High Temperature Oxidation Behavior of Alloy 617 and Haynes 230 in Impurity-Controlled Helium Environments

The high temperature oxidation behavior of alloy 617 and Haynes 230 have been investigated for VHTR intermediate heat exchanger applications. Oxidation tests were carried out for up to 500 h at 900 °C and 1000 °C in impure helium environments containing H₂, H₂O, CO, CO₂, and CH₄. The oxidation kinetics of the alloys followed a parabolic rate law in all cases. In the impure helium environments with very low oxygen, the external oxides of alloy 617 were composed of a Cr₂O₃ layer, TiO₂ ridges on the grain boundaries, and isolated MnCr₂O₄ grains on top of the Cr₂O₃ layer. On the other hand, those of Haynes 230 consisted of a Cr₂O₃ inner layer and a protective MnCr₂O₄ outer layer, which increased the oxidation resistance. The effect of small amounts of CH₄ and H₂ on the oxidation kinetics of the alloys was insignificant. Irregular oxide morphology, such as cellular Cr₂O₃ oxides for alloy 617 and MnCr₂O₄ platelets for Haynes 230, was formed in the impure helium environment at 900 °C. For Haynes 230, along with platelets, whiskers were frequently found at the tip of the MnCr₂O₄ oxide crystals.     

Oxidation Characteristics and Oxide Layer Evolution of Alloy 617 and Haynes 230 at 900 °C and 1100 °C

To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900 °C and 1100 °C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900 °C, for Alloy 617, a mixture of the extensive NiO–Cr₂O₃ double layer and isolated NiO–NiCr₂O₄–Cr₂O₃ triple layer were observed at a steady-state condition. For Haynes 230, a MnCr₂O₄ layer was formed on top of the Cr₂O₃ layer, resulting in a lower oxidation rate. At 1100 °C, both alloys showed a double layer consisting of an inner Cr₂O₃ and outer MnCr₂O₄ or TiO₂. The spallation of outer layer and subsequent volatilization of the Cr₂O₃ layer produced a rugged surface and interface as well as internal oxidation.     

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.     

Corrosion of Alloy Haynes 230 in High Temperature Supercritical Carbon Dioxide with Oxygen Impurity Additions

The corrosion of Ni-based alloy Haynes 230 in supercritical carbon dioxide at temperatures of 650 and 750 °C at a pressure of 20 MPa was investigated. In high-purity research grade CO₂, the corrosion performance of the alloy was excellent with a thin, uniform, protective chromium-rich oxide layer forming on the surface. Introduction of 10 and 100 ppm O₂ impurity in the CO₂ environment noticeably enhanced oxidation with evidence of oxide spallation and nodule formation. In these oxygen impurity added tests, increased oxidation led to subsurface voids due to the more rapid outward diffusion of chromium as well as intergranular alumina and chromia. The oxygen concentration at the inlet and the outlet of the autoclave was measured and used to support the results of characterization of the surface oxide to develop a more holistic understanding of the role of oxygen impurity on the corrosion process. In all cases, there some carbon was observed, which manifested as slightly higher concentration of chromium–carbide phase at the grain boundaries compared to the unexposed alloy.     

Metallic interconnects for solid oxide fuel cell: Performance of reactive element oxide coating during long time exposure

One of challenges in improving the performance and cost‐effectiveness of SOFCs (solid oxide fuel cells) is the development of suitable interconnects materials. Chromia‐forming alloys and especially ferritic stainless steels, like Crofer22APU, are considered to be among the most promising candidate materials as interconnects in SOFC stacks. However, the performance of chromia‐forming materials can be limited by the low electronic conductivity of the oxide scale (high ASR – area specific resistance – value). Such degradation are unacceptable regarding the long‐term operation (>40 000 h). A previous study 1 demonstrated that in air, the addition of a nanometric reactive element oxide (La₂O₃) layer applied by metal organic chemical vapor deposition (MOCVD) drastically improved both corrosion rate and electrical properties of Crofer22APU and Haynes230 alloys for 100 h at 800 °C. In this present study coating performances were checked after 10 months (7500 h) and 20 months (15 000 h) at 800 °C in air. The corrosion products were carefully analyzed by SEM, EDX, and XRD. ASR measurements are realized after long time exposure. This study demonstrates that the Crofer22APU alloy has a good oxidation resistance after 15 000 h in air but this alloy has an ASR value equal to 0.370 Ω cm². The coatings composed of a thin reactive element oxide such as La₂O₃ resulted in an important improvement in the high temperature oxidation resistance; the ASR values are equal to 0.154 Ω cm². Haynes230 alloy has a better oxidation resistance but the formation of an insulating Al₂O₃/SiO₂ layer could be detrimental.     

