جستجوی مقالات مرتبط با کلیدواژه « اکسیداسیون » در نشریات گروه « مواد و متالورژی »

As a result of their low weight and significant physical, mechanical and ablation behaviors, the carbon fabric composites and reinforced felt with ultrahigh temperature ceramics (UHTCs) have become the focus of aerospace industries. In this research, carbon fabric composites and resin have been used as matrix and the combination of ZrB2 76 wt%, ZrC 10 wt%, SiC 10 wt% powders as reinforcement and B4C 4% powder has been utilized as additive the mixture of mentioned powders with weight ratio of 25, 50, 75 precent was splashed on the carbon fabric and felt layers. for comparison, samples of 100% ceramic powder and 100% carbon fabric and felt were made as control samples. When the samples become dried up, they were pressed and sintered by pressureless sintering method. The results indicated that increasing the amount of powders led to samples density and shrinkage increase. The highest density is related to the sample without carbon fabric and felt with a relative density of 98.5. Also, the porosity decreased with the increase in the weight percentage of the powder, and the lowest porosity is related to the sample without carbon fabric and felt with a value of 0.5%. In this research, it was found that the lowest linear oxidation is related to the sample containing 75% ceramic powder in the carbon fabric group with a value of 3.17×10-2  (s ) in the felt group.   mm s )  and the sample containing 75% powder with a value of 3.61×10-2 ( mm.

In this research, transparent multi-layer graphene nanocoating was used to protect copper against oxidation at low temperature and the role of this coating on maintaining brightness and preventing copper color change was evaluated. For this purpose, multilayer graphene coatings were applied on copper substrate using chemical vapor deposition (CVD). The multilayer graphene coating before and after the oxidation test was characterized using Raman analysis, optical and scanning electron microscopes. In addition, the color changes in the CIE-L*a*b* color space and the reflectance percentage in the range of the visible spectrum were studied. The results showed that the transparent multilayer graphene coating can maintain the appearance properties of copper after thermal oxidation for up to 4 hours. The results of the present research show that the multilayer graphene coating is effective in protecting the oxidation and as a result maintaining the color of copper. In addition, the results of this research showed that graphene oxidation starts and spreads from the defects and graphene grains boundaries.

In this research, pure CoNiCrAlY and composite CoNiCrAlY-2, 4 wt% Al2O3 powders were deposited by the HVOF process on IN738 nickel-based superalloy substrates. Composite powders were produced using a water-based solution method and polyvinyl alcohol adhesive to distribute oxide nanoparticles on CoNiCrAlY microparticles. An oxidation test was performed on the coatings at 1050°C and 50, 100, 150, and 200 hours in the stationary air. The microstructure and phase composition of the coatings were investigated by SEM and XRD before and after the oxidation test. The results showed that nanoparticles are present with relatively uniform distribution around the powders and the microstructure of the corresponding coatings. Porosity and surface roughness of pure and composite coatings (2, 4 wt%) were measured as 0.6, 1, and 2.7 vol.% and 4.4, 4.8, and 7.1 µm, respectively. After 200 hours of oxidation test, the thickness of the oxide layer formed on pure and composite coatings (2, 4 wt%) was measured as 5.2, 4.3, and 5.9 micrometers, respectively. CoNiCrAlY coating containing 2 wt% of Al2O3 showed the best resistance to oxidation due to the creation of diffusion barriers by nanoparticles. In contrast, the composite coating containing 4 wt% of Al2O3 had the weakest resistance to oxidation, which was caused by the rough surface morphology and the occurrence of internal oxidation in it.

