High temperature oxidation of Fe–Ni-base alloy HR120 and Ni-base alloy HAYNES 282 in steam

High temperature oxidation of Fe–Ni-base alloy HR120 and Ni-base alloy HAYNES 282 in steam at 800–1000 °C was investigated and compared. Results show that the oxidation kinetics of HAYNES 282 and HR120 followed a parabolic law. Temperature affected the stability of formed chromia, leading to the observation of (Fe, Cr)₂O₃, Fe₂NiO₄ on the surface of Fe–Ni-base alloy, which is not the case for Ni-base alloy. The strengthening element titanium accelerated the growth of chromia formed on the Ni-base alloy. It is concluded that the resistance of high temperature oxidation of HR120 is superior to HAYNES 282.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

Oxidation behaviour of nanocrystalline Fe–Ni–Cr–Al alloy coatings

Abstract

Nanocrystalline Fe–Ni–Cr–Al alloy coatings with ～4 wt-%Al were produced using the unbalanced magnetron sputter deposition technique with a composite 310S stainless steel target embedded with aluminium plugs. The oxidation behaviour of the coatings was studied, during which complete external α-Al₂O₃ scales were formed. During isothermal oxidation tests at 950, 1000, and 1050°C, the oxidation kinetics followed an essentially parabolic rate law, and the oxidation constants were measured to be 2·06 × 10^-3, 4·23 × 10^-3, and 1·14 × 10^-2 mg² cm^-4 h^-1 respectively. During a cyclic oxidation test at 1000°C the α-Al₂O₃ scale showed good scale spallation resistance. The surface hardness of the coatings was measured with a Knoop indentor before and after oxidation. After oxidation, the coating surface hardness was still significantly higher than that of the uncoated specimen, demonstrating the potential this coating has in the improvement of high temperature erosion resistance.     

Oxidation behaviour of TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings with Al2O3 topcoat deposited on γ-TiAl alloys

Abstract

TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl specimens using magnetron sputtering techniques. The nitride layers were manufactured by unbalanced magnetron sputtering (UBM) and high power impulse magnetron sputtering (HIPIMS). The CrAlYN/CrN coatings had an oxy-nitride overcoat. On some of the coated samples an additional alumina topcoat was deposited. The oxidation behaviour of the different coatings was investigated at 750 and 850°C performing quasi-isothermal oxidation tests in laboratory air. Mass change data were measured during exposure up to failure or the maximum exposure length of 2500 h. When exposed to air at 750°C, the Ti-based nitride films exhibited higher oxidation resistance than the Ti – 45Al –8Nb substrate material. The alumina topcoat enhanced the oxidation protection of this coating system, acting as diffusion barrier to oxygen penetration. At 850°C, the TiAlYN/CrN films exhibited poor stability and rapidly oxidised, and therefore were not applicable for long-term protective coatings on γ-TiAl alloys. The beneficial effect of the additional Al₂O₃ layer was less pronounced at this exposure temperature. The Cr-based nitride films exhibited high oxidation resistance during exposure at 850°C. HIPIMS deposition improved the oxidation behaviour of the CrAlYN/CrN nanoscale multilayer coatings in comparison to UBM coatings. For these coatings, the decomposed and partially oxidised nitride films were an effective barrier to oxygen inward diffusion. The alumina topcoat did not significantly increase the oxidation resistance of the γ-TiAl alloy coated with Cr-based nitride films.     

Improved surface properties of D2 steel by laser surface alloying

The wear and the high-temperature oxidation resistance of the D2 steel (Fe-1.5 C-12 Cr-0.95 Mo-0.9 V-0.3 Mn) were increased by laser surface alloying after coating the surface with SiC or Cr₃C₂ powder. The surface alloys exhibit two microstructures: hypoeutectic and hypereutectic, respectively, all containing iron solid solutions and iron-chromium carbides, (Fe,Cr)₇C₃. The oxidation resistance of these alloys was measured in isothermal and cyclic conditions, and was shown to increase with silicon or chromium additions, particularly due to the formation of a chromia scale with excellent behaviour during thermal shoks. The surface alloy obtained with Cr₃C₂ also has shown a better resistance to wear due to its hypereutectic microstructure.     

Secondary ion mass spectrometry and multireflection infrared analyses of conversion coatings on Fe–Cr–Al stainless steels for catalysis: study of thermal stability

Abstract

The thermal stability of three stainless steel conversion coatings for high temperature applications (e.g. photothermal conversion catalysis) are investigated. The thermal oxidation in air up to 1000°C of Fe–17Cr, Fe–18Cr–1·3Al, and Fe–22Cr–5Al coatings (all wt-%) are compared. This study has revealed a critical temperature below which the coating thickness is preserved; the critical temperature increases and the thermal oxidation of the conversion coating decreases with higher chromium and aluminium content. This is attributed to the difference in the substitution ratio of γ lacunar phase (additionally oxidised substituted magnetite), which is the main component of the conversion coatings. The thermal stability of this phase is higher when it is richer in chromium or aluminium. Higher contents of these elements raise the temperature of formation of chromite (FeCr₂O₄) and alumina, the occurrence of which causes thickening of the coating during thermal treatment.