Effects of Corrosion in Supercritical CO<Subscript>2</Subscript> on the Microstructural Evolution in 800H Alloy

This study investigates corrosion of Fe–Ni-based alloy 800H that were exposed to supercritical CO₂ (sCO₂), ambient air and argon gas at pressures up to 20 MPa, at 650 and 750 °C for up to 1000 h. This alloy and other comparable metal alloys are expected to be used in sCO₂ heat exchanger cycles as proposed in the DOE Advanced Ultra-Supercritical program. Alloy 800H is considered for this application, because it meets the high-temperature strength and creep rupture requirements and is a lower cost alternative to other Ni-based alloys. The oxidation performance and microstructural changes due to exposure in sCO₂ have been evaluated and compared with exposures in air and Ar. The 800H alloy showed similar oxide scale thicknesses in sCO₂ as in air. A recrystallized zone was observed beneath the oxide formed in air and sCO₂. No such zone was observed after exposure to Ar, suggesting this recrystallization was associated with the oxidation process and not simply an effect of surface finishing. A wider recrystallized zone was observed underneath the oxide formed in sCO₂ than in air. The effect of air and sCO₂ on internal oxidation and carburization was investigated as well, showing that air led to more internal oxidation but less internal carburization than sCO₂. It was concluded that the carbon species provided by the sCO₂ atmosphere in conjunction with the increased grain boundary density in the recrystallized zone allowed for more ingress of carbon into the base metal, which resulted in a higher densities of carbides beneath the oxide scale.     

Oxidation of orthorhombic titanium aluminide Tl-22AL-25NB in air between 650 and 1000 °C

The oxidation behavior of orthorhombic titanium aluminide alloy Ti-22Al-25Nb was studied in air between 650 and 1000 °C by isothermal thermogravimetry and postoxidation scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti₃Al-based titanium aluminides at and below 750 °C. Between 800 and 1000 °C, the oxidation resistance of Ti-22Al-25Nb was as good as that of γ-TiAl based aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1000 °C. The mixed oxide scale consisted of TiO₂, AlNbO₄, and Al₂O₃, with TiO₂ being the dominant oxide phase. Underneath the oxide scale, a nitride-containing layer formed in the temperature range investigated, and at 1000 °C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 μm below the oxide scale when exposed to air at 900 °C for 500 h. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions.     

Oxide-Layer Formation and Stability on a Nickel-Base Alloy in Impure Helium at High Temperature

The corrosion behavior in impure helium of Haynes 230, a nickel base alloy candidate for heat exchangers in Very High Temperature Reactors (VHTR), has been investigated. The study focused on the formation and the subsequent destruction of the surface oxide layer at 900 °C and 980 °C. In-situ gas-phase analysis coupled to post-exposure surface analyses showed that a chromium-rich surface oxide formed on Haynes 230 at 900 °C but was unstable above a critical temperature T _A : the chromium-rich oxide reacted with carbon in solution in the alloy to produce chromium and CO(g). The effect of carbon monoxide partial pressure in the gas phase as well as the influence of chromium and carbon activities in the alloy on T _A are discussed taking thermodynamics and kinetics aspects into account.     

