting its development of intensive processing. In the present work, the changes in the structure of waxy rice starch after ozone treatment were systematically researched by various characterization methods. The study has shown that with the increase in ozone oxidation time, the granules of oxidized starch were polygons with multiple face depressions. It was also observed that ozone first attacked the amorphous zone of the starch granules and then penetrated the crystalline zone. Combining 1D and 2D NMR (1H NMR, 13C NMR, HSQC and HMBC) and other methods, it was proved that ozone oxidation led to ring splitting between C2 and C3 of the glucose unit. The resulting hemiacetal groups showed different types of structures. Among them, the main structures were intramolecular acetals and intermolecular hemiacetals. This research offered theoretical guidance for the utilization of ozone oxidation technology for starch modification and the development of waxy rice new foods.

Effect of coating on the oxidation behavior of turbine rotor material in supercritical water

AbstractHigh‐temperature oxidation behavior of IBN‐1 alloy, Ni–Cr thermally sprayed coating, Ni–Cr thermally sprayed coating with surface shot peening and grinding treatment in supercritical water have been investigated at 650°C under 25 MPa. The oxidation kinetics of IBN‐1 steel follows the parabolic law. Compared with substrate metals, the oxidation resistance of IBN‐1 steel with the three kinds of Ni–Cr coating treatment has been greatly improved due to the formation of Cr‐rich oxide and the oxidation kinetics deviated from the parabolic behavior seriously. The deviation may be attributed to differences in the oxide phase with an increase in oxidation time. Furthermore, the difference in oxidation rate among different Ni–Cr coating surface treatment processes can be observed. The related mechanism has been discussed.

Oxidation Behaviors of ZrB2–SiC Ceramics with Different Porosity

ZrB2–SiC ceramics have great application potential in thermal protection system of hypersonic aircraft, due to their excellent oxidation resistance and mechanical properties. Porosity is one of the main factors which will obviously affect the oxidation behaviors of ZrB2–SiC ceramics. Herein, ZrB2–SiC ceramics with three different porosity are sintered by spark plasma sintering at different temperatures. Then, a series of oxidation tests of the ceramics with different porosity are carried out at different temperatures and with different time. Morphologies and microstructures of the oxidation surface and cross section, as well as the oxide layer thickness and weight change are discussed in detail. Results show that the porosity of ZrB2–SiC ceramics has great influence on the oxidation behaviors of ceramics. The oxide layer thickness and weight change gradually increase with the increase of oxidation temperature and oxidation time for ZrB2–SiC ceramics with different porosity. As the porosity of ZrB2–SiC ceramics decreases, the oxide layer thickness and weight change generally decrease, which shows that the ceramics with lower porosity have stronger oxidation resistance. This work provides a relative systematic analysis of the effects of porosity on the oxidation behaviors of ZrB2–SiC ceramics.

Synthesis and kinetic analysis of green AA-type monomers using linoleic acid through bromide promoted transition metal-catalyzed oxidation with air

The present study explores the renewable route to synthesize AA-type monomers for condensation polymerization, namely adipic and azelaic acid, from linoleic acid (LA) using a bromide-promoted transition metal catalyst. For this, liquid-phase catalytic oxidation of LA is carried out using a catalyst system involving the acetates of Cobalt(II) and Manganese(II) and hydrogen bromide in an acetic acid green solvent at temperature: 393–423 K and a pressure of 5.8–8.8 bar in the air. The oxidation products are confirmed by a gas chromatography-mass spectrometer and carbon dioxide analyzer. At the early stage of LA oxidation (i.e.,<5.0 h), the identified major products are malonic acid, azelaic acid, and adipic acid, whereas malonic acid decomposes to carbon dioxide after increasing oxidation time. The influence of process variables, i.e., (i) the loading of LA, bromide, Cobalt(II), and Manganese(II), (ii) temperature, and (iii) pressure on LA conversion and yield of adipic acid and azelaic acid have also been carried out, and the loading of catalyst, pressure, and temperature plays a vital role on the LA conversion and product yields. The temperature-dependent kinetic parameters are also determined, and the rate-controlling step for LA oxidation is analyzed and further extended by evaluating the Hatta number and enhancement factor. The lower values of the Hatta numbers and enhancement factors suggest that the oxidation using air is controlled by the diffusion of oxygen and reaction in the film, and adequate gas-liquid mixing will improve the rate of LA oxidation.

