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.     

Thermogravimetric Study of Oxidation-Resistant Alloys for High-Temperature Solar Receivers

Three special alloys likely to be suitable for high-temperature solar receivers were studied for their resistance to oxidation up to a temperature of 1050°C in dry atmospheres of CO₂ and air. The alloys were Haynes HR160, Hastelloy X, and Haynes 230, all nickel-based alloys with greater than 20% chromium content. The oxidation rate of specimens cut from sample master alloys was followed by thermogravimetry by continuously monitoring the weight change with a microbalance for a test duration of 10 h. The corrosion resistance was deduced from the total weight increase of the specimens and the morphology of the oxide scale. The surface oxide layer formed (scale) was characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy and in all cases was found to be chromia. Oxidation was analyzed by means of parabolic rate law, albeit in some instances linear breakaway corrosion was also observed. For the temperature range investigated, all alloys corroded more in CO₂ than in air due to the formation of a stronger and more protective oxide scale in the presence of air. At 1000°C, the most resistant alloy to corrosion in CO₂ was Haynes 230. Alloy Haynes HR160 was the most oxidized alloy at 1000°C in both CO₂ and air. Hastelloy X oxidized to a similar extent in CO₂ at both 900°C and 1000°C, but in air, it resisted oxidation better at 1000°C than either at 900°C or 1000°C.     

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.     

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900�C1000?��C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates ??-Ni₃Nb and/or (Ni, Mo)₆C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of ??-Ni₃Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The ??-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)₆C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.     

Aufkohlung von Chrom-Nickel-Eisen-Stählen in der Kohlenstoffpackung

Carburization of chromium nickel steels in a carbon bed Annealing the samples in carbon powder is a simple method for testing the carburization behaviour of heat resistant CrNiFe steels. It is shown that according to the thermodynamic conditions during this test a very thin layer of Cr₂O₃ is formed on the surface of the samples in the temperature range 900° to 1050°C. This layer virtually prevents carburization. Above 1050°C. The oxide layer is transformed to a carbide and carburization of the alloy can take place without restraint. However, the influence of temperature on carburization as described may not apply to service conditions, in particular, carburization may also occur if the oxide layer is thermodynamically stable but porous and fissured. Upon carburization carbon is diffusing into the alloy and reacting with chromium and iron under formation of the carbides M₇C₃ in an outer zone and M₂₃C₆ in an inner zone. The penetration of these zones is also determined by the solubility of carbon in the carburized region. By a high nickel content of the alloy the carbon solubility is diminished and therefore the rate of carburization is retarded. This influence of the nickel content also shows under service conditions.     

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.     

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.     

Oxidation of FeCrAl alloys at 500–900°C in dry O2

This paper reports on the oxidation of a commercial FeCrAl alloy, Kanthal AF, in the temperature range 500–900°C. The samples are exposed isothermally in dry oxygen for up to 72 h using a thermo‐balance. In addition, 168 h exposures are carried out in a tube furnace. The exposed samples are investigated by grazing angle X‐ray diffraction (XRD), scanning electron microscopy/energy dispersive X‐ray analysis (SEM/EDX), and auger electron spectroscopy (AES). The rate of oxidation increases with temperature, the kinetics being parabolic in the range 700–900°C. At all exposure temperatures, most of the sample surface is covered by a thin smooth base oxide. In addition, RE‐rich particles, with a typical size of 1–3 μm form. At 800 and 900°C patches of thick oxide appear, featuring needle‐formed crystallites situated on top of the base oxide. The thick oxide usually forms around Y‐rich oxide particles. The concentration of iron and chromium in the oxide decreases with increasing temperature. XRD proves the formation of α‐Al₂O₃ already at 700°C. The low temperature of formation of α‐Al₂O₃ is attributed to the presence of chromium in the initial oxide. It is proposed that corundum nucleation is facilitated on a surface consisting of the isostructural escolaite, (Cr₂O₃). After exposure at 900°C AES shows large amounts of Mg in the outer part of the oxide, MgAl₂O₄ being detected by XRD together with γ‐ and γ‐Al₂O₃.     