Oxidation behaviour of some nickel base alloys in carbon dioxide at 900–1000°

Abstract

The oxidation behaviour of three nickel base alloys, EPE 16, Nimonic 75 and Hastelloy X, has been studied in 1 atm of CO₂ at 900°, and in the case of Hastelloy X at 1000°, for a maximum exposure period of 8000 h. Weight gain data were obtained and changes in the subsurface of the metal were examined. The structure of the metal could be altered up to a maximum depth of 200 μ by void formation, internal oxidation and decarburisation. Hastelloy X was the most resistant alloy and an aluminised coating greatly increased the resistance of EPE 16. With the exception of one batch of Nimonic 75, the addition of 5–10% of CO to the gas did not influence the behaviour. The decarburisation of the surface of Nimonic 75 in 3 × 10^-3 atm of CO₂ was studied at 900° using the radioactive tracer ¹⁴C. Carbon was transferred both to the gas phase by the reaction CO₂ + C (in the metal) → 2CO, and to the centre of the specimen. After the initial oxidation, the rate of carbon transfer to the gas was approximately 6 × 10^-3 μg/cm² h decreasing to 3 × 10^-3 μg/cm² h at the end of the 1900 h exposure. Over half the carbon removed from the surface diffused into the centre of the specimen. This diffusion has been attributed to chromium depletion at the surface to form a chromium-rich oxide, being primarily responsible for the decomposition of the chromium carbide phase in the metal.     

The Isothermal and Cyclic Oxidation Behavior of a Titanium Aluminide Alloy at Elevated Temperature

The isothermal and cyclic oxidation behavior of Ti-47Al-2Mn-2Nb with 0.8 vol.% TiB₂ particle-reinforced alloy was investigated in air between 700 and 1000 °C. In the study, the kinetics of isothermal and cyclic oxidation were performed by using a continuous thermogravimetric method which permits mass change measurement under oxidation conditions. The oxide scales and substrates were characterized by scanning electron microscopy with energy-dispersive x-ray analysis and x-ray diffraction. At 700 and 800 °C, the alloy showed an excellent oxidation resistance under isothermal and cyclic conditions. After exposure to air above 800 °C, the outer scale of the alloy was dominated by a fast-growing TiO₂ layer. Under the coarse-grained TiO₂ layer was the Al₂O₃-rich scale, which was fine-grained. At 900 and 1000 °C, the extent of oxidation increased clearly. The oxidation rate follows a parabolic law at 700 and 800 °C. However, the alloy, upon isothermal oxidation at 900 °C, can be divided into several stages. During the cyclic oxidation at 900 and 1000 °C, partial scale spallation takes place, leading to a stepwise mass change.     

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.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

The Effect of Si Additions on the High-Temperature Oxidation of a Ternary Ni–10Cr–4Al Alloy in 1 ATM O2 AT 900–1000 °C

The oxidation of four Ni–10Cr–ySi–4Al alloys has been studied in 1 atm O₂ at 900 and 1000 °C to examine the effects of various Si additions on the behavior of the ternary alloy Ni–10Cr–4Al, which during an initial stage formed external NiO scales associated with an internal oxidation of Cr + Al, later replaced by the growth of a chromia layer at the base of the scale plus an internal oxidation of Al. The addition of 2 at.% Si was able to prevent the oxidation of nickel already from the start of the test, but was insufficient to form external alumina scales at 1000 °C, while at 900 °C alumina formed only over a fraction of the alloy surface. At 1000 °C the addition of 4 at.% Si produced external chromia scales plus a region of internal oxidation of Al and Si, a scaling mode which formed over a fraction of the alloy surface in combination with alumina scales also by oxidation at 900 °C. Conversely, the presence of about 6 at.% Si produced external alumina scales over the whole sample surface at 900 °C, but only over about 60 % of the alloy surface at 1000 °C. The changes in the oxidation modes of the ternary Ni–10Cr–4Al alloy produced by Si additions have been interpreted by extending to these quaternary alloys the mechanism of the third-element effect based on the attainment of the critical volume fraction of internal oxides needed for the transition to the external oxidation of the most-reactive-alloy component, already proposed for ternary alloys.     