Among the applications of austenitic stainless steels, it can be mentioned the application in solid oxide fuel cells and boiler tubes. It is necessary to protect these steels at high temperatures. One of the best effective methods to increase the life of these steels at high temperature is to apply surface coatings. In this research, Ni-Co-CeO2-ZrO2 composite coating was fabricated by electroplating method on the surface of AISI 304 austenitic stainless steel. In order to investigate the high temperature corrosion resistance, isothermal oxidation tests at 800°C for 300 hours and cyclic oxidation at 800°C for 50 cycles were performed on uncoated and coated samples. Scanning electron microscope (SEM) was used to observe the morphology and X-ray diffraction (XRD) was used to determine the formed phases. The results showed that the formed coating at the current density of 15 mA/cm2, pH of 3, and a concentration of 25 g.l-1 of ZrO2 particles and a concentration of 10 g.l-1 of CeO2 particles was without holes and cracks. In isothermal and cyclic oxidation tests, the coated samples showed a lower weight gain than the uncoated samples due to the formation of NiFe2O4 and CoFe2O4 spinels. These spinels prevented the outward diffusion of chromium cations from and improved the oxidation resistance of the 304 steel substrate.

A new method has been presented for the synthesis of copper (Cu)/copper oxide (CuO)-nanoparticles (NPs), based on the process of corrosion and oxidation of Cu-NPs on the surface of the gold electrode by nitric acid. Cu-NPs were deposited on the surface using potentiometric method. The high concentration of Cu-NPs was estimated by Differential Pulse Voltammetry (DPV). The process of growth and distribution of CuO-NPs on the surface of Cu-NPs using structural analysis of Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) showed that nitrate was well absorbed and a sharp hydroxyl peak appeared and a phase of CuO NPs formed on the electrode surface. The surface morphology indicated that the average size reduced from about 150 nm to 50 nm in the presence of nitrate. This can be due to the oxidation of Cu nanoparticles on the surface and reduction of particle size compared to the absence of nitric acid. This simple and low-cost method can be used as a surface modification of antibacterial and active catalyst electrodes.

The aim of this study was to investigate the behavior of hot deformation and occurrence of restoration phenomena during the deformation of AISI H10 hot work tool steel. For this purpose, hot tensile test was performed on the steel in the temperature range of 900-1150 ºC with a temperature interval of 50 ºC and at a constant strain rate of 0.1s-1. The microstructures were examined and the curves of hot flow and ductility were drawn. According to the curves and microstructures, ductility was lower at temperatures of 900 ºC and 950 ºC due to inactivity of repair processes and the presence of carbides. Ductility increased in the temperature range of 1000-1100 ºC due to the occurrence of dynamic recrystallization. Finally, ductility decreased in the temperature of 1150 ºC due to the dissolution of carbide particles and grain growth. The results obtained from hot tensile test and microstructural studies at a constant strain rate of 0.1s-1 revealed that the appropriate temperature range for deformation of AISI H10 hot work tool steel was 1000-1100 ºC.

In this research, NiCrAlY powder was applied on steel, and Hastelloy X substrates with solid shielding shrouded plasma spray (SSPS) process and compared with atmospheric plasma spraying (APS). The high-temperature oxidation test was also performed on the coatings, and the microstructure of coatings was studied by optical microscopy (OM) and scanning electron microscopy (SEM). To investigate the influence of the SSPS process on the properties of metallic coatings, variable parameters; such as type of shroud gas (Ar, H2), the gas injection method (internal, external or simultaneous) and the flow rate of that, were examined. During the use of shroud gas, the temperature of the plasma jet has increased significantly. The oxidation test results showed the proper performance of NiCrAlY coating under the protection of argon internal shroud gas with a flow rate of 75SLPM, which was able to perform the best plasma flame protection during spraying. It can lead to a reduction in oxide and porosity of coating up to 8%. Also, the lowest thermally grown oxide (TGO) thickness was obtained for this sample after 200 hours of oxidation, indicating its excellent performance in maintaining the Al for the formation of the continuous α-Al2O3 layer during high-temperature oxidation.

