Early stages of the oxidation of a FeCrAlRE alloy (Kanthal AF) at 900 °C: A detailed microstructural investigation

Early stages of the evolution of Al₂O₃ scales formed on a FeCrAlRE alloy (Kanthal AF) have been investigated by analytical TEM. The samples were oxidized isothermally at 900 °C in dry O₂ or O₂ + 40% H₂O for 1 h or 24 h. All oxide scales exhibited a two-layered structure, with a continuous inward growing α-Al₂O₃ inner layer and an outward growing outer layer. After 1 h, the outer oxide layer consisted of γ-Al₂O₃ in both environments. After 24 h exposure in dry O₂, the γ-Al₂O₃ in the outer oxide layer was partly transformed to α-Al₂O₃ and spinel oxide (Mg_1−xFe_xAl₂O₄). In contrast, the γ-Al₂O₃ in the outer layer was not transformed after 24 h in O₂ + 40% H₂O, showing that water vapour stabilizes γ-Al₂O₃. All oxide scales contained a Cr-rich band, a product of the initial oxidation. The inner α-Al₂O₃ layer is suggested to nucleate on Cr₂O₃ or Cr_2−xFe_xO₃ in the initial oxide.     

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr₂AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr₂AlC films form a dense continuous oxide scale consisting of α-Al₂O₃ on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol^− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10^− 10 kg² m^− 4 s^− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr₇C₃ increases at the expense of Cr₃C₂. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr₂O₃-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)₂O₃ solid solution support this notion. For comparison bulk Cr₂AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr₂AlC is due to the considerably larger amount of Al available.     

Early high temperature oxidation behaviour of Ti(C,N)-based cermets in air

Isothermal oxidation behaviour of two Ti(C,N)-based cermets (TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32-x)Ni-xCr, x = 0 and 6.4 wt%) was investigated in air at 800-1100 °C up to 2 h. Mass gains exhibited neither linear nor parabolic law during isothermal oxidation. The oxide scales formed at 800-1100 °C were multi-layered, consisting of NiO outerlayer, NiTiO₃ interlayer and TiO₂-based innerlayer. The internal oxidation zones formed at 1000-1100 °C consisted of Ti-based, Ni-based and Mo-based complex oxides. Cermet with 6.4 wt% Cr exhibited superior oxidation resistance, due to the presences of Cr_0.17Mo_0.83O₂ in TiO₂-based innerlayer of the oxide scale and Cr-rich Ti-based complex oxide in the internal oxidation zone.     

Oxidation of Cr2AlC at 1300 °C in air

High purity, dense Cr₂AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr₂O₃ but Al₂O₃. Since Al was consumed to produce the Al₂O₃, Al-depletion and Cr-enrichment occurred underneath the Al₂O₃ layer. This led to the formation of a Cr₇C₃ layer containing voids. These grew during oxidation, eventually destroying the Cr₇C₃ layer formed on the unoxidized Cr₂AlC matrix.     

Corrosion behavior of NiCrFe Alloy 600 in high temperature, hydrogenated water

The corrosion behavior of Alloy 600 (UNS N06600) is investigated in hydrogenated water at 260 °C. The corrosion kinetics are observed to be parabolic, the parabolic rate constant being determined by chemical descaling to be 0.055 mg dm⁻² h^−1/2. A combination of scanning and transmission electron microscopy, supplemented by energy dispersive X-ray spectroscopy and grazing incidence X-ray diffraction, are used to identify the oxide phases present (i.e., spinel) and to characterize their morphology and thickness. Two oxide layers are identified: an outer, ferrite-rich layer and an inner, chromite-rich layer. X-ray photoelectron spectroscopy with argon ion milling and target factor analysis is applied to determine spinel stoichiometry; the inner layer is (Ni_0.7Fe_0.3)(Fe_0.3Cr_0.7)₂O₄, while the outer layer is (Ni_0.9Fe_0.1)(Fe_0.85Cr_0.15)₂O₄. The distribution of trivalent iron and chromium cations in the inner and outer oxide layers is essentially the same as that found previously in stainless steel corrosion oxides, thus confirming their invariant nature as solvi in the immiscible spinel binary Fe₃O₄-FeCr₂O₄ (or NiFe₂O₄-NiCr₂O₄). Although oxidation occurred non-selectively, excess quantities of nickel(II) oxide were not found. Instead, the excess nickel was accounted for as recrystallized nickel metal in the inner layer, as additional nickel ferrite in the outer layer, formed by pickup of iron ions from the aqueous phase, and by selective release to the aqueous phase.     

