Temperature dependence of metastable alumina formation during thermal oxidation of FeCrAl foils

Industrial FeCrAl foils were isothermally oxidized during 5 hours between 850 and 1000°C in atmosphere of pure oxygen. Characterization of transition and α‐alumina phases was performed by XRD and XPS, using reference spectra obtained by various air annealing treatments of pure γ‐Al₂O₃. An original model was proposed to deconvolute XPS spectra to obtain quantification of transition alumina formation and transformation. At 850°C, oxide scales on FeCrAl consisted of transition alumina, whereas higher temperature treatments resulted in decreased amounts of transition aluminas and in increasing α‐alumina formation. At 1000°C, the highest temperature studied, the scale could be described by XPS and XRD as pure alpha.     

The cyclic oxidation behavior of the single crystal TMS‐82+ superalloy in humidified air

The cyclic oxidation behavior of a single crystal Ni‐based superalloy TMS‐82+ was studied at 800 and 900 °C for 200 h in water vapor (air plus 15% H₂O). Regardless of the exposure temperature, time‐dependence of the growth rate of the scale for the superalloy was fitted by a subparabolic relationship. The oxidation rate was enhanced with increase in exposure temperature, which was evidenced by a higher mass gain and thicker scale. The oxides on the specimen at 800 °C consisted of (Ni,Co)O, CrTaO₄, AlTaO₄, Cr₂O₃, and θ‐Al₂O₃, whereas for the specimen exposed at 900 °C, spinels of NiCr₂O₄ and (Ni,Co)Al₂O₄ as well as α‐Al₂O₃ were observed. An innermost dense α‐Al₂O₃ layer was responsible for a stable growth rate of the scale after the initial rapid oxidation.     

Oxide growth stress measurements and relaxation mechanisms for alumina scales grown on FeCrAlY

Early‐stage tensile stress evolution in α‐Al₂O₃ scales during oxidation of FeCrAlY at 1000, 1050, 1100, and 1200 °C was monitored in situ by use of synchrotron radiation. Tensile stress development as a function of oxidation temperature indicated a dynamic interplay between stress generation and relaxation. An analysis of the time dependence of the data indicated that the observed relaxation of the initial tensile stress in the oxide scales at 1100 and 1200 °C is dominated by creep in the α‐Al₂O₃. A thin layer of a (Fe,Cr,Al) oxide was observed at the oxide‐gas interface, consistent with a mechanism whereby the conversion of (Fe,Cr,Al)₂O₃ to α‐Al₂O₃ produces an initial tensile stress in the alumina scale.     

Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr₂O₃ and an inner zone of isolated α‐Al₂O₃ precipitates in the alloy. The amounts of Cr₂O₃ and NiO in the oxide scale attain their maximum values when successively continuous Cr₂O₃ and α‐Al₂O₃ layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al₂O₃) in the oxide scale maintain at an approximately constant value. The formation of NiCr₂O₄ and subsequently NiAl₂O₄ happens as a result of solid‐state reactions between the oxide phases within the oxide scale.     

Isothermal oxidation behavior of Ti3Al-based alloy at 700–1000 °C in air

The isothermal oxidation behavior of a Ti₃Al-based alloy (Ti-24Al-14Nb-3V-0.5Mo-0.3Si, molar fraction, %) at 700– 1 000 °C in air was investigated. The oxidation kinetics of tested alloy approximately obeys the parabolic law, which shows that the oxidation process is dominated by the diffusion of ions. The oxidation diffusion activity energy is 241.32 kJ/mol. The tested alloy exhibits good oxidation resistance at 700 °C. However, when the temperature is higher than 900 °C, the oxidation resistance becomes poor. The XRD results reveal that the oxide product consists of a mixture of TiO₂ and Al₂O₃. Serious crack and spallation of oxide scale occur during cooling procedure after being exposed at 1 000 °C in air for 16 h. According to the analysis of SEM/EDS and XRD, it is concluded that the Al₂O₃ oxide forms at the initially transient oxidation stage and most of it keeps in the outer oxide layer during the subsequent oxidation procedure.     

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.     