In the present study, kinetic of oxidation process in Mg-0.15Al powders under non-isothermal condition was ‎studied to determine the empirical kinetic triplets (activation energy (E), pre-exponential factor (A), and ‎kinetic models of reaction (f(α))). In order to determine the mechanism of this process, the phase and ‎structural studies were done by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) ‎methods, respectively. The obtained results showed that magnesium oxide (MgO) and spinel (MgAl2O4) ‎phases were formed from the initial Mg and Al12Mg17 phases during the oxidation process. Microstructural ‎observation confirmed the phenomena occurring during oxidation by the evolution of morphology of ‎particles. Also, the isoconversional, invariant kinetic parameters (IKP) and fitting models were used to ‎investigate the kinetic of this process and to determine the kinetic parameters. The obtained results revealed ‎that this reaction is controlled by a nucleation and growth mechanism with the activation energy (Eα) in the ‎range of 150-320 kJ/mol.‎

In this study, three nickel-based superalloys consist of Colmonoy 6 ،Inconel 625 and Hastelloy C276 was coated by GTAW process method on 310 stainless steel. Evaluation of hardness, wear behavior and wear mechanism of samples was performed with Vickers, pin-on-disc method and using SEM images, respectively. The high temperature oxidation behavior was also studied at 1050°C in 8 cycles of 24hours. Also, to investigate corrosion behavior of samples, potentiodynamic polarization test was used in a solution of sulfuric acid and sodium chloride. The surface hardness results showed higher hardness of the Hastelloy C276 (727 HV) and Colmonoy 6 (612 HV) than the Inconel 625 (338 HV) and the steel substrate (217 HV). The best wear behavior was related to Hastelloy C276 with friction coefficient of 0.28. Surface morphology studies of the samples show a change in the wear-resistance mechanism from sliding to adhesive by applying nickel-base coatings on 310 stainless steel. In high temperature oxidation test, minimum weight gain was calculated for Hastelloy C276 (0.5mg/cm2). Corrosion test shows that fabrication of coatings improved 310 stainless steel corrosion resistance which maximum corrosion potential (-119/9 mV) and minimum corrosion current density (0.14 µA.cm-2) was owned by Hastelloy C276. Finally with considering the results, it seems that Hastelloy C276 be as the most desirable option with the aim of coating on 310 stainless steel in high-temperature conditions.

Oxidation resistance and electrical conductivity of stainless steels used as interconnects in solid oxide fuel cells can be improved by using a conductive and protective coating layer. In this study, AISI 304 austenitic stainless steel was coated in a cobalt powder base pack mixture. Isothermal oxidation test was used to investigate the oxidation resistance of coated and non-coated steel. Area specific resistance (ASR) was also measured as a function of oxidation time at 800 °C. Microstructure of coated and oxidized samples was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD) device. The results showed that cobalt coated samples showed better oxidation resistance than non-coated samples. The results showed that increasing the temperature would increase the area specific resistance. The cobalt coating layer also converted to CoFe2O4, Co3O4 and CoCr2O4 spinels during the isothermal oxidation process. Cobalt spinels exhibited less ASR (27.4 mΩ cm2) compared to the uncoated sample (60.5 mΩ cm2).

In this research, the effect of heating rate on oxidation kinetics of magnesium powder particles under non-isothermal conditions was studied. For this purpose, differential thermal analysis (DTA) and thermogravimetry analysis (TGA) was done on magnesium powder particles at three heating rates of 5, 10 and 20 K min-1 up to 1000 °C under air atmosphere. Also, in order to better understand the oxidation process of magnesium powder, three temperatures were selected according to the DTA curve at a heating rate of 20 K min-1. Then, samples of magnesium powder were heated up to these three temperatures with heating rate of 20 K min-1 and were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) for phase and microstructural analysis. Then, kinetic studies were performed using some isoconversional methods such as Starink and Friedman as well as direct and indirect fitting methods. The activation energy (E) and pre-exponential factor (lnA) for oxidation of magnesium powder were in the range of 327-956 kJ mol-1 and 45-135 min-1, respectively. The reaction models for heating rates of 5, 10 and 20 K min-1 were obtained to be A3/2, R2 and D1, respectively.