MST/1891     

High-temperature degradation mechanism of aluminide coatings on titanium alloys under cyclic oxidation at 750°C

Aluminide coatings prepared on Ti-6Al-4V substrate were able to improve oxidation resistance of the alloy under cyclic oxidation at 750°C both in dry and moist air conditions due to aluminide’s ability to form a stable alumina oxide scale. However, degradation of the coating due to spallation, cracking, internal oxidation and formation of voids with increased cyclic oxidation reduced the lifespan of the coating and the underneath substrate. The main cause of coating degradation for hot-dip specimens is cracks that initiated and propagated perpendicular to the surface. For the plasma spray specimens, the cracks are parallel to the surface. Initiation of cracks in hot-dip coatings are more accredited to residual stresses due to cooling and presence of brittle intermetallic phases TiAl₂ and TiAl. For plasma spray coatings, initiation and propagation of cracks are attributed to presence of entrapped oxides, pores and grain boundaries of the deposited splats whose flattened edges are parallel to the surface of the coating. Presence of water vapor, too, acts as an oxygen carrier and thus promotes oxidation internally, inhibits growth of continuous protective alumina oxide scales and weakens the scale/alloy interfacial toughness. Water vapor therefore accelerates degradation by increasing spallation and cracking rate of the coating.     

Optimising oxidation resistance of MCrAl coating systems using vapour phase alloy design

Abstract

The potential for developing new overlay coatings by codeposition from segmented or multiple sources has been investigated. The main focus of this work was the development of new NiCrAl based alloys with improved resistance to high temperature oxidation. Initially, studies concentrated on the deposition of a wide range of ternary alloy compositions by magnetron sputtering from a segmented target consisting of pure Ni, Cr, and Al segments. Hence, alloy compositions were produced by mixing of these source materials in the vapour phase. These coating compositions were deposited onto high purity Al₂O₃ to ensure that no interaction between the coating compositions and substrate occurred during subsequent oxidation trials. Oxidation trials at 950°C for periods up to 100 h have been undertaken. Modelling of the oxidation behaviour along quasibinary sections has allowed the construction of a ternary iso-oxidation contour map for the NiCrAl system. This approach has identified new oxidation resistant materials with a twofold improvement in oxidation resistance over typical commercial overlay coatings.

MST/1093     

Effect of aluminium in thermal oxidation of Fe–Cr–Al conversion coating for catalysis

Abstract

The use of coatings having special properties for photothermal conversion or catalysis is often complicated by problems of high temperature stability. This study deals with the effect of aluminium, as an additional element inferritic stainless steel, on the behaviour of Fe–Cr–Al conversion coatings during their thermal oxidation in air up to 1000°C. X-ray diffraction and infrared spectrometry show that the main component of the coating is a chromium, aluminium substituted magnetite and that the presence of aluminium slows down the oxidation rate of the coating-substrate system mainly owing to the formation of mixed iron oxides containing aluminium and of alumina.

MST/1400     

Improvements in honeycomb abradable seals

Abstract

The concept of utilising honeycomb abradable seals to improve gas turbine engine performance has been under development for many years. Engine operating temperatures, in the region of the seals, have been restricted to below 950°C by the reliance on a chromia scale for degradation protection. The introduction of nickel brazed FeCrAlY based alloys within the honeycomb seal could facilitate a safe increase in operating temperatures to over 1100°C. This is aided by the formation of a more stable, α-alumina scale.

These Fe–20Cr–5Al–0.5Y foils, including the commercially produced variant designated MI2100, have been designed for a service lifetime of up to 24,000 hours. However, burner tests and isothermal oxidation tests in laboratory air at 1100 and 1200°C have shown them to fail after much shorter times. The major degradation of the foils occurs adjacent to the brazed region and limits the lifetime of the honeycomb seal.

Cross-sectional analysis in a scanning electron microscope of seals manufactured from MI2100 foils, after oxidation testing at 1200°C, has shown that voids form beneath the protecting α-alumina scale. In some cases, these voids are filled with silica, with some chromia present, and may be the origin of the subsequent degradation process. The results contrast with tests on free-standing thin foils (100 μm thickness) of MI2100 and other FeCrAlY alloys, where a continuous layer of chromia is formed below the alumina outer scale, once the aluminium content of the alloy drops below a critical composition. Although there is a small amount of silicon in MI2100, the main source of the high level of silicon found in the honeycombs is most likely to be the brazing alloy, since both nickel and silicon from the braze are very mobile in FeCrAlY alloys at high temperatures.

