The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000

Abstract

Preliminary studies have been undertaken on cyclic and isothermal oxidation at 1,300°C of thin (125 μm) samples of commercial ODS alloy PM2000 for up to 350h in two different oxidising environments; dry and moist air. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) have been used to study the influence of such environments on alumina scale formation and growth. Initial mass gain observations showed that the alumina scale, which formed on the samples oxidised in air+2.5vol% H₂O grew faster in the early stages of oxidation than in the case of dry air. However the SEM analysis revealed that the scale morphologies in both dry air and air+2.5vol% H₂O were similar. In both cases the scales consisted of equiaxed grains at the scale–gas interface with Ti-rich particles in the outermost part of the scale. The major factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in moist air than in dry air     

Kinetic and metallographic study of oxidation at high temperature of cast Ni 25Cr alloy in water vapour rich air

Abstract

The high temperature oxidation behaviour of a Ni–25 wt-%Cr alloy in air enriched with water vapour (180 mbars H₂O) was studied at 1000, 1100, 1200 and 1300°C. The oxidised samples were characterised by X-ray diffraction, electron microscopy and wavelength dispersion spectroscopy. The obtained data were compared to the ones earlier obtained for the same alloy oxidised in dry air. Water vapour globally induced at all temperatures a decrease of the parabolic constant Kp and an increase in the chromia volatilisation constant Kv. The oxide scales do not present morphologic difference between the two atmospheres. After oxidation in humidified air the scale thickness is thinner and the Cr depleted depth is lower than in dry air.     

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

Abstract

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO₂ and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO₂–30% H₂O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO₂ and Ar–H₂O. It was found that in the CO₂- and/or H₂O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H₂O-containing gases and carbide formation in the CO₂-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO₂(–H₂O)-rich gases appeared to be permeable to CO₂ molecules resulting in substantial carburization of the steel.     

The isothermal and cyclic oxidation behaviour of Ti-48Al-2Cr at 700°C

The effect of the microstructure on the isothermal and cyclic oxidation behaviour of the intermetallic Ti–48A1–2Cr (at.%) alloy was investigated at 700°C in air up to 3000 h. Different microstructures, i.e., duplex, near gamma, nearly lamellar, and fully lamellar, obtained by various heat treatments, were used. Results of thermogravimetry showed a good oxidation resistance at 700°C against both isothermal and cyclic oxidation. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. After equal exposure times, cyclic oxidation induced a higher oxidation rate compared to isothermal oxidation. Oxidation of Ti–48A1–2Cr in air, initially resulted in the formation of α-A1₂O₃, TiO₂ (rutile), Ti₂A1N and TiN, with the latter two near the scale/substrate interface. After longer exposure times, the mixed corrosion scale was overgrown by fast growing TiO₂. The oxide scales, formed under isothermal as well as under cyclic conditions, were uniform.     

The behaviour of commercial FeCrAlRE alloys in nitrogen-oxygen – water vapour bioxidant environments

Abstract

The corrosion behaviour of a range of commercial FeCrAlRE alloys (MA956, ODM751, PM2000, Kanthal AF, Kanthal APM and Aluchrom YHf) have been examined in nitrogen–oxygen-H₂O or N₂–H₂–H₂O bioxidant environments, at temperatures between 1100°C and 1350°C. The corrosion behaviour is governed by the competition between oxidation leading to protective alumina formation/ maintenance and nitrogen ingress leading to nitridation of the matrix alloy. Key issues addressed by four series of experiments, have included: the influence of a pre-formed protective alumina scale; the oxidant level required to form/reheal a protective oxide scale; the role of mechanical failure of the scale above the critical thickness for cracking/spallation in oxygen rich environments; chemical failure of the protective oxide scale leading to breakaway (non-protective) attack and in particular, the potential roles in such failure processes of nitridation concurrent with, and following defective oxide scale formation, and of oxidation following nitridation.

Detailed characterisation of the chemical composition and physical microstructure of the attack of the respective alloys was undertaken using a range of surface analytical techniques, including X-ray diffraction, optical and scanning electron microscopy and energy dispersive X-ray analysis.     

Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Abstract

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.

Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.     

Influence of the oxidation environment on scale morphology and oxidation rate of Fe–22Cr

Abstract

The oxidation behaviour of a commercial Fe–Cr alloy with 22 wt% Cr was investigated at 1173K in Ar–9 H₂ with 1% H₂O (pO₂ = 9.8 × 10^?19), in air with 1% H₂O (pO₂ = 0.208), and in a combination of the two atmospheres. The oxide morphology was investigated with X-ray diffraction and scanning electron microscopy. The oxide layer consisted of MnCr₂O₄ on top of Cr₂O₃.