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。     

High‐temperature oxidation of Ti3Al0.7Si0.3C2 compound at 900 and 1000 °C in air

The compound, Ti₃Al_0.7Si_0.3C₂, was synthesized by hot pressing a powder mixture of TiC_X (x = 0.6), Al and Si. Its oxidation at 900 and 1000 °C in air for up to 50 h resulted in the formation of rutile‐TiO₂, α‐Al₂O₃ and amorphous SiO₂. The oxide scales formed consisted of triple layers, viz, an outer TiO₂ layer containing Al₂O₃ particles, an intermediate Al₂O₃ layer, and an inner mixed layer that was rich in TiO₂, but deficient in Al₂O₃ and SiO₂. During oxidation, Ti diffused outwards to form the outer TiO₂ layer, and oxygen was transported inwards to form the inner mixed layer. At the same time, carbon was liberated from Ti₃Al_0.7Si_0.3C₂. Ti₃Al_0.7Si_0.3C₂ oxidized slower than the TiO₂‐forming kinetics, but faster than the (α‐Al₂O₃ or SiO₂)‐forming kinetics.     

Si对625合金激光熔覆层高温氧化特性的影响

利用循环氧化法,研究了不同Si含量（0%,1%,3%,质量分数）的625合金熔覆层在700、800、900 ℃下氧化144 h后的高温氧化行为。用XRD分析了氧化物相。通过SEM/EDS研究了氧化物表面和截面的形貌、元素组成和氧化膜的厚度。结果表明,不同温度下试样的氧化动力学都保持抛物线规律,随着温度的升高,氧化增重逐渐增加。通过观察,在900 ℃时,0% Si含量的625合金熔覆层出现了氧化膜大面积剥落的情况,3% Si含量的合金熔覆层氧化膜保持完整。在700 ℃时,随着Si含量增加,氧化膜表面的氧化颗粒尺寸减小且更加致密,同时促进了Cr₂O₃氧化物的生成。在700 ℃下,0 % Si含量的试样出现了大片的内氧化区域;1% Si含量的试样基体部分出现了2处条状的含Ni,Cr,Mo的氧化物相区;而3% Si含量的试样氧化后由于生成了富Si的内氧化层,这阻止了内氧化的发生。外层Cr₂O₃氧化膜和内层SiO₂的联合作用既阻止了O阴离子的渗入也抑制了Fe等金属离子的扩散,提高了合金熔覆层的抗氧化性。     

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.     

Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina‐forming alloy. Part III: The use of two stage oxidation experiments and in situ X‐ray diffraction to understand the oxidation mechanisms

The aim of this work was to investigate several different yttrium introduction routes to improve the high temperature oxidation resistance of a Fe‐20Cr‐5Al model alloy. Y₂O₃ sol‐gel coatings, Y₂O₃ metal‐organic chemical vapor deposition (MOCVD) coatings, yttrium ion implantation and yttrium as alloying element (0.1 wt.%) were the different methods of introduction of the reactive element. Both isothermal and cyclic oxidation tests showed that the surface introduction of yttrium or yttrium oxide did not drastically improve the oxidation behavior of the steel. Complementary experiments were performed to understand the lack of major beneficial effects of the so‐treated samples. Two stage oxidation experiments under 200 mbar ¹⁶O₂ and ¹⁸O₂ followed by secondary neutral mass spectrometry (SNMS) were performed to understand the alumina scale growth mechanisms, according to the introduction route of the reactive element. The results exhibited that the yttrium induced an increase of the inward transport of oxygen through the alumina scale compared to the untreated specimen. Nevertheless, the outward transport of aluminum was generally observed, except for the specimen containing Y as alloying element, which exhibited only a single¹⁸O peak close to the metal/oxide interface. Phase transformations during the oxidation at 1100°C were registered by in‐situ X‐ray diffraction (XRD). The untreated alloy was only covered by a thin layer of α‐Al₂O₃. For implanted specimens, yttrium was incorporated in Y₃Al₅O₁₂ and YAlO₃ phases. All the YAlO₃ is transformed into Y₃Al₅O₁₂ after less than 10 h. For the MOCVD or the sol‐gel coated samples, the primary formed YAlO₃ phase was progressively transformed into Y₃Al₅O₁₂. For the Fe‐20Cr‐5Al‐0.1Y alloy, no yttrium containing phases could be detected, even after 40 h of oxidation test at 1100°C.     