High temperature corrosion of alloy Haynes 556 in carburizing/oxidizing environments

The iron based alloy Haynes 556 has been recently used in hydrocarbon and carbonaceous environments due to its excellent resistance to carburization. This alloy outperforms stainless steels and some of the best commercial carburization‐resistant nickel‐based alloys. This paper is concerned with the behavior of alloy Haynes 556 in high temperature carburizing environments containing trace amounts of oxygen. Thermal cyclic exposures were conducted in 2% and 10% CH₄/H₂ gas mixtures at 800, 900, 1000, and 1100°C for 10 cycles, 50 h each. Carbon activities, oxygen partial pressures, and stabilities of oxides and carbides were used to identify the role played by the reaction products in providing protection. Thermodynamic analyses, weight changes, and microstructural characterization were correlated with environmental parameters and alloy composition to elucidate the causes of its marked resistance to carburization. The results indicate protective character in both gas mixtures under all exposure conditions except the most aggressive, namely 1100°C in 10% CH₄/H₂ gas mixture. Below 1000°C, the formation of Cr, Al, and Si oxides along with Cr carbides provides the primary means of protection. Catastrophic failure at 1100°C was manifested by extensive fracture and crack development within the outer substrate surface in the 10% CH₄/H₂ gas mixture resulting in a dramatic increase in weight gain. This has been attributed to the increased carbon pick‐up, coupled with the loss of the protective outer scales.     

Selective Oxidation of Chromium by O2 Impurities in CO2 During Initial Stages of Oxidation

This study shows that the corrosion behaviour of 12 wt% Cr steel in CO₂ at 550 °C is determined in the first stage of oxidation by reaction with O₂ impurities. Depending on the amount of theses impurities and the thermal ramp rate, selective oxidation of chromium could lead to the formation of a protective chromium-rich oxide. An oxidation model describing qualitatively the nature of the oxide layer formed in the initial period of oxidation is presented. From these observations, surface engineering processes for protecting 9–12 wt% chromium steels from fast corrosion rate have emerged.     

Water Vapour Effect on Ferritic 4509 Steel Oxidation Between 800 and 1000?��C

The 4509 alloy (Fe?C18Cr?CNb?CTi) was oxidised in dry and wet air in the 800?C1000 °C temperature range. Results showed that the formation of a chromia layer acts as a good diffusion barrier under isothermal conditions at 800 and 900 °C, under 7.5 vol.% water vapour and dry air. Nevertheless, a breakaway is generally observed at 1000 °C, under wet air 7.5 vol.% H₂O. It is proposed that the oxidant H⁺/OH^? species react at the internal interface with iron in the chromium-depleted alloy zone. Wüstite reacts with Cr₂O₃ to form FeCr₂O₄. Outward iron diffusion leads to Fe₃O₄ and Fe₂O₃ formation. The chromia scale was consumed by reaction with wüstite, but chromia also internally forms owing to a chromium oxidation process with the inner chromium-rich alloy area.     

Effect of CeO<Subscript>2</Subscript> Coating on the Isothermal Oxidation Behaviour of Ni-Based Alloy 230

Ceria coating was deposited on alloy 230 using an electro-deposition process in a cerium nitrate electrolyte. Reactive element effects were investigated by comparing the oxidation behaviour of the samples with and without the ceria coating. The prepared ceria coating reduced the oxidation rate and improved the adherence of the oxide layer. A thicker oxide layer with large spallation areas formed on the uncoated sample, while a thin and protective oxide layer was found on the coated sample after oxidation for 1000 h at 900 °C. The oxidation mechanisms of alloy 230 with and without ceria coating were discussed. Furthermore, as the ceria coating changed the grain shapes of chromium oxides from columnar to an equiaxed structure, discussion also advanced proposals regarding the mechanisms of formation of these different oxides. The equiaxed grains enhanced the adhesion of the oxide to the alloy surface.     

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.     

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。     

Metal dusting of nickel-base alloys

Metal dusting, the disintegration of metallic materials into fine metal particles and graphite was studied on nickel, Fe Ni alloys and commercial Ni-base alloys in CO H₂ H₂O mixtures at temperatures between 450–750°C. At carbon activities a_c > 1 all metals can be destroyed into which carbon ingress is possible, high nickel alloys directly by graphite growth into and in the material, steels via the intermediate formation of instable carbide M₃C. Protection is possible only by preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of the alloy and by a surface treatment which generates fast diffusion paths for the supply of chromium to the surface. The metal dusting behaviour of Alloy 600 is described in detail. A ranking of the metal dusting resistance of different commercial nickel-base alloys was obtained by exposures at 650°C and 750°C.     