Small oxide whiskers were present at the surface after oxidation in Ar–9 H₂ with 1% H₂O but not after oxidation in air with 1% H₂O. For samples initially oxidised in air with 1% H₂O, the oxide/alloy interface was wrinkled and covered with a SiO₂ layer. SiO₂ particles had developed at a rather flat oxide/alloy interface for samples initially oxidised in Ar–9% H₂ with 1% H₂O. The results obtained can be explained assuming that oxide growth occurs by cation diffusion only in Ar–9 H₂ with 1% H₂O, whereas both cation and anion diffusion contribute to the growth in air/H₂O.     

The effect of H2-anneal on the adhesion of Al2O3 scales on a Fe3Al-based alloy

Abstract

The cyclic (1,100°C, air) and isothermal (1,000°C, O₂) oxidation behavior of a Fe-28Al-5Cr (at%) alloy, with and without a prior H₂-anneal heat treatment at 1,200°C for 100 h, was studied. Changes in interfacial chemistry were evaluated using Scanning Auger Microscopy after removal of the oxide film in ultra high vacuum. This was achieved by making a scratch on the specimen surface, which caused spallation of the film at various locations along the scratch. The scale thickness and the temperature drop at which spallation took place during cooling were utilized to semi-quantitatively compare the adherence of the scales. Porosity at the scale–alloy interface and the scale microstructure were determined from scanning electron microscope observations. It was found that H₂-anneal greatly improved scale adhesion and resulted in a pore-free and sulfur-free interface. The effects were similar to that of a 0.1 at% Zr-containing alloy, except that the improvement in scale adhesion was not as great as that from Zr doping. This implies that oxide/alloy interfaces are not intrinsically strong and the effect of reactive elements, such as Zr, is more than preventing impurity from segregating to the interface. Results are also compared with the effect of H₂-anneal on other model alloys, such as NiCrAl, FeCrAl and NiAl, and on single crystal superalloys.     

In situ ESEM investigations of the oxide growth on hot work tools steel: effect of the water vapour

Abstract

Tempered martensitic steel modified AISI H11 is used in forging processes where tool failure can result from the combination of thermo-mechanical and chemical damage. A better knowledge of the oxidation mechanisms in this material could be useful for a better appreciation of its service behaviour and lifetime. The low chromium content of this Fe–Cr type steel allows the development of mainly Fe_2–xCr_xO₃ oxides with corundum structure and leads to enhanced oxidation in the presence of water vapour.

In situ FEG–ESEM images show the scale microstructural modifications during high temperature exposure, as well as the lateral growth of oxide particles. Together with GIXRD, SEM/EDS and SIMS analysis, FEG–ESEM also allows assessment of the H₂O effect on oxidation behaviour during high temperature exposures (600 and 700°C). Water vapour induces either pores or crystallites size increase, favours faceted oxides particles with enhanced density at the highest partial pressure. At this microscopic scale, anisotropic growth of crystallites is observed, and size expansion rates are found to be linear and characteristic of each individual particle.

Temperature acts principally on oxide film microstructure. Whatever the environment, homogeneous scale growth is observed at 600°C whereas the steel surface is heterogeneously covered by oxides at 700°C.     

High temperature oxidation behaviour of directionally solidified nickel base superalloy CM–247LC

Abstract

The present paper describes the isothermal and cyclic oxidation behaviour of the technologically important nickel base directionally solidified superalloy CM-247LC in air in the temperature range 1000-1200°C. This superalloy behaves as a transition nickel base alloy under isothermal oxidation conditions and exhibits a fairly long transient oxidation period (~20 h at 1100°C). Irrespective of the temperature of exposure and nature of oxidation (isothermal or cyclic), a composite oxide scale develops on CM-247LC. While the outer portion of the oxide scale consists of either spinel (NiAl₂O₄) or a mixture of spinel and NiO, depending on oxidation temperature, the inner portion is always constituted of alumina. Beyond the transient period, the alloy is found to follow parabolic oxidation kinetics. The oxide layer that forms is invariably very non-uniform in thickness, and is dispersed with two types of oxide particles. While tantalum rich oxide particles are found scattered in the outer zone of the oxide layer, hafnium rich oxide particles lie close to the oxide/metal interface. Results also reveal that the nature of oxidation associated with the CM-247LC superalloy causes entrapment of metal islands in the oxide layer.     

