Oxidation Mass Gain Research Articles

Oxidation behaviour and mechanism of a cobalt based superalloy between 1050 and 1250 °C

A new Co-base superalloy 22Co–21Ni–23Fe–29Cr–2.2Nb (Co22 for short) was developed, and the influence of a trace amount of rare earth (RE) on the oxidation resistance was investigated. Isothermal oxidation behaviour of this Co-base superalloy was investigated at 1050–1250°C in air, analyzed with scanning electronic microscopy and X-ray diffraction. The results show that oxidation mass gain of the samples with or without RE elements obeyed secondary parabolic law, and the effect of 0.03wt.% RE on oxidation resistance is small. The oxidation rate constant of the 0.03wt.% RE added Co22 alloy decreases by about 6.1% to 9.0% compared to that of the Co22 alloy without RE. Oxidation at 1050–1250°C for 100h results in the formation of MnCr2O4, Cr2O3 and SiO2. A continuous and protective MnCr2O4 spinel layer forms as outer layer. The continuous middle oxide layer is confirmed to be Cr2O3, and the innermost layer consists of discontinuous SiO2. The outer and middle layers strongly improve oxidation resistance and oxide layer adhesion because of their continuity and compactness, and the innermost SiO2 layer forms a diffusion barrier to prevent further oxidation. Formation and growth mechanisms of oxide layers were also discussed.

Effects of Y2O3 on SiC/MoSi2 composite by mechanical-assistant combustion synthesis

MoSi2 based materials are considered as a potential high temperature structural parts. In this work, a 0.5wt% Y2O3–20vol% SiC/MoSi2 composite was successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. Adding a small amount of Y2O3 to the SiC/MoSi2 composite decreased the apparent activation energy of sintering by 10.4%, resulting in a denser composite with finer grains. The relative density, flexural strength, Vickers hardness and fracture toughness of 0.5wt% Y2O3–20vol% SiC/MoSi2 increased by 5.3%, 27.7%, 27.2% and 35.8% as compared to 20vol% SiC/MoSi2, respectively. The oxidation mass gain of Y2O3–20vol% SiC/MoSi2 at 1200°C was higher than that of 20vol% SiC/MoSi2 for 16.9%, while it still exhibited very good oxidation resistance at this temperature.

Preparation and performance of coating on rare-earth compounds-immersed magnesium alloy by micro-arc oxidation

A composite ceramic coating containing Y2O3–ZrO2–MgO (YSZ–MgO) was prepared on AZ91D magnesium alloy, which was immersed in Y(NO3)3 aqueous solution as pretreatment, by micro-arc oxidation (MAO) process. The morphology, elemental and phase compositions, corrosion behavior and thermal stability of the coatings were studied by SEM, EDX, XRD, electrochemical corrosion test, high temperature oxidation and thermal shock test. The results show that the coating mainly consists of ZrO2, Y2O3, MgO, Mg2SiO4, and MgF2. Among these compounds, Y2O3 accounts for 26.7% of (Y2O3 + ZrO2). The thickness of YSZ–MgO coating is smaller than that of ZrO2–MgO coating, but its compactness and surface roughness are better than those of ZrO2–MgO coating. YSZ–MgO coating has a good corrosion resistance, and its corrosion rate in 5% NaCl aqueous solution is lower than that of ZrO2–MgO and only about 8.5% of that of AZ91D magnesium alloy. After oxidation at 410 °C, the mass gain of AZ91D magnesium alloy presents a linear increase with the oxidation time. The YSZ–MgO coating and ZrO2–MgO coating can remarkably decrease the oxidation mass gain. The oxidation mass gain of YSZ–MgO coating is lower than that of ZrO2–MgO coating, especially during a long oxidation period. The thermal shock resistance of YSZ–MgO coating is superior to ZrO2–MgO coating.