Short‐term oxidation and hot corrosion resistance of a gradient CrN/Cr1−xAlxN coating

A CrN/Cr_1?xAl_xN coating comprised of an inner layer of CrN and an outer layer of Cr_1?xAl_xN with a gradient distribution of Al was deposited on two different alloys by a reactive sputtering method. Oxidation and hot‐corrosion tests of the gradient CrN/Cr_1?xAl_xN coating were performed at different temperatures. The phase compositions and morphologies of the as‐deposited coating and the corrosion products were investigated by using XRD and SEM/EDS. The results showed that the gradient CrN/Cr_1?xAl_xN coating exhibited good oxidation resistance at temperatures above 1000 °C owing to the formation of an α‐Al₂O₃‐rich oxide scale. The coating possessed good hot‐corrosion resistance in molten sulfate because the inner CrN layer could supply enough Cr to form a relatively protective Cr₂O₃ after the Al₂O₃‐enriched scale failed due to its dissolution in the molten sulfate.     

α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.     

The high temperature oxidation of 11% chromium steel: Part I – Influence of pH2O

The oxidation of 11% Cr steel (X20 11Cr1MoV) in the presence of dry O₂ and O₂ + 10 and 40% H₂O was investigated at 600°C. The exposure time was between 1 and 672 hours. The oxidized samples were investigated by a number of surface analytical techniques including GI‐XRD, SEM/EDX, GDOES and Auger spectroscopy. X20 steel (11Cr1MoV) forms a protective chromium rich α‐(Cr,Fe)₂O₃ oxide in dry O₂ at 600°C. In mixtures of oxygen and 10 or 40% H₂O, at the same temperature, the material is affected by chromium vaporization because of the formation of CrO₂(OH)₂(g). The loss of chromium tends to deplete the oxide in chromium. The formation of a more iron‐rich oxide may result in a loss of the protective properties of the oxide scale. The loss of chromium and the tendency to destabilize the protective oxide increases with the concentration of water vapour. The material suffers breakaway corrosion after 336 hours in an O₂/H₂O (60/40) mixture while the rate of oxidation is only marginally increased in the presence of 10% H₂O. The thick oxide formed in O₂/H₂O (60/40) environment features an inner layer consisting of FeCr spinel and an outer layer which is almost pure hematite.     

Influence of a TiO2 surface treatment on the growth and adhesion of alumina scales on FeCrAl alloys

Deposits of TiO₂ on FeCrAl alloys were obtained by surface TiO₂ slurry application or by immersion of samples in tetraisopropylorthotitanate (TIPT) solution followed by air dry which gave the thinnest coatings. Isothermal oxidation of treated samples showed strong modification compared to non‐treated ones, particularly in the temperature range of 850–925 °C where parabolic rate constants rapidly decreased when alloys were TiO₂ treated. SEM surface observation, X‐ray diffraction and ruby fluorescence showed that the presence of TiO₂ promoted the formation of α‐Al₂O₃ whereas non‐treated samples exhibited large amounts of transition aluminas. An interesting effect of the rapid change from metastable to stable α‐alumina was a strong increase of scale adhesion, determined by tensile testing, from 300–400 to 2000 J/m² for scales grown at 850 °C on Aluchrom YHfAl. This was explained not only by the change from outward to mainly inward growth but also by the volume reduction at the transition to alpha transformation.     

Behaviour of ferritic stainless steels subjected to dry biogas atmospheres at high temperatures

The objective of this study is to understand the high temperature corrosion behaviour of the ferritic stainless steel type AISI 441 (18CrTiNb), a candidate for SOFC interconnectors, under dry synthetic fermentation biogas (CH₄ + CO₂ mixtures), possibly used at the anode side of the cell. Thermodynamic analysis showed that, in such mixtures, the partial pressure of oxygen lies in the range of 10^?23 to 10^?20 bar for temperature between 700 and 900 °C and that the formation of solid carbon may take place in several conditions. XRD results confirmed the formation of Cr₂O₃ and Mn‐Cr spinel, with a mixture of internal carbides. In this temperature range, kinetic experiments showed linear mass change. Comparing with the linear rate constants of 441 oxidised in pure CO₂, corrosion in biogas was larger and increased with increasing the methane content in the biogas. The surface morphology of the corroded specimens showed a dense oxide scale at temperatures less than 800 °C, serving as an efficient barrier to carbon penetration. However, when the temperature reaches 900 °C, cracks and pores appear in the oxide scale, carbon can precipitate and diffuse easier than at 800 °C and may lead to internal carbide formation. In such biogas atmospheres, 800 °C seems the maximum operating temperature of devices containing this ferritic stainless steel.     