One of the most effective ways to improve oxidation resistance of interconnects used in solid oxide fuel cells (SOFCs) is to apply a layer of conductive protective coating. In this study, Crofer 22APU ferritic steel was coated in a titanium- based powder mixture by pack cementation method. The powder composition for titanium coating was Ti 20 wt.%, NH4Cl 5 wt.% (activator) and Al2O3 75 wt.%. The optimum temperature and time to obtain the best coating quality in terms of adhesion and porosity were 800 °C and 7 hours, respectivly. The obtained titanized coating consisted of TiFe, TiFe2 and TiCr2 phases. The results of isothermal and cyclic oxidation tests carried out at 900 °C, showed that titanium-coated samples had better oxidation resistance than non-coated samples. Microstructural and phase studies of coated and oxidized samples were performed by scanning electron macroscopy (SEM) and X-ray diffraction analysis (XRD). During oxidation process, the coating layer was converted into TiFe, TiFe2, TiFe2O5, TiO2 and TiCr2O4 phases. The coated specimens had lower weight gains relative to uncoated samples showing that coating effectively protects the substrate against oxidation. Moreover, coated samples had higher electrical resistance than uncoated ones.

Crofer 22 APU ferritic stainless steel has been evaluated as one of the favorable materials for utilization in Solid oxide fule cell (SOFC) interconnects. However, there are difficulties in utilizing these metallic interconnects, including the quick  decrease of their electrical conductivity and evaporation of Cr species. To overcome the above problems, the application of protective coatings has been proposed. In this work, Co/Y2O3 composite coatings were deposited onto Crofer 22 APU stainless steels by direct current electrodeposition method. Oxidation and electrical properties of uncoated and coated steels were evaluated. Surface and cross-section of the bare and coated steels were characterized using scanning electron microscopy and X-ray diffraction techniques. Results showed that oxidation rate of the coated specimen was reduced by about 4 times, as compared to the uncoated one after 500 h isothermal oxidation in air at 800˚C. Formation of Co3O4 and MnCo2O4 spinel compositions improved electrical conductivity of the coated sample. After 500 h of isothermal oxidation at 800˚C, ASR value of the Co/Y2O3-coated and uncoated steels was 15.8 mΩ·cm2 and 25.9 mΩ·cm2 , respectively.

AISI 430 ferritic stainless steel is used as interconnects in solid oxide fuel cells. In order to improve the oxidation resistance of steel in operating conditions of solid oxide fuel cells, protective coatings can be employed. In this study Ni-TiO2 coating was deposited on an AISI 430 stainless steel substrate by electrocodeposition in Watt’s plating bath containing TiO2 particles. Composition and properties of Ni-TiO2 coating were investigated as a function of current density. The volume percentage of embedded TiO2 was raised by increasing in current density until 4 A/dm2 and beyond that reduced. Also grain size of Ni-TiO2 composite coating decreased until 3 A/dm2 and above this current density, the again grain size raised. Maximum value of microhardness obtained in 4 A/dm2 current density. In order to investigate the oxidation resistance, coated and uncoated specimens were subjected to isothermal oxidation at 800 ºC for 300 h. Results showed that the coated specimens at the bath containing of SDS surfactant and without SDS surfactant had less mass gain compared to uncoated ones. The microstructure of composite coatings, before and after oxidation was investigated by scanning electron microscope (SEM) and X-ray systems.

In order to improve the oxidation and hot corrosion resistance of steels, various elements including aluminum, chromium, silicon, titanium or combination of these elements can be diffused on to the surface of steel. In this study, aluminum and titanium were simultaneously co-deposited onto the AISI 430 ferritic stainless steel substrate by the pack cementation process. Coating was examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The coating consised of two layers with the thickness of approximately 14 microns. The results obtained by XRD showed the existence of FeTi, TiO2, AlTi, Al3Ti and Al5Ti phases in the coating. Isothermal oxidation and cyclic oxidation were carried out at 1000○C. It was showed that the diffusional coating of aluminum-titanium led to the improvement of cycle and isothermal oxidation resistance.