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.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Oxidation of 316 stainless steel in supercritical water

The oxide scales of 316 stainless steel (316 SS) have been examined after exposure to supercritical water (SCW) with 2.0% H₂O₂ for up to 250 h. The exposed samples were analyzed using weight measurement, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). It was found that mass gain of all samples increased with increasing temperature and exposure time. Higher temperature SCW resulted in rougher surfaces and thicker oxide scales. Duplex layer oxide structures with Ni-enrichment at the oxide/metal interface developed on all samples exposed to SCW, which were identified as Fe₂O₃/Fe₃O₄ + spinel/Cr₂O₃/Ni-enrichment/316 SS from the outer to inner layer. The possible oxidation mechanisms are also discussed.     

KCl-induced high temperature corrosion of the austenitic Fe-Cr-Ni alloys 304L and Sanicro 28 at 600 °C

The influence of KCl(s) on the high temperature oxidation of the austenitic alloys 304L and Sanicro 28 at 600 °C in O₂ + H₂O environment is reported. 0.10 mg/cm² KCl(s) was added before exposure. The samples are investigated by grazing angle XRD, SEM/EDX, and AES. In the absence of KCl, both alloys show protective behaviour in dry O₂. In O₂ + H₂O environment, alloy 304L suffers local breakaway corrosion while Sanicro 28 still shows protective behaviour. The oxidation of both alloys is strongly accelerated by KCl. KCl reacts with chromium in the normally protective corundum-type oxide, forming K₂CrO₄. This depletes the scale in chromia and leads to the formation of a non-protective, iron-rich scale. The significance of KCl-induced corrosion in real applications is discussed and the oxidation behaviour of the two steels is compared.     

Corrosion stability of ferritic stainless steels for solid oxide electrolyser cell interconnects

Long-term oxidation behaviour of eight ferritic steels with 20-29 wt.% chromium (F 20 T, TUS 220 M, AL 453, Crofer 22 APU, Crofer 22 H, Sanergy HT, E-Brite and AL 29-4C) has been studied. The samples were cut into square coupons, ground and annealed for 140-1000 h at 1173 K in flowing, wet hydrogen, air and pure oxygen. The reaction kinetics was followed by mass increase of individual samples over time. Parabolic rate law was observed for most measurements. The respective rate constants have been evaluated and compared. The chemical composition of the oxide scale was investigated by XRD and SEM/EDXS. The major constituent is chromium oxide. Other oxides, such as (Mn, Cr)₃O₄, MnTiO₃, SiO₂ or Al₂O₃, are also present in different amounts depending on the chemical composition of the steel. The oxidation rate increases with increasing oxygen partial pressure and decreasing chromium concentration. Chromium diffusion coefficients in Cr₂O₃ and parabolic rate constants are compared. The reaction mechanism for the chromia formation is suggested. The results are discussed with respect to the applications of the steels in a working solid oxide electrolyser cell stack. Furthermore, suggestions for the development of a superior alloy composition are given.     