A Study on Effect of Reactive and Rare Earth Element Additions on the Oxidation Behavior of Mo–Si–B System

Mo–9Si–8B–1Ti, Mo–9Si–8B–1.8Ti, Mo–9Si–8B–0.2La and Mo–9Si–8B–0.4La₂O₃ (at.%) alloys were prepared using mechanical alloying followed by hot isostatic pressing and field assisted sintering. XRD, SEM and EBSD analysis confirmed the formation of Mo solid solution, A15 and T2 phases in the alloys. Isothermal oxidation behavior of the specimens was studied in the temperature range from 750 to 1,300 °C for up to 100 h. Both the Ti and La containing alloys showed superior oxidation behavior compared to unalloyed Mo–Si–B at 900 °C at the initial periods of oxidation. Ti-added alloys suffered higher rate of weight loss at higher temperatures (1,000–1,300 °C) due to the formation of non-protective low viscosity SiO₂-TiO₂-B₂O₃ scale. La-alloyed Mo–Si–B showed superior oxidation resistance at intermediate temperatures (900 °C) as well as at higher temperatures. Enrichment of La at the oxide/alloy interface was found to be the reason for improved oxidation behavior of La-alloyed Mo–Si–B. Amongst the four materials studied, the La₂O₃ containing alloy showed the best oxidation resistance at 900 °C.     

Oxidation Kinetics of AISI 441 Ferritic Stainless Steel at High Temperatures in CO2 Atmosphere

Ferritic stainless steels used as interconnectors in SOFC stacks are subjected to air and fuel atmospheres at 800 °C. The use of hydrogen as fuel gas may be substituted by fermentative biogas consisting of mainly CO₂ and CH₄. In this gas mixture, carbon dioxide leads to steel oxidation whereas methane induces carburization. The objective of this study was to investigate the oxidation kinetics of the AISI 441 ferritic stainless steel under pure CO₂ in order to understand oxidation mechanisms. The results show that the kinetic behaviour is linear at low temperatures (800–900 °C) and initially linear then parabolic at higher temperatures (925–1,000 °C). Oxide scale consisted of major Cr₂O₃-rich oxide, topped with MnCr₂O₄ and a dispersion of TiO₂. The chromium-rich oxide was analysed by using the photoelectrochemical method. It exhibits N-type semi-conductor. Oxidation kinetics is modelled by the mixed surface and oxide-diffusion limited steps.     

Oxidation of Minor Elements from an Iron–Nickel–Chromium–Cobalt–Phosphorus Alloy in 17.3% CO2–H2 Gas Mixtures at 700–1000 °C

Fe–Ni–Cr–Co–P alloys were exposed to 17.3% CO₂–H₂ gas mixtures to investigate the oxidation of minor elements in metallic alloys in the early solar system. Reaction temperatures varied between 700 and 1000 °C. Gas-phase equilibrium was attained at 800, 900, and 1000 °C, yielding H₂–H₂O–CO–CO₂ gas mixtures. Experiments at 700 and 750 °C did not achieve gas-phase equilibrium and were performed in H₂–CO₂ gas mixtures. Reaction timescales varied from 1 to 742 h. The experimental samples were characterized using optical microscopy, electron microprobe analysis, wavelength-dispersive-spectroscopy X-ray elemental mapping, and X-ray diffraction. In all experiments Cr experiences internal oxidation to produce inclusions of chromite (FeCr₂O₄) and eskolaite (Cr₂O₃) and surface layers of Cr-bearing magnetite [(Fe,Cr)₃O₄]. At 900 and 1000 °C, P is lost from the alloy via diffusion and sublimation from the metal surface. Analysis of P zoning profiles in the remnant metal cores allows for the determination of the P diffusion coefficient in the bulk metal, which is constant, and the internally oxidized layer, which is shown to vary linearly with distance from the metal surface. At 800 and 900 °C, P oxidizes to form a surface layer of graftonite [Fe₃(PO₄)₂] while at 700 and 750 °C P forms inclusions of the phosphide-mineral schreibersite [(Fe,Ni)₃P].     