Influence of SO2 on the Oxidation of 304L Steel in O2 + 40%H2O at 600 °C

The effect of sulphur dioxide on the oxidation of alloy 304L in O₂ + 40%H₂O has been investigated at 600 °C. A protective chromium-rich corundum-type oxide forms in clean dry O₂. Exposure to O₂ + 40%H₂O environment results in chromium vaporization in the form of CrO₂(OH)₂. This causes local failure of the protective oxide and the formation of 10 μm thick oxide islands on the alloy grain centers. The oxide islands are layered, the outer part consisting of hematite while the inner part is FeCrNi spinel oxide. The addition of 100 ppm SO₂ to O₂ + 40%H₂O reduces the corrosion rate compared to O₂ + 40%H₂O. SO₂ is suggested to influence oxidation by two separate effects. Firstly, SO₂ forms surface sulfate on the oxide surface that impedes the vaporization of chromium from the protective oxide. This slows down the breakdown of the protective oxide. Secondly, SO₂ also influences the rapid oxidation that ensues once the protective oxide has been destroyed. In this case, the presence of surface sulfate interferes with the surface reactions involved in oxidation. In this way, SO₂ slows down the growth of the oxide islands.     

Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment

Corrosion of four alloys has been studied in supercritical carbon dioxide at 650 °C and 20 MPa, specifically AL-6XN stainless steel and three nickel-based alloys, PE-16, Haynes 230, and Alloy 625. The tests were performed for exposure durations of up to 3000 h with samples being removed for analyses at 500 h intervals. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide layers were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Weight gain measurements showed that the Al-6XN stainless steel exhibited the least corrosion resistance while the weight gains were nearly similar for the other alloys. The oxide layer in AL-6XN stainless steel was composed of large equiaxed grained outer layer of Fe₃O₄ (magnetite) and an inner layer of FeCr₂O₄. Oxide spallation was observed in this stainless steel even after 500 h exposure. In all alloys, Cr-rich oxides phases of Cr₂O₃ and Cr_1.4Fe_0.7O₃ were identified as the protective layers. In alloy PE-16 a thin layer of aluminum oxide formed that promoted the corrosion resistance of the alloy. Cr₂O₃ was identified as the main protective oxide layer in nickel base alloys Haynes 230 and 625.     

Oxidation mechanism of cobalt based alloy at high temperatures (800–1100°C)

Abstract

A cobalt based Phynox alloy has been oxidised in the 800–1100°C temperature range. Kinetic results show that the parabolic behaviour is followed under isothermal conditions. The scale growth mechanism of cobalt based Phynox alloy in air is consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Thermal cycling tests show that the best scale adherence is found at 1000°C. This temperature permits a rapid chromium supply from the substrate to form a continuous chromia scale. A keying effect at the internal interface is promoted by the presence of silicon and molybdenum. At 900°C, CoCr₂O₄ cobalt containing oxide formation is favoured and leads to a bad scale adherence. At 1100°C, thermal cycling conditions lead to scale spallation and chromium depletion. Then, important weight losses are registered corresponding to the oxidation of cobalt and molybdenum to induce CoCr₂O₄ and CoMoO₄ formation.     

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.     

Effects of cold work on the oxidation behavior and carburization resistance of Alloy 800

The structure of the oxide layer formed on Alloy 800 at 600 °C in superheated steam markedly indicates the role of the grain boundaries as easy diffusion paths of Cr and Mn to the alloy/oxide interface. Increasing the number of grain boundaries by 10-90% cold work leads to increasing Cr- and Mn-content in the scale and to decreasing oxide growth rates. Variation of the grain size by different annealing treatments leads – since the Cr-content in the scale is decreasing with the grain size – to a linear relation of growth rate and grain size. The effect of cold work was also demonstrated on the protectiveness of the oxide scale towards carbon uptake and carburization of Alloy 800. After preoxidation of differently deformed specimens at 900 °C, these were exposed to a CO-CO₂H₂O-H₂ mixture at 700 °C for long time. The gas mixture was tagged with ¹⁴C so that the C-ingress into the oxide scale and into the alloy could be sensitively monitored by autoradiography and (upon stepwise polishing) radioactivity measurements of the carbon penetration. The carbon uptake is effectively reduced with cold working; in contrast a non-deformed, electropolished and preoxidized specimen shows relatively high C-content after exposure. The investigations prove the highly favorable effect of mechanical pretreatment on the formation of the oxide scale on an austenitic Fe-Ni-Cr alloy. Cold work and other methods of surface deformation (grinding, polishing, sand blasting, shot peening) generate easy diffusion paths for fast Cr-diffusion to the surface and sufficient supply of Cr to form a protective oxide layer.