Simultaneous analysis of the oxidation of solvent-extracted and cold-pressed green coffee oil during accelerated storage using 1H NMR and 13C NMR spectroscopy

Green coffee oil (GCO) extracted from green coffee beans, is known for its antioxidant and anticancer properties, and has been increasingly utilised in cosmetic and other consumer products. However, lipid oxidation of GCO fatty acid components during storage may be harmful to human health, and there remains a need to understand the evolution of GCO chemical component oxidation. In this study, proton nuclear magnetic resonance (1H and 13C NMR) spectroscopy was used to investigate the oxidation status of solvent-extracted and cold-pressed GCO under accelerated storage conditions. Results show that the signal intensity of oxidation products gradually increased with increasing oxidation time, while unsaturated fatty acid signals gradually weakened. Five different types of GCO extracts were clustered according to their properties, except for minor overlapping in the two-dimensional plane of the principal component analysis. Partial least squares-least analysis results demonstrate that oxidation products (δ = 7.8–10.3 ppm), unsaturated fatty acids (δ = 5.28–5.42 ppm), and linoleic acid (δ = 2.70–2.85 ppm) in 1H NMR can be used as characteristic indicators of GCO oxidation levels. Furthermore, the kinetics curves of unsaturated fatty acids, linoleic, and linolenic acyl groups all fit an exponential equation with high coefficients of GCO for 36 days under accelerated storage conditions. Our results show that the current NMR system is a fast, easy-operated and convenient tool for the oxidation process monitoring and quality control of GCO.

Effect of grain size on oxidation behaviour of Ag-20Cu-30Cr alloys in 0.1 MPa pure O2 at 700 and 800 °C

Abstract Bulk three-phase nanocrystalline (NC) Ag-20Cu-30Cr alloy was obtained by hot-pressing mechanically alloyed powders, and oxidation tests were completed in 0.1 MPa pure O2 at 700 and 800 °C. The oxidation behaviour of the alloy and the effect of the grain size were also studied in comparison with the previous coarse-grained (CG) Ag-20Cu-30Cr alloy prepared by a powder metallurgy route. At the two temperatures the oxidation kinetic curves of the NC Ag-20Cu-30Cr alloy are composed of three parabolic stages, and their parabolic rate constants become smaller as the oxidation time increases. The oxidation rate at 700 °C is higher before 5 h, but lower after 5 h than that at 800 °C. Furthermore, the oxidation rate of the NC Ag-20Cu-30Cr alloy is lower than that of the CG Ag-20Cu-30Cr alloy at the same temperature. Moreover, the NC Ag-20Cu-30Cr alloy forms an outer oxide layer composed of the Cu oxides and double oxides of the metallic Ag, Cu and Cr as well as an inner regular, continuous and protective chromia layer. Thus, the Ag-20Cu-30Cr alloy can complete the transition from an internal to external oxidation of the reactive component Cr, and in the end form a regular, continuous and protective chromia layer after nanocrystallization.

Effect of Pulse Duty Cycle and Oxidation Time on Microstructure and Properties of Micro-arc Oxidation Coatings on Ti6Al4V Alloy

Micro-arc oxidation (MAO) technology can in-suit generate coatings with high hardness and strong adhesion on the surface of titanium alloy, thereby greatly improve the application range of titanium alloy. The morphology and properties of coatings are controlled by changing electrical parameters such as duty cycle and oxidation time. This work mainly aims to study the influence of different duty cycles and oxidation times on MAO coatings microstructures and properties of Ti6Al4V alloy under the condition of constant voltage and pulse frequency. The scanning electron microscopy (SEM) and X-ray Diffractometer (XRD) were used to investigate into MAO coatings microstructure and phase composition. The roughness, porosity, thickness, and microhardness of MAO coatings were evaluated respectively by roughness tester, thickness gauge, and microhardness tester. The results show that surface morphology of the MAO coatings change from crater-like micropores to foam-like protrusions with the enhancement of duty cycle and oxidation time. When duty cycle is 50% and oxidation time is 30 min respectively, the ejection of the molten material made the MAO coating surface appear foam-like protrusions, and the surface morphology is seriously damaged. When the oxidation time and the duty cycle are 30 min and 10% respectively, the rutile phase intensity is the maximum, and further increasing the duty cycle will lead to a rutile phase decrease. In a limit range, the hardness rise with increase of oxidation time and duty cycle. The highest hardness is 668.85 HV.