Corrosion behavior of Hastelloy C-276 in supercritical water

The corrosion behavior of a nickel-based alloy Hastelloy C-276 exposed in supercritical water at 500–600 °C/25 MPa was investigated by means of gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. An oxide scale with dual-layer structure, mainly consisting of an outer NiO layer and an inner Cr₂O₃/NiCr₂O₄-mixed layer, developed on C-276 after 1000 h exposure. Higher temperature promoted oxidation, resulting in thicker oxide scale, larger weight gain and stronger tendency of oxide spallation. The oxide growth mechanism in SCW seems to be similar to that in high temperature water vapor, namely solid-state growth mechanism.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Continuous and cyclic oxidation of T91 ferritic steel under steam

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 °C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)₃O₄. The intermediate layer was porous magnetite (Fe₃O₄) followed by a compact thinner layer of hematite (Fe₂O₃). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Zinc treatment effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 °C were found to lower the corrosion rate of austenitic stainless steel (UNS S30400) by about a factor of five, relative to a non-zinc baseline test [S.E. Ziemniak, M. Hanson, Corros. Sci. 44 (2002) 2209] after 10,000 h. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed that miscibility gaps in two spinel binaries—Fe(Fe_1−mCr_m)₂O₄ and (Fe_1−nZn_n)Fe₂O₄—play a significant role in determining the composition and structure of the corrosion layer(s). Although compositions of the inner and outer corrosion oxide layers represent solvus phases in the Fe₃O₄-FeCr₂O₄ binary, zinc(II) ion incorporation into both phases leads to further phase separation in the outer (ferrite) layer. Recrystallization of the low zinc content ferrite solvus phase is seen to produce an extremely fine grain size (∼20 nm), which is comparable in size to grains in the inner layer and which is known to impart resistance to corrosion. Zinc(II) ion incorporation into the inner layer creates additional corrosion oxide film stabilization by further reducing the unit cell dimension via the substitution reaction

0.2Zn²⁺(aq)+Fe(Fe_0.35Cr_0.65)₂O₄(s)?0.2Fe²⁺(aq)+(Zn_0.2Fe_0.8)(Fe_0.35Cr_0.65)₂O₄(s)     

Microstructure and analysis of oxide scales formed on Cr-Si-Ni compacts in air and H2O-containing atmosphere

The purpose of this study was to characterize the oxide scales formed on various Cr-Si-Ni compacts at 1273 K in air and H₂O-containing atmosphere by TEM. It was found that CrSi₂-(5-20)mass%Ni compacts form double layer scales, consisting of an outer Cr₂O₃ layer and an inner SiO₂ layer. The oxide scale changed from SiO₂- to Cr₂O₃-based scale with an increase in the Ni concentration. However, it was observed that the oxide scale formed in H₂O-containing atmospheres showed local SiO₂ growth into the substrate. This result suggests that the inward oxidant diffusion promotes the local growth of SiO₂ in the H₂O-containing atmospheres.     

Corrosion behavior of NiCrMo Alloy 625 in high temperature, hydrogenated water

The corrosion behavior of NiCrMo Alloy 625 (UNS N06625) has been characterized in a 10,000 h test conducted in hydrogenated water at 260 °C. The corrosion kinetics were observed to be parabolic, the parabolic rate constant being determined by chemical descaling to be 0.074 mg dm⁻² h^−1/2. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed the presence of two spinel oxide phases and significant amounts of recrystallized nickel. Based on the distribution of three oxidized alloying constituents (Ni, Cr, Fe) with respect to depth and oxidation state, it was concluded that: (a) corrosion occurs in a non-selective manner, but significant amounts of nickel(II) ions are released to the water, and (b) the spinel oxides exist as a chromite-rich inner layer (Ni_0.7Fe_0.3)(Cr_0.8Fe_0.2)₂O₄ underneath a coarser, ferrite-rich outer layer (Ni_0.9Fe_0.1)(Cr_0.1Fe_0.9)₂O₄. The trivalent cation distribution in each of these phases appears to represent a solvus in the immiscible NiCr₂O₄-NiFe₂O₄ binary.