The formation and filling of voids with silica may be associated with the subsequent failure of the protective alumina scales on these brazed alloys, and this mechanism will be developed further in this paper.     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Characterisation studies of chromia formation on commercial FeCrAlRE alloy foils following chemically induced failure at 900°C

Abstract

The chemical failure of commercial FeCrAlRE (where RE is a reactive element) alloys, such as Aluchrom YHf and Kanthal AF foils (30–50 μm thick), is induced by oxidation, in air, at 800–950°C. This is also the life limiting process of this family of alloys at higher temperatures (≥1100°C) and occurs when the Al activity in the alloy becomes depleted below a critical level, as a result of oxidation. At this junction, the integrity of the protective alumina scale is not sustainable but continued ‘pseudo-protection’ can be provided by chromia formation beneath the alumina layer. Oxidation continues at a slow linear rate controlled by transport through the alumina scale. The duration of the pseudo-protection region can extend to several hundred hours and has considerable technological implications, if it could also be used in defining component lifetimes.

Greater understanding is, therefore, required concerning the formation and growth of the chromia subscale, as this has received only limited attention to date. Of particular possible significance is the mode of formation of the subscale. At temperatures above 1000°C, this forms a continuous, essentially uniform layer. In contrast, at ～900°C scalloped shaped pits of chromia develop beneath the outer alumina scale. Hence a detailed characterisation study has been undertaken of the formation and development of this chromia attack; the results of which form the basis of this paper. The study has involved the deployment of a range of microscopy and analytical techniques, including optical and scanning electron microscopy, electron microprobe analysis (EPMA) and energy dispersive X-ray analysis (EDX).     

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.     

Effect of the La<Subscript>2</Subscript>O<Subscript>3</Subscript> sol–gel coating on the alumina scale adherence on a model Fe–20Cr–5Al alloy at 1100 °C

The effect of lanthanum sol–gel coatings was studied in order to improve the alumina scale adherence during the model Fe–20Cr–5Al alloy oxidation, at 1100 °C, in air. Various sol–gel coating procedures were applied. Argon annealing of the lanthanum sol–gel coating was tested at temperatures ranging between 600 and 1000 °C. The coating crystallographic nature was characterized by X-ray diffraction (XRD) depending on the annealing temperature. The oxidation process has been examined at 1100 °C by in situ XRD on blank Fe–20Cr–5Al, sol–gel coated and argon-annealed specimens. This study shows that the coating argon annealing at 1000 °C leads to the preferential formation of LaAlO₃ instead of La₂O₃. This coating procedure leads to an alumina scale formation showing the best adherence under thermal cycling conditions at 1100 °C.     

FEG-SEM examination of alumina scales formed on FeCrAl alloys

Abstract

Field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) has been used to investigate the microstructure and the oxidation behaviour of ultra-high purity Fe–20Cr–5Al model alloys and a commercial Fe–20Cr–5Al alloy. The model alloys contain controlled additions of phosphorus and carbon impurities and increased levels of more beneficial elements including yttrium, hafnium and titanium. The samples studied were oxidised at 800°C and 1200°C in humidified air for up to 3100 h, and 900°C and 1000°C for 1 h in laboratory air. At the higher temperature, well-adhered, compact and highly protective α-alumina scales formed, whereas at the lower temperature the scales formed were a less protective type of metastable alumina.

Preliminary examination showed that the texture of the formed alumina scale was unaffected by the texture of the underlying substrate and the substrate compositions. At the higher temperature, the study revealed that the alumina scale comprised two distinct regions; the outer region at the scale/gas interface contained small, equiaxed (0.5–1 micron) grains and the inner region at the scale/metal interface contained, columnar grains that are 2–3 times larger than the equiaxed ones. However, at the lower temperature these two distinct regions were not apparent. Instead, grains of predominantly metastable alumina were observed. The links between texture morphology and oxide growth mechanisms will be discussed in this paper.     

Effects of surface-applied ceria on the stability of thermally growing chromia scale of FeCr alloys and 310steel

The influence of surface-applied ceria on the oxidation behavior of FeCr alloys and 310 stainless steel at 1000°C and 1100°C has been studied. The surface-applied ceria were beneficial in reducing the growth rate of chromia scale, and were particularly effective in inhibiting the accelerated breakaway oxidation of Fe20Cr alloy in wet oxygen and spalling and cracking of the scale under cyclic oxidation. The beneficial effects of the ceria have been attributed to the improvement in the stability of the thermally growing chromia scale.     

Protective coatings on orthorhombic Ti2AlNb alloys

Abstract

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)₂ Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α₂-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.     

Laser raman spectroscopic studies of the surface oxides formed on iron chromium alloys at elevated temperatures

Binary iron-base alloys containing chromium additions of 3, 9, 12 and 18 % were oxidized in air at elevated temperatures. Laser Raman spectroscopy has been used to determine the chemical compounds of the oxides of these alloys. It is found that the oxides formed on Fe-3Cr alloy at various elevated temperatures consist mainly of iron. However, for Cr additions ≥12 %, the surface oxide formed at 400°C consists of αFe₂O₃, Fe₃O₄ and spinel phases. With increasing oxidation temperature up to 850°C, the oxide scale consists of Cr₂O₃ and spinel phases only.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.