Oxidation Behavior of Tribaloy T-800 Alloy at 800 and 1,000 °C

The oxidation behavior of Co-based Tribaloy T-800 alloy has been studied isothermally in air at 800 and 1,000 °C, respectively. The results showed that the oxidation mechanism was dependent on the exposure temperature. The oxidation of the alloy followed subparabolic oxidation kinetics at 800 °C. The oxide scale at this temperature exhibited a multi-layered structure including an outer layer of Co oxide, a layer composed of complex oxide and spinel, a nonuniform Mo-rich oxide layer, an intermediate mixed oxides layer and an internal attacked layer with different protrusions into Laves phase. During 1,000 °C exposure, it followed linear kinetics. The oxidation rendered a relatively uniform external Cr-rich oxide layer coupled with a thin layer of spinel on the top surface and voids at local scale/alloy interface and intergranular region together with internal Si oxide at 1,000 °C.     

Oxidation Behavior of the Nickel-Based Superalloy DZ125 at 980℃

The isothermal oxidation behavior of the nickel-based superalloy DZ125 was investigated at 980 °C through SEM/EDX and XRD. The weight loss process exhibited three periods—initial, transient and steady stages, which correspond to the formation of three layers on the surface. The outer layer was composed of Ni O, whereas the intermediate layer mainly consisted of spinels and was accompanied with Ta-rich oxide. Alumina was evident as the inner continuous layer close to the substrate. The first formation of alumina was responsible for the initial stage of weight loss, and the formation of Ni O and intermediate layer may dramatically affect on the transient and steady stage, respectively. The oxide scales effect on growth mechanism was discussed.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

Oxidation and Creep Limited Lifetime of Kanthal APMT®, a Dispersion Strengthened FeCrAlMo Alloy Designed for Strength and Oxidation Resistance at High Temperatures

Kanthal APMT^® is an FeCrAlMo alloy optimized for continuous service up to 1,250 °C (~2,300 °F). Rapid solidification powder metallurgy applied on this FeCrAlMo composition provided an oxide dispersion strengthened microstructure. The alloy exhibits an attractive combination of resistance to oxidation and corrosion and excellent form stability. In this study, oxidation and corrosion properties were investigated, as well as mechanical properties at elevated temperature. It was shown that an adherent alumina layer on the alloy surface formed during service that provided excellent resistance to corrosion attacks in most industrial atmospheres and gave great advantages compared to chromia forming high temperature Ni-base alloys in terms of maximum operating temperature and life. Focus was set on oxidation and creep properties but also other important aspects are discussed.     

Kinetic and Morphological Study of the Oxidation of an Fe–36% Ni Alloy in Carbon Dioxide at 750–1000°c

Abstract

The oxidation of ‘Nilo’ alloy 36, an Fe–36% Ni alloy, has been studied by thermogravimetric, metallographic and electron-probe microanalysis techniques. At 750°–950°, after a short period of ill-defined oxidation the parabolic law was obeyedthroughout the entire exposure period which vaned from ～150h at 750° to ～25h at 950°. At 1000° there was a paralinear kinetic transition after 2–3 h (wt. gain 8–9 mg/cm²). The activation energy for the oxidation reaction derived from the parabolic rate constants was 52 ± 9·3 kcal/mole.

During the parabolic stage at 1000° the scale was mainly magnetite with pockets of wustite along the scale/metal interface. There was also considerable intergranular oxidation and nickel enrichment of the alloy grains in the region of the alloy/scale interface. The intergranular oxide was wustite. This pattern of oxidation was followed at lower temperatures. Followmg the transition from parabolic to linear kinetics at 1000° the scale was found to consist of wustite containing ≤0·4% Ni. The manganese concentration in the scaleswas similar to that in the alloy. The results are discussed and mechanisms of oxidation are suggested.     

Isothermal and Cyclic Oxidation Behavior of Fe–B Based Alloys at 927 K

The high temperature oxidation behavior of an Fe–16Cr binary alloy, oxidized under different compressive stresses in air at 900 °C, was investigated. Surface and cross-sectional micrographs, observed by scanning electron microscopy, indicated that the resulting morphology of the thermally grown oxide scale depended on the compressive stress. Results showed that oxide scales were infact below 5 MPa stress after 10 h of oxidation. Delamination developed at the outer/inner oxide scale interface in the case of compressive stress above 5 MPa. Growth kinetics measurements revealed that the rate of oxide-scale growth increased by the compressive stress.     