The Oxidation of cobalt-tantalum base alloys containing carbon

The oxidation of cobalt-tantalum carbon alloys, containing 10 and 15 wt.% Ta and carbon in the range 0–1 wt%, was carried out in oxygen and air at atmospheric pressure at 900, 1000 and 1100°C. The alloys oxidised according to the parabolic rate law with activation energy of about 38 Kcal/mole. In general, the addition of tantalum decreases the oxidation rates, in comparison with cobalt and with the same mass of chromium added to cobalt. Again, the presence of carbon in the Co-Ta alloys decreases its oxidation rates in comparison with carbon-free alloys. The scales formed on Co-Ta and Co-Ta-C alloys consist mainly of an outer layer of cobalt oxide, CoO, and an inner porous layer of mixture of oxides: cobalt oxide; CoO, tantalum oxide; Ta₂O₅, and solid solution of these two oxides; CoTaO₄ at all temperatures in the range of 900°-1100°C. The binary Co ?10% Ta and Co ?15% Ta show an internal oxidation along the internal phase, increasing of alloy tantalum content increases the density of the internal phase. The presence of carbon in the ternary Co-Ta-C alloys has little effect and there is no apparent preferential penetration along the tantalum carbide network. In contrast to carbide present in Co-Cr-C alloys, where these carbides were preferentially attacked, the outer scale was disrupted, due to the formation of carbon gaseous oxides.     

Increasing the Upper Temperature Oxidation Limit of Alumina Forming Austenitic Stainless Steels in Air with Water Vapor

A family of alumina-forming austenitic (AFA) stainless steels is under development for use in aggressive oxidizing conditions from ~600?C900 °C. These alloys exhibit promising mechanical properties but oxidation resistance in air with water vapor environments is currently limited to ~800 °C due to a transition from external protective alumina scale formation to internal oxidation of aluminum with increasing temperature. The oxidation behavior of a series of AFA alloys was systematically studied as a function of Cr, Si, Al, C, and B additions in an effort to provide a basis to increase the upper-temperature oxidation limit. Oxidation exposures were conducted in air with 10% water vapor environments from 800?C1000 °C, with post oxidation characterization of the 900 °C exposed samples by electron probe microanalysis (EPMA), scanning and transmission electron microscopy, and photo-stimulated luminescence spectroscopy (PSLS). Increased levels of Al, C, and B additions were found to increase the upper-temperature oxidation limit in air with water vapor to between 950 and 1000 °C. These findings are discussed in terms of alloy microstructure and possible gettering of hydrogen from water vapor at second phase carbide and boride precipitates.     

两种铸造镍基合金的高温氧化行为

利用热重分析法、XRD和SEM (EDS)对比研究了700℃超超临界发电机组用K317和K325铸造合金在900和1000℃大气环境下氧化行为。结果表明,K317的氧化性能要优于K325。在900℃氧化时,2种合金的氧化增重遵循抛物线规律,而在1000℃氧化时,氧化增重均分段遵循抛物线规律。K317的氧化膜分3层,外层是NiO、TiO_2和NiCr_2O_4,中间层是致密的Cr_2O_3,内层是内氧化产物Al_2O_3。而K325的氧化膜分2层,外层是NiO, NiCr_2O_4和Nb_2O_5,内层是致密的Cr_2O_3和嵌入的Nb_2O_5颗粒,没有内氧化现象发生。在1000℃氧化时,K325中的Mo严重被氧化形成挥发性MoO_3;同时氧化膜发生了局部剥落现象,氧化膜的附着性相对较差。     

Hot Corrosion Behavior of a Ni3Al-Based IC21 Alloy in a Molten Salt Environment

Ni₃Al-based alloys have become important candidates for hot components in turbine engines, owing to their low densities and outstanding mechanical properties in service environments. The hot corrosion behavior of a Ni₃Al-based IC21 alloy in a molten salt environment of 75 wt% Na₂SO₄ and 25 wt% NaCl at 900 °C was studied, via oxidation kinetics analyses, scanning electron microscope observations and energy dispersive as well as diffraction analyses by X-ray. A multilayer corrosion oxide scale and dendritic morphology internal corrosion zone formed after hot corrosion, and inter-phase selective corrosion phenomena were also observed. Salt fluxing and oxidation-sulfidation processes were inferred to be the essential hot corrosion mechanisms of the alloy. Moreover, additions of Cr and Y proved to be beneficial to the hot corrosion resistance of the IC21 alloy, while the Mo content should be strictly controlled.