Ni-Cr-Al-Hf alloys containing 0, 0.1, 0.2 and 0.4 wt.% Hf were produced and their isothermal oxidation behavior and electrical resistance has been investigated in air for 75 h at the temperature of 1000 °C. Microstructures of the oxidized samples were examined using scanning electron microscopy (SEM) and electrical resistance was measured. The samples showed different microstructures, and oxidation behavior depending on their chemical composition. The results indicated that the sample containing 0.4 wt.% Hf had the lowest weight gain, while the weight gain of the sample without Hf was the highest. An external scale of spinel overlying a region of internal oxides precipitates formed on Ni-12Cr-4.5Al. Conversely, an external Al2O3 formed on the samples containing Hf additions. In absence of Hf, Cr2O3 was the major scale that formed on surface. The improvement in the oxidation resistance is believed to be due to the transition between the internal and external oxidation of aluminum, adherent and protective Al2O3 oxide layer on the surface of the Ni-12Cr-4.5Al-0.4Hf alloy. It was found that the scale adhesion can be affected by mechanical keying at the alloy/scale interface resulting from the pegs’ formation during oxidation. Higher electrical resistance of Alloy without Hf is due to higher thickness of oxide scale formation on surface and sample with 0.4 wt.% Hf showed lowest electrical resistance due to thinner scale of Al2O3.

Mg alloy is easily oxidized during melting in the unprotected atmosphere. This affects on the quality and cleanness of the alloy. In addition, the presence of oxide layer on the surface of melt decreases its fluidity so use of magnesium alloys in thin-film casting is faced with Serious limitation. In this paper, effect of adding calcium on the oxidation resistance and fluidity of AZ91 alloy melt have been investigated. For this purpose, samples of AZ91 alloy were prepared with different amounts of calcium. Afterward samples were stored at 700 ° C for different periods of time in furnace atmosphere .Microstructural, GI-XRD and surface oxidation tests showed that, a dense and compact oxide film composed of CaO is formed on surface, which performs melt protection and prevents penetrating of oxygen into the melt .As a result, the melt stables at a temperature of 700 ° C without presence of a protective gas .The most important advantage of use of calcium in magnesium alloys is not applying protective gases such as SF6. To investigate the fluidity of AZ91 alloy in the presence of calcium, the fluidity test was performed by using spiral pattern. The test results showed that by adding calcium, length of fluidity increased from about 100 mm to 340 mm.

This study has examined the properties of Crofer 22APU stainless steel produced by mechanical alloying for using as interconnect plates in solid oxide fuel cells.This alloy was produced by mixing the source powders and mechanical alloying for 40 hours. For creating a sample with high density, spark-plasma sintering was applied at 1100 °C and 50 Mpa stress for 10 minutes. To achieve the desired properties such as low electrical resistance and high oxidation resistance, a number of samples were coated by manganese-cobalt using electrodiposition technique at current density of 150 mA/cm 2 for 40 minutes. Then, considering the properties required for an interconnect plate of solid oxide fuel cell, oxidation resistance and electrical resistance of the coated and uncoated samples were investigated. Oxidation behavior of the coated and uncoated samples, after 100 hours oxidation in air at 800°C did not follow any rule and its curve was a sinus type. The electrical resistance of uncoated samples was in the range of 0.1-0.2 mOhm.cm2, but the electrical resistance of the coated samples after 100 h oxidation reached to a less ammount than that of the corresponding uncoated ones. The alloy produced by mechanical alloying method, compared with commercial ones produced by casting methods, showed similar oxidation behavior after 100 h oxidation, but it had a surface electrical resistance far less than its commercial ones.