Oxidation Behavior of Sputtered DD98M Nanocrystalline Coating at 1000 °C

Increasing the efficiency of coal fired steam power plants is an important contribution towards clean coal power. In fact, new ferritic steels are expected to withstand 325 bar and 650 °C. Moreover, in order to facilitate CO₂ capture oxygen can be used instead of air for combustion (oxycombustion) so that no NO_X emissions are produced. Boiler components, such as superheater tubes, are exposed to both steam and fireside corrosion and at higher temperatures, ferritic steels corrode at very fast rates under both atmospheres. A solution can be found in the use of protective coatings, a number of which, applied by techniques capable of depositing said coatings both on the inner and outer surfaces of tubes, are being studied within nationally and European funded projects. In particular, two new Ni and Cr modified aluminide coatings deposited on P92 by non-line-of-sight hybrid processes have been produced and the preliminary results of on-going laboratory testing, both under oxycombustion model atmospheres as well as under pure steam at 650 °C are promising, in particular those exhibited by the Cr enriched aluminide coating. Moreover, results obtained in a pilot oxycombustion boiler operated by CIUDEN in Leon, Spain are also shown.     

Oxidation Behavior of β-21 S Titaniu m Alloy over Tem perture Range 600 ℃to 800 ℃

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Oxidation Behavior of a PVD-Processed Mg–10.6Zr Alloy

The oxidation behavior in air of a physical vapor-deposited (PVD) Mg–10.6Zr (wt. %) alloy was studied in the 325–450°C temperature range. The oxidation rate of this alloy remains low at temperatures below 375°C. However, at higher temperatures, the alloy experienced extremely high oxidation rates, which can even lead to disintegration of the sample. Oxidation is controlled by fast inward oxygen transport along the open boundaries of the alloy, leading to the formation of cracks throughout the sample, and subsequent formation of a thin MgO at crack interfaces. The MgO layer remains protective while coarsening of zirconium precipitates at the open boundaries does not take place. Thickening of Zr precipitates over a critical size induces impairment of the MgO layer at crack interfaces, facilitating inward oxygen diffusion. The volume increase resulting from the formation of new oxide at open boundaries favors decohesion of open boundaries, leading to accelerated oxidation.     

Oxidation of pre-oxidized GH128 alloy implanted with Ce+ at 1000℃

The influence of Ce implantation into preformed scales with a dose of 1×1017 ions/cm2 on the subsequent oxidation behavior of GH128 alloy at 1000℃ in air was investigated. The pre-oxidation was carried out at 1000℃ in air for 1h and 5h respectively. Cr2O3, NiO and NiCr2O4 formed on the surface of all specimens. Ce implantation decreased the subsequent oxidation rate of both the alloy and the 1h pre-oxidized alloy, however, had no effect on that of the 5h pre-oxidized alloy. The beneficial effect was most obvious in the directly implanted alloy. During the cyclic oxidation for 600h.Ce implantation for all specimens with or without preferential oxidation played a similar beneficial effect on the oxide spallation resistance. The results indicate that Ce incorporated into the oxide scales affects the diffusion of the reaction species to some extent, the wavy interface and small grain structure make a significant contribution to improving the spallation resistance of the oxide scales.     

Pack Aluminide Coatings Formed at 650℃ for Enhancing Oxidation Resistance of Low Alloy Steels

This study aims to investigate the feasibility of forming iron aluminide coatings on a commercial 9Cr-1Mo (wt.%) alloy steel by pack cementation at 650℃ in an attempt to improve its high temperature oxidation resistance. Pack powders containing Al, Al2O3 and a series of halide salts were used to carry out the coating deposition experiments, which enabled identification of the most suitable activator for the pack aluminising process at the intended temperature. The effect of pack aluminium content on the growth kinetics and microstructure of the coatings was then studied by keeping deposition conditions and pack activator content constant while increasing the pack aluminium content from 1.4 wt.% to 6 wt.%. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the phases and microstructures of the coatings formed and to determine depth profiles of coating elements in the coating layer. Oxidation resistance of the coating was studied at 650 ~C in air by intermittent weight measurement at room temperature. It was observed that the coating could substantially enhance the oxidation resistance of the steel under these testing conditions, which was attributed to the capability of the iron aluminide phases to form alumina scale on the coating surface through preferential A1 oxidation.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

A Modified Johnson–Cook Model for Flow Behavior of Alloy 800H at Intermediate Strain Rates and High Temperatures

A modified Johnson–Cook model for the flow behavior of alloy 800H at intermediate strain rates and high temperatures is presented. The modification is based on a study of the relation between strain hardening and both strain rate and softening parameters. The predicted stresses obtained using the modified model are compared to those obtained using the original Johnson–Cook model. The parameters constitute the two models are determined using the inverse method, Kalman filter. The results show that the modified model fits the experimental data very well for different combinations of strain rates and temperatures, with a mean value of R-squared regression of 0.90 for the modified model and 0.74 for the original Johnson–Cook model.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Effect of Surface Roughness on the Oxidation Behavior of a Directionally Solidified Ni-Based Superalloy at 1,100℃

The effect of surface roughness on the oxidation behavior of a directionally solidified Ni-based superalloy was investigated by surface mapping microscope,scanning electron microscope and X-ray diffraction.It was found that specimens with surface roughness of 0.05 urn exhibit the best oxidation resistance,while specimens with surface roughness of 0.14 μm behave worse than specimens with surface roughness of 0.83 μm.The specimens with surface roughness of 0.05 μm have the best oxidation resistance,which is mainly due to the smallest surface area exposed in air and thinnest work-hardening layer.The Al_2O_3 layer alleviates the oxidation process of the specimens with surface roughness of 0.83 μm,and this is the possible reason for the better oxidation resistance of samples with surface roughness of 0.83 μm than samples with surface roughness of 0.14 μm.     