High temperature oxidation behavior of directionally solidified NiAl–31Cr–2.9Mo–0.1Hf–0.05Ho eutectic alloy

The isothermal oxidation behavior of NiAl–31Cr–2.9Mo–0.1Hf–0.05Ho directional eutectic alloy was investigated with the help of scanning electron microscopy and X-ray diffraction. The results revealed that a continuous Al2O3 scale was formed and owned excellent oxidation resistance in the temperature range of 900–1100 °C. When the temperature was up to 1150 °C, the continuous Al2O3 oxide film ruptured. Trace rare earth element Ho distributed uniformly in the alloy and relatively high level of Al in Cr(Mo) phase are beneficial to the formation of continuous and compact Al2O3 scale. During the oxidation, a phase transformation from θ-Al2O3 to α-Al2O3 existed on the surface of oxidation film. It resulted in the abnormal oxidation mass gain happening when the alloy was oxidized at 1000 °C or 1050 °C.

Evolution of Al<sub>2</sub>O<sub>3</sub> Scale Formed on Ni-Al-Pt Alloy in Atmospheres Containing Water Vapor

High temperature cyclic oxidation behavior of γ'-base Ni-25Al-10Pt (in at.%) alloy was investigated at 1000°C in air with and without 30vol.%H2O. The oxidation mass gain during the initial stage of oxidation was similar in both atmospheres, but the oxidation rate in air+H2O was lower in the longer steady-state oxidation stage. Metastable Al2O3, which formed during the initial stage of oxidation, transformed completely after about 100hr of oxidation in dry air. The transformation to α-Al2O3also occurred in air+H2O, but complete transformation to α-Al2O3was not observed during the oxidation time in the present study. θ-Al2O3grains remained for longer on the α-Al2O3layer in air+H2O and became significantly coarser with oxidation time. The present results indicate that water vapor delays the metastable to α-Al2O3phase transformation, and decreases the growth rate of α-Al2O3.

High Temperature Corrosion Resistance of Siliconized Stainless Steel under Continuous Deposition of Salt

The surface alloying of Si into SUS304 austenitic stainless steel was carried out by a halide-activated pack-cementation method. By this treatment, the silicon diffusion layer containing about 13 at.% Si was formed. The high temperature corrosion resistance of this specimen was evaluated under the continuous deposition of salt. The result of the corrosion test showed that the oxidation mass gain of the siliconized stainless steel was lower than that of non-treated stainless steel. It was found from the observation of the cross-section of the specimen after the corrosion test that a thin scale was formed on the silicon diffusion layer and silicon oxide was formed as an inner layer of the scale. A mechanism of the oxidation suppression for the siliconized steel under the continuous deposition of salt was investigated by the oxidation test of pure silicon, iron, chromium or nickel powder mixed with equimolar NaCl-KCl. As a result, it was found that the high corrosion resistance of the siliconized steel was attributable to the fact that the silicon oxide formed on the silicon diffusion layer was inert to the chemical reaction with the NaCl-KCl salt.

The effect of Al and Cr additions on pack cementation zinc coatings

Zinc is widely used as a protective coating material due to its corrosion resistant properties. The structure and oxidation resistance of Al and Cr mixed zinc coatings, deposited by pack cementation process, is thoroughly examined in this work. The morphology and chemical composition of the as-deposited and oxidized samples was accomplished by electron microscopy while the phase identification was performed by XRD diffraction analysis. The experimental results showed that the addition of aluminum or chromium in the pack mixture forms only Al and Cr rich phases on the surface of the specimens without affecting significantly the phase composition of the rest zinc coatings. In the case of Zn–Al coatings, the overlying layer contains high concentrations of Al together with lower amounts of zinc and iron and in Zn–Cr coatings this layer contains Cr, Fe and Zn atoms and has much smaller thickness. The presence of these additional layers promotes significantly the oxidation resistance of the zinc pack coatings and they preserve most of their initial thickness and chemical content when exposed to an aggressive environment while their oxidation mass gain was measured at low levels during the oxidation tests.