Oxidation mechanism of the Inconel 601 alloy at high temperatures

The Inconel 601 alloy oxidation was performed in air, in the temperature range 1000–1150 °C, during 90 h. Kinetic results show that the parabolic behavior is always followed in this temperature range. The Arrhenius plot of the k_p values shows two different activation energies. Between 1000 and 1050 °C the activation energy is E_a1 = 160 ± 10 kJ/mol. In the 1050–1150 °C temperature range a higher value is calculated E_a2 = 252 ± 20 kJ/mol. The E_a2 value and the X‐ray diffraction (XRD) results and scanning electron microscope (SEM) energy dispersive X‐ray spectroscopy (EDS) examinations are in accordance with a scale growth mechanism limited by a growing Cr₂O₃ scale acting as a diffusion barrier. In the 1000–1050 °C temperature range the activation energy is lower and the structural analyses show that the oxide scale is not only composed of Cr₂O₃. Then, the oxide scale is composed of titanium oxides (TiO₂ and Ti₂Cr₇O₁₇) and chromia mixed together. A doping effect of the chromia scale by titanium can be envisaged. Our results also show the presence of some Mn_1.5Cr_1.5O₄ at the external interface. This external subscale spalls off easily during cooling after the highest temperature oxidation tests. Nevertheless, XRD results and SEM–EDS observations show that the Cr₂O₃ scale remains very adherent on the substrate and can give a good oxidation protection. This good adherence can be related to the presence of a low amount of aluminum in the Inconel 601 alloy composition.     

The high temperature oxidation of 11% chromium steel: Part II – Influence of flow rate

The oxidation of type X20 CrMoV 11 1 steel at 600°C in the presence of dry O₂ and O₂ + 10 or 40% H₂O was investigated. The flow rate was varied between 0.25 to 10.0 cm/s. Exposure time was 168 hours. The oxidized samples were investigated gravimetrically and by a number of surface analytical techniques including grazing angle SEM/EDX, GDOES and XRD. Oxidation is strongly influenced by pH₂O and flow rate. In O₂ + H₂O environment at 600°C, the protective Cr‐rich α‐(Cr,Fe)₂O₃ oxide loses chromium by vaporization of CrO₂(OH)₂. When chromium loss is limited (e. g., in 10% H₂O and in 40% H₂O at low flow rates) the supply of chromium from the alloy compensates for chromium vaporization and the oxide retains its protective properties, resulting in slow oxidation. In 40/60 H₂O/O₂ and high flow rates chromium evaporation becomes so rapid that the protective properties of the oxide are lost and a thick duplex (Fe₂O₃/Fe₂CrO₄) scale develops.     

Effect of stabilising heat treatment on characteristics of electrolytic alumina coating on ferritic stainless steel

The effect of stabilising thermal treatment on the characteristics of conversion coatings modified by electrolytic alumina deposits have been studied using SIMS analysis and X‐ray diffraction in grazing incidence. Before heating, alumina deposit obtained is in form of amorphous boehmite gel. Heat treatment at 800°C in air leads to a very uniform coating morphology and induces diffusion phenomena. After heating (24 h), the coating is constituted of two layers: an alumina (δ‐Al₂O₃) outer layer and a chromium oxide (α‐Cr₂O₃) inner layer. At the coating/steel interface mixed oxide (iron, chromite) are present. The electrolytic alumina deposit blocks the chromium oxide in the deep zone.     

High temperature oxidation of Ti−48%Al−2%W intermetallics

Powder metallurgically produced Ti-48% Al-2%W alloys were oxidized between 800 and 1050°C in air. The W-addition was quite effective in providing isothermal and cyclic oxidation resistance. The alloys oxidized parabolically up to 1050°C during isothermal oxidation, with small weight gains. The scales were adherent up to 900°C during cyclic oxidation. Oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. Tungsten was present below the intermediate Al₂O₃ layer. and also at the scale-matrix interface as W-enriched compounds. Below the oxide scale, a Ti₃Al zone containing some W and O existed.