A study of breakaway oxidation of 9Cr–1Mo steel in a Hot CO2 atmosphere using Raman spectroscopy

Abstract

The microstructure of the oxide scale and parent metal in ferritic 9Cr–1Mo steel was observed to explore the oxidation and carburisation mechanisms upon exposure to a CO₂ gas environment at high temperature and high pressure. An experimental 9Cr–1Mo steel sample that had been oxidised at 580 °C for more than 165,000 hrs in coolant gas consisting primarily of CO₂ was analysed. To elucidate the oxidation characterisation, scanning Raman spectrometry was used to analyse the oxide close to the metal/oxide interface. Carbon was found to be deposited in the spinel layer. The microstructure and the distribution of elemental chemical composition were examined and analysed using optical and scanning electron microscopy combined with energy dispersive X-ray analysis. The results are discussed with relation to understanding the mechanisms of oxidation and carburisation which aim to underpin extension of the service life of components fabricated from this steel.     

Oxidation behaviour and microstructural evolution of FeCrAl ODS alloys at high temperature

Abstract

The oxidation behaviour of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (MA 956) and of a dispersion-free FeCrAl alloy (APM) were studied during isothermal exposures in air between 1,100°C and 1,300°C. After short heat treatments, the oxide film developed on MA 956 was more protective than that developed on APM. Longer isothermal exposures were conducted on MA 956 in order to examine the long-term behaviour of the oxide layer and the microstructural evolutions of the alloy. Chemical analyses of the substrate for increasing treatment duration revealed a continuous decrease of the matrix aluminium content due to oxide growth. After 4 months at 1,300°C, aluminium content of the alloy decreased to a critical value below which a continuous and protective oxide film could no longer be formed. Longer exposure times were carried out in order to relate the different stages of the catastrophic breakaway oxidation and identify the parameters involved in this phenomenon. Other substrate evolutions like cavity growth and nature and morphology changes of the Y₂O₃ particles were observed and are discussed in relation with the oxidation behaviour.     

Evolution of oxide scale on a Ni–Mo–Cr alloy at 900 °C

The cyclic oxidation behavior of a Ni–Mo–Cr alloy was studied in air at 900 °C for exposure periods of up to 1000 h. The morphology, microstructure and composition of the oxide scale was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Oxidation kinetics was determined by weight gain measurements. The results show that steady state oxidation was achieved within 1 h of exposure. During transient oxidation, the alloy grain boundaries intersecting the alloy surface became depleted in Ni and enriched in Mo and Cr. The scale initially formed at the surface was NiO which grew outwardly. However, a protective Cr₂O₃ layer developed, rapidly retarding the rate of oxidation. Formation of NiMoO₄ was also observed. The presence of Mo in the alloy facilitated the formation of a Cr₂O₃ layer at an early stage of oxidation. The alloy exhibited considerable oxide spalling during prolonged exposure.     

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

Abstract

The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co₃O₄, Co(Ni)O and Ni-doped CoCr₂O₄, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr₂O₃ layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.     

Effect of Mo-alloyed layer on oxidation behavior of TiAl-based alloy

A Mo-alloyed layer was prepared on a TiAl-based alloy using plasma surface metallurgy technique. The microstructure and oxidation behavior of the alloyed TiAl were investigated. The surface alloyed layer with a thickness of approx. 40 μm and a gradient concentration distribution was composed of Ti₂MoAl, TiAl and Mo phases. Cycling oxidation test was carried out at 850 °C in air under an atmospheric pressure. The result of the oxidation kinetics test shows that the mass gain of the Mo-alloyed TiAl after exposure for 100 h was less than the value of the untreated TiAl substrate, exhibiting an improved oxidation resistance. The Al₂O₃-rich mixed oxide scale formed on the alloyed TiAl is supposed to be the main reason of the beneficial effect on the improvement of oxidation resistance for TiAl-based alloy.     

Breakdown of the protective oxide on 11 % Cr steel at high temperature in the presence of water vapor and oxygen,the influence of chromium vaporization

Abstract

We report on the effect of water vapor and oxygen on the oxidation of a ferritic/martensitic 11 % Cr steel (CrMoV11 1). The influence of pH₂O, exposure time, gas velocity and temperature was investigated. The samples were exposed to dry O₂, O₂+10 or 40 % H₂O for up to 336 hours. Total pressure was 1 atm (1.02 × 10⁵ Pa). The gas velocity was between 0.05 and 10 cm/s while temperature was in the range 450–700°C. The samples are investigated by thermogravimetry, GI-XRD, SEM/EDX, GDOES, FIB and TEM/EDX. Oxidation is strongly affected by the vaporization of CrO₂(OH)₂ in H₂O/O₂ environment. The mechanism of vaporization of CrO₂(OH)₂ from a Cr₂O₃ surface is modelled by DFT calculations. In the absence of chromium vaporization the alloy forms a protective oxide consisting of a corundum-type solid solution (Fe_1–xCr_x)₂O₃. The vaporization of chromium tends to deplete the oxide in chromium. In some cases the oxide remains protective in spite of chromium depletion while in other cases there is a transition to breakaway oxidation. In the latter case a thick layered scale forms, consisting of an outer hematite part and an inner iron-chromium spinel. Oxidation behavior in an O₂+H₂O environment is to a large extent determined by the ability of the metallic substrate to supply the oxide with chromium by diffusion in order to compensate for the losses by vaporization. The corrosivity of the environment increases with the concentration of water vapor and oxygen, with the gas velocity and with temperature.     