High-temperature oxidation behavior of CeO 2-SiO 2/Ni-W-P composites

Ni-W-P matrix composites containing CeO 2 and SiO 2 nano-particles were prepared on common carbon steel surface by means of pulse electrodeposition, and the high-temperature oxidation behavior was investigated. The results show that when the oxidation time is controlled in 1 h, oxidation kinetics curve between oxidation mass gain rate and oxidation temperature of CeO 2-SiO 2/Ni-W-P composites accords with the index increasing law. When the oxidation temperature is controlled at 300 °C, the kinetics curve between oxidation mass gain rate and oxidation time accords with the linear increasing law. The composites as-deposited are in the amorphous state and turn into the crystal state at 400 °C. The microstructures of oxidation film on the composites will change from the compact state to the loose state with increasing oxidation temperature to 800 °C. They are still continuous and compact, and there are no crackle, strip and falling-out. CeO 2 and SiO 2 nano-particles co-deposited into Ni-W-P alloy can improve the high-temperature oxidation resistance.

Oxidation behavior of in-situ Al 2O 3/TiAl composites at 900°C in static air

In-situ Al 2O 3/TiAl composites were successfully synthesized from the starting powders of Ti, Al, TiO 2 and Nb 2O 5. The oxidation behavior of the composites at 900°C in static air was investigated. The results indicate that the composite samples present a much lower oxidation mass gain. Under long-time intensive oxidation exposure, the formed oxide scale is multi-layer. The formation of the outer TiO 2 layer is fine and dense, the internal Al 2O 3 scale has good adhesiveness with the outer TiO 2 scale, and the TiO 2+Al 2O 3 mixed layer forming the protective oxide scale is favorable for the improvement of oxidation resistance. It is believed that the incorporation of Al 2O 3 particulates into the metal matrix decreases the coefficient of thermal expansion of the substrate, and forms a local three-dimensional network structure that can hold the oxide scale. The formation of the oxide scale with finer particle size, stronger adherence, less micro-defects and slower growth rate can contribute to the improvement of oxidation resistance. Nb element plays an important role in reducing the internal oxidation action of the materials, restraining the growth of TiO 2 crystals and promoting the stable formation of the Al 2O 3-riched layer, which is beneficial to improve the oxidation properties.

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The effect of reactive air aluminizing (RAA) on the high temperature oxidation behavior of the H230 superalloy is studied at 850 °C for 600 h. Mass gain values are measured and microstructure is studied by XRD, SEM, and EDX. Post oxidation mass gain for the aluminized H230 is reduced compared to wrought H230. The RAA process lead to formation of a three-zone diffusion coating with dominant β-NiAl in the two outer zones and increased γ'-Ni3Al after high temperature oxidation, which is attributed to the alumina scale growth during the high temperature oxidation.

Microstructure and wear properties of WC–TiC–Co/CuZnNi composite surface coating

The microstructure and wear behavior of WC–TiC–Co/CuZnNi composite surface coating has been investigated by means of scanning electron microscope (SEM), electron dispersion X-ray analysis (EDXA) and wear test. The oxidation behavior of WC–8TiC–8Co and WC–8Co hard metal in the temperature range between 600 and 1000 °C was also researched. The oxidation mass gain of WC–8TiC–8Co hard metal is less than the WC–8Co hard metal. The particle of hard metal is surrounded by the α-Cu + β-Zn phase in the surface coating. The size of hard metal is almost unchanged from its original one. There exists an inter-diffusion zone at the interface of the hard metals and Cu-based matrix. The wear volume of both the WC–8TiC–8Co/CuZnNi and WC–8Co/CuZnNi composite surface coatings increased with the increasing of applied load. However, the composite surface coating of WC–8TiC–Co/CuZnNi exhibited lower wear volume than that of WC–8Co/CuZnNi composite surface coating. According to the results of engineering application, the working efficiency and employing life of milling tools, which were surfaced WC–8TiC–8Co/CuZnNi surface coating, is by approximately 2–3 times the milling tools surfaced WC–8Co/CuZnNi surface coating.