Effects of Ga,Sn Addition and Microstructure on Oxidation Behavior of Near-α Ti Alloy

We investigated the effects of adding Ga or Sn, with almost the same Al equivalent, on the oxidation behaviors of near-α Ti alloy with the bimodal structure and lamellar structure. The replacement of Sn with Ga decreased the alloy weight gain during oxidation, suppressed oxide growth, and improved adherence between the oxide and substrate. A lamellar alloy structure showed a lower weight gain during oxidation compared to the bimodal structure. Unlike conventional near-α alloys, recrystallization occurred near the oxide/substrate interface in Ga-modified alloy, which may contribute to the release of stress, improvement of the adherence between the oxide and substrate, and prevention of oxide-scale spallation from the Ga-modified alloy. A possible mechanism for the recrystallization in the Ga-modified alloy was also discussed.     

Oxidation of AM60B Mg Alloys Containing Dispersed SiC Particles in Air at Temperatures Between 400 and 550 °C

AM60B magnesium alloys, with and without dispersed SiC particles, were oxidized between 400 and 550 °C in air. The scales generated consisted primarily of MgO and a small amount of Mg₃N₂ formed by the outward diffusion of cations (Mg, Al, Mn) and the inward diffusion of anions (N, O). The SiC particles were stable in the AM60B alloy during oxidation and increased its oxidation resistance to a certain extent. However, given the predominance of the non-protective MgO as the main oxide, the SiC_p/AM60B composites were inevitably destroyed as oxidation progressed.     

Effect of Shot Blasting on Oxidation Behavior of TP304H Steel at 610℃-770℃ in Water Vapor

Shot blasting on a tube steel of TP304H can greatly improve the oxidation resistance at 650℃-770℃ in water vapor, by the presence of an appropriate blasting intensity. SEM and EPMA analysis indicated that the structure of the oxide scale turned from multi-layer to monolayer after blasting and Cr2O3 in predominant. The untreated sample characterized in multi-layer oxide was composed of Fe in the outer layer and Fe and Cr underneath. For blasted one, the increase of diffusion rate of Cr and the decrease of effective diffusion energy of scale led to a uniform Cr-rich oxide layer, which was dense and protective and the oxidation resistance was increased.     

Positron Annihilation Study of High-Temperature Oxidation Behavior of Zr–1Nb Alloy

The oxidation behavior of Zr–1Nb alloys exposed at 873 and 973 K in air was investigated by positron annihilation lifetime spectroscopy together with mass gain, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mass-gain results showed that during the oxidation process, a transition of the oxidation rate occurred. The transition times of the specimens oxidized at 873 and 973 K were 30 and 6 h, respectively. In the pre-transition stage, the mass-gain curves obeyed the subparabolic law (n?=?2.3), while at the post-transition stage, the mass-gain curves obeyed the linear law. The positron lifetime measurements indicated that in pre-transition stage, the formed oxide scale mainly consisted of a compact layer that only contained small-size vacancy defects. The accumulation of these vacancy defects together with the high compressive stress might cause the breakaway of the oxide layer. During the post-transition stage, the thickness of the porous oxide layer with more and larger-size defects such as voids and pores increased rapidly as increasing the oxidation time. These large-size defects, together with the cracks produced during the transition from protective to breakaway-type oxide, increased the oxygen absorption rate and accelerated the diffusion of oxygen. The formation of cracks in the porous layer was confirmed by SEM examinations.     

Steam Oxidation Behavior of Advanced Steels and Ni-Based Alloys at 800 °C

This publication studies the steam oxidation behavior of advanced steels (309S, 310S and HR3C) and Ni-based alloys (Haynes^® 230^®, alloy 263, alloy 617 and Haynes^® 282^®) exposed at 800 °C for 2000 h under 1 bar pressure, in a pure water steam system. The results revealed that all exposed materials showed relatively low weight gain, with no spallation of the oxide scale within the 2000 h of exposure. XRD analysis showed that Ni-based alloys developed an oxide scale consisting of four main phases: Cr₂O₃ (alloy 617, Haynes^® 282^®, alloy 263 and Haynes^® 230^®), MnCr₂O₄ (alloy 617, Haynes^® 282^® and Haynes^® 230^®), NiCr₂O₄ (alloy 617) and TiO₂ (alloy 263, Haynes^® 282^®). In contrast, advanced steels showed the development of Cr₂O₃, MnCr₂O₄, Mn₇SiO₁₂, FeMn(SiO₄) and SiO₂ phases. The steel with the highest Cr content showed the formation of Fe₃O₄ and the thickest oxide scale.