Role of molybdenum on the AISI 316L oxidation at 900 °C

In situ X-ray diffraction was used to study the oxide formation on AISI 316L stainless steel (SS) specimens during isothermal oxidation at 900 °C in air. Results were compared with those obtained on AISI 304 SS to determine the role of molybdenum on the oxidation process for the AISI 316L SS specimens. Our results show that molybdenum plays a major protective role during steel oxidation. This element is found in a NiMoO₄ phase at the internal oxide–metal interface. The high molybdenum content of the alloy hinders the outward diffusion of iron and leads to a lower growth rate and better scale adherence. The oxide scale is then composed of Cr₂O₃ with a small amount of Mn_1.5Cr_1.5O₄ at the external interface. The improved scale adherence appears to be due to a keying effect at the scale/alloy interface promoted by molybdenum.     

Oxidation of potential SOFC interconnect materials,Crofer 22 APU and Avesta 353 MA,in dry and humid air studied in situ by X-ray diffraction

Abstract

Several stainless steels have been developed for the use as interconnect materials in solid oxide fuel cells in order to reduce costs, while maintaining the required performance. The materials however, are subjected to humid air, at high temperatures up to 800°C leading to enhanced oxidation. High temperature X-ray diffraction, combined with field emission – scanning electron microscopy, has been applied to study in situ the influence of water vapour on the chromia scale formation on the ferritic Crofer 22 APU and the austenitic Avesta 353 MA alloys in comparison to their dry air oxidation behaviour. Both materials form at 800°C, during the first 100h exposure, Cr₂O₃ and MnCr₂O₄, the latter mostly in the surface region of the oxide scale. Additionally, Crofer 22 APU forms internal Al₂O₃ precipitates, while on Avesta 353 MA a SiO₂ layer is found beneath the outer oxide scale. High temperature XRD indicates stress formation and relaxation in the Cr₂O₃ scale formed in humid air, especially for Crofer 22 APU.     

Oxidation of Ni-based single crystal after grit-blasting during exposure at high temperature

The effect of grit-blasting on the oxidation of Ni-based single crystal at 1150 and 1250 °C was studied. The oxide scales formed on the samples with or without grit-blasting were characterized by SEM, XRD, EDS, and EPMA. The results indicate that grit-blasting introduces a plenty of oxide nodules with lamellar structure into the oxide scale of samples after oxidation. The oxide nodule is composed of external and internal part. The external part is multi-layered with outer Cr₂O₃ layer, inner Al₂O₃ layer and transitional layer. The internal part includes several alternative alloy layers lack of Al and Al₂O₃ layer. Hf and Ta segregate at the oxide/alloy and oxide/oxide interface in two parts. Two recrystallized grains are formed under the blasted alloy surface after grit-blasting and heat exposure. The formation of oxide nodules accelerates the development of equiaxed recrystallization. Grit-blasting introduces abundant paths of oxygen diffusion and residual stress into the alloy, promoting the formation of oxide nodules and recrystallization.     

Effect of oxygen partial pressure on the oxidation behaviour of an yttria dispersion strengthened NiCr-base alloy

An yttria dispersion strengthened NiCr-base alloy was studied with respect to isothermal oxidation behaviour at 1000 °C and 1050 °C in high- and low-pO₂ gases, i.e. Ar–O₂ and Ar(−H₂)–H₂O. The scale growth kinetics, morphology and composition were studied by thermogravimetry in combination with SEM/EDX and SNMS. Due to Y doping the surface scale is very protective and initially grows predominantly by inward oxygen diffusion. Local formation of mainly outwardly growing oxide nodules occurs after longer oxidation times and is related to metallic protrusions formed as a result of internal oxidation of the minor alloying addition aluminium. The differences in scale morphology in the various environments are related to the effect of the gas composition on scale grain size and on the relative amounts of inward scale growth. Possibly the pO₂ dependence of the Ti-solubility in the chromia scale and/or hydrogen doping of the oxide plays an additional role in the scale growth process.