The effects of milling conditions on the subsequent oxidation behaviour of mechanically alloyed Fe3Al-based powders

Mechanically alloyed, Fe 3 Al-based, oxide dispersion strengthened alloys form a surface oxide scale during powder processing. This scale becomes entrained in the consolidated alloy, and may have a significant effect on subsequent recrystallisation behaviour. The high oxidation rates found in these alloys are mainly due to the bulk alloy composition. However, batch-to-batch differences in oxidation mass gain occur in powders with ostensibly identical compositions. Batches PMWY2 and PMWY3 were studied and parameters such as alloy composition and homogeneity, powder surface area to volume ratios and scale thickening rates considered. Batch PMWY2 showed 20-90% faster weight gain than PMWY3 and reached the onset of breakaway oxidation approximately twice as quickly. PMWY2 was found to contain aluminium-depleted regions, whereas PMWY3 is much more homogeneous. The surface area to volume ratio for PMWY2 was 44% higher than that of PMWY3, and batch PMWY2 was found to contain extremely fine powder particles. The scale on batch PMWY2 thickened more quickly than that on batch PMWY3, with rates 20-40% higher at different stages in the oxidation. The major contributory factor to the difference in oxidation mass gain between the two alloy batches is scale thickening rate and factors influencing thickening rates are discussed.

Injection and stabilization of mesophase pitch in the fabrication of carbon–carbon composites: Part II. Stabilization process

In the fabrication of carbon–carbon composites by mesophase injection through a fiber preform, it is essential to stabilize the flow-induced microstructure in the flow channels and to prevent relaxation and exudation of the mesophase. Oxidation stabilization studies were conducted on preforms injected with the naphthalene-based AR mesophase pitch. Oxidation mass gain (OMG) curves at 170, 222, and 270 °C were generated for 60°-wedges cut from full size composite disks. The rates of OMG at 170 °C of first- and second-cycle injection wedges and full-size disks were comparable to those using as-spun filaments 30 μm in diameter, and particles sieved to 200 to 340 μm. The results suggest that oxygen is accessible deep into a mesophase matrix and the transport is facilitated by connected array of shrinkage cracks. Oxidation at 170 °C has strong advantage over higher oxidation temperatures by having a higher carbon yield and lower OMG threshold and thus oxidation time required for stabilization. The 60°-wedges could be stabilized at 170 °C after a 25 h oxidation with a 7.2% OMG and attaining a carbon yield above 85%.

High temperature oxidation behaviour of a TiAl–Al 2O 3 intermetallic matrix composite

A TiAl-based intermetallic matrix composite has been produced through sintering of mechanically milled Al/TiO 2 composite powder. The composite contains 42–50 vol.% of α-Al 2O 3 as the particulate reinforcement phase. Oxidation experiments were carried out at 800–900 °C in air up to 500 h to evaluate its oxidation and scale spallation resistance. A cast Ti–50at.%Al alloy was also tested for comparison. The composite samples showed much lower oxidation mass gain than the cast alloy under all testing conditions. Moreover, the composite samples exhibited extremely strong scale spallation resistance. Spallation could never be recorded and observed even under long-time intensive cyclic oxidation exposure. Based on the kinetic and microstructural studies, the mechanisms for the improved oxidation and spallation resistance are discussed.

Effect of Mo Addition on the Oxidation Behavior of TiAl Intermetallic Compound

The oxidation behavior of the Mo-modified Ti-34.5 mass% Al intermetallic compound in air at 1073, 1173 and 1273 K was investigated by comparing with that of a binary Ti-34.5 mass% Al. The oxidation mass gain of the Mo-modified TiAls isothermally oxidized below 1273 K decreased with increasing Mo content. At 1273 K, however, a minimum in a mass gain-Mo content curve was observed at 2 mass% Mo content. For the Mo-modified TiAls parabolic oxidation curves obtained under the cyclic oxidation condition at 1173 K, in contrast to a linear curve for the binary TiAI. As a result of SEM-EPMA observation, the scales on the Mo-modified TiAls were found to have a layered structure in the following order from the outer side; TiO 2 /Al 2 O 3 /TiO 2 +Al 2 O 3 /Ti 3 Al (Mo-enriched phase)/TiA1(matrix). Al 2 O 3 in the scale/metal interface zone formed along the Mo-enriched phases. The second Al 2 O 3 layer from outer and the Al 2 O 3 in the inner layer were much more continuous and must have acted as protective layers. The oxygen solid solubility decreased in the Mo-modified TiAl and further in the Mo-enriched phase region, which must be the cause for the Al 2 O 3 formation in the external oxidation manner.

TiAl金属間化合物の耐酸化性に及ぼすW添加の影響

The oxidation behavior of the Mo-modified Ti-34.5 mass% Al intermetallic compound in air at 1073, 1173 and 1273 K was investigated by comparing with that of a binary Ti-34.5 mass% Al. The oxidation mass gain of the Mo-modified TiAls isothermally oxidized below 1273 K decreased with increasing Mo content. At 1273 K, however, a minimum in a mass gain-Mo content curve was observed at 2 mass% Mo content. For the Mo-modified TiAls parabolic oxidation curves obtained under the cyclic oxidation condition at 1173 K, in contrast to a linear curve for the binary TiAI. As a result of SEM-EPMA observation, the scales on the Mo-modified TiAls were found to have a layered structure in the following order from the outer side; TiO 2 /Al 2 O 3 /TiO 2 +Al 2 O 3 /Ti 3 Al (Mo-enriched phase)/TiA1(matrix). Al 2 O 3 in the scale/metal interface zone formed along the Mo-enriched phases. The second Al 2 O 3 layer from outer and the Al 2 O 3 in the inner layer were much more continuous and must have acted as protective layers. The oxygen solid solubility decreased in the Mo-modified TiAl and further in the Mo-enriched phase region, which must be the cause for the Al 2 O 3 formation in the external oxidation manner.

High‐Temperature Active Oxidation of Chemically Vapor‐Deposited Silicon Carbide in CO─CO2 Atmosphere

Active oxidation behavior of CVD‐SiC in CO─CO2 atmospheres was investigated using a thermogravimetric technique in the temperature range between 1823 and 1923 K. The gas pressure ratio, PCO2/PCO, was controlled between 10−4 and 10−1 at 0.1 MPa. Active oxidation rates (mass loss rates) showed maxima at a certain value of PCO2/PCO, (PCO2/PCO)*, In a PCO2/PCO region lower than the (PCO2/PCO)* a carbon layer was formed on the SiC surface. In a PCO2/PCO region higher than the (PCO2/PCO)*, silica particles or a porous silica layer was observed on the SiC surface.

The oxidation behaviour of some cobalt-based amorphous alloys

The oxidation behaviour of three amorphous and pre-crystallized cobalt-based alloys (Co 58Ni 10Fe 5Si 11B 16, Co 66Ni 1Fe 4Si 15B 14 and Co 76Mn 4Fe 2Si 6B 12) have been studied in air at 400 °C. A combination of thermogravimetry, scanning and transmission electron microscopy, and differential scanning calorimetry have been used to investigate the oxidation kinetics, oxide composition and oxide morphology. The oxidation of the amorphous alloys obeyed a parabolic rate law for oxidation mass gain vs. oxidation time, whereas crystallized samples showed more complicated oxidation kinetics with an approximate logarithmic rate law. In all three alloys, the amorphous samples showed poorer oxidation resistance than the crystallized samples did. Electron microscopy showed that both amorphous and crystallized alloys form multiphase microcrystalline oxide layers with complex morphology.

Accelerated oxidation rate of chromium induced by sodium chloride

The oxidation rate of NaCl-coated chromium was measured over the temperature range of 823–1043 K. Although the melting point of NaCl is 1074 K, accelerated oxidation was observed at every temperature. Oxidation mass-gain curves were divided into two types. Type I involved only accelerated oxidation during the initial stage followed by the formation of a thin protective Cr2O3 film. Type II pertained to an acceleration over a long time, forming a thick and nonprotective Cr2O3film containing Na2CrO4. The former type of oxidation occurred at lower temperatures or with a small amount of NaCl, whereas the latter occurred at higher temperatures and with large amounts of NaCl. A comparison of these oxidation processes with those by CaCl2 or BaCl2, revealed two problems: (1) Why was the oxidation rate of chromium so high in the presence of NaCl?, and (2) Why did the high oxidation rate continue for such a long time?