Evolution of Oxide Film of T91 Steel in Water Vapor Atmosphere at 750 °C

In order to investigate the evolution of oxide film on T91 steel, oxidation tests were conducted in water vapor atmosphere at 750 °C. The phase compositions and microstructures of the oxide scales for early stage oxidation were investigated by using glancing angle XRD and SEM equipped with EDS. The results showed that during the initial oxidation stage Cr-rich oxide film formed and then it covered the sample surface rapidly. The initial Cr-rich oxide film was mainly composed of FeCr₂O₄, (Fe,Cr)₂O₃ and Fe₂O₃. This oxide film acted as a barrier against outward diffusion of iron and inward diffusion of oxygen. During the initial oxidation stage, chromium in the sample surface was consumed gradually, and then a large amount of iron ions penetrated the oxide film and diffused rapidly to the sample surface, resulting in forming an outer “non-protective” Fe₂O₃ layer.     

High-Temperature Oxidation Behavior of SIMP Steel at 800 °C

In this work, the high-temperature oxidation behavior of SIMP and commercial T91 steels was investigated in air at 800 °C for up to 1008 h. The oxides formed on the two steels were characterized and analyzed by XRD, SEM and EPMA. The results showed that the weight gain and oxide thickness of SIMP steel were rather smaller than those of T91 steel, that flake-like Cr₂O₃ with Mn_1.5Cr_1.5O₄ spinel particles formed on SIMP steel, while double-layer structure consisting of an outer hematite Fe₂O₃ layer and an inner Fe–Cr spinel layer formed on T91 steel, and that the location of the oxide layer spallation was at the inner Fe–Cr spinel after 1008 h, which led the ratio between the outer layer and the inner layer to decrease. The reason that SIMP steel exhibited better high-temperature oxidation resistance than that of T91 steel was analyzed due to the higher Cr and Si contents that could form compact and continuous oxide layer on the steel.     

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.     

Formation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Diffusion Barrier Between Cr-Contained Stainless Steel and Cold-Sprayed Ni Coatings at High Temperature

A novel approach to prepare a coating system containing an in situ grown Cr₂O₃ diffusion barrier between a nickel top layer and 310SS was reported. Cold spraying was employed to deposit Ni(O) interlayer and top nickel coating on the Cr-contained stainless steel substrate. Ni(O) feedstock was prepared by mechanical alloying of pure nickel powders in ambient atmosphere, acting as an oxygen provider. The post-spray annealing was adopted to grow in situ Cr₂O₃ layer between the substrate and nickel coating. The results revealed that the diffusible oxygen can be introduced into nickel powders by mechanical alloying. The oxygen content increases to 3.25 wt.% with the increase of the ball milling duration to 8 h, while Ni(O) powders maintain a single phase of Ni. By annealing the sample in Ar atmosphere at 900 °C, a continuous Cr₂O₃ layer of 1-2 μm thick at the interface between 310SS and cold-sprayed Ni coating is formed. The diffusion barrier effect evaluation by thermal exposure at 750 °C shows that the Cr₂O₃ oxide layer effectively suppresses the outward diffusion of Fe and Cr in the substrate effectively.     

Cyclic Oxidation Kinetics and Oxide Scale Morphologies Developed on the IN600 Superalloy

The oxidation behavior of the IN600 Ni?CCr?CFe superalloy was investigated in air at temperatures ranging from 750 to 950 °C, for up to 12 cycles. Oxidation kinetics and oxide scale morphologies were examined using weight gain measurements, SEM-EDS, and X-ray diffraction. The cyclic oxidation kinetic results suggested that the oxidation behavior of the IN600 alloy approximately followed a sub-parabolic rate and the scaling process was controlled by the formation of a chromia scale. At 850 °C, SEM-EDS observations indicated that the formed oxide scale was primarily composed of Cr₂O₃, and the internal oxidation of Cr and Ti occurred. At 950 °C, a fast initial stage with high weight gain was observed, followed by a steady-state stage with gradual weight gain. Additionally, a considerable change in the oxidation kinetic occurred. SEM-EDS observations and XRD results indicated that the external scale was relatively thick with a localized porous, preferential adherent, and a complex oxide scale was developed. This complex oxide scale consisted of an outermost thin layer composed of MnCr₂O₄?CCr₂O₃ mixed together with a small amount of isolated TiO₂, an intermediate relatively thick layer, composed of Cr₂O₃, and an innermost discrete layer formed at the scale/alloy interface, which enriched by Ni/NiO mixed with Ti-, Al-, and Fe-oxides. Finally, only the Al alloying element was internally oxidized to form Al₂O₃ fingers, which create a discrete and narrow internal oxidation zone. Al oxide was observed as a dark area and primarily grows along the alloy grain boundaries in the vicinity of the inward chromia pegs.     

The Effect of Water Vapor on the Transition from Internal to External Oxidation of Austenitic Steels at 1,073 K

The effect of water vapor on the transition from internal to external oxidation of austenitic alloys has been conducted at 1,073 K under the equilibrium oxygen partial pressure for the coexistence of Fe and FeO. Critical Cr concentrations in the Fe–Cr–30Ni (at.%) austenitic alloys were determined to be 30 at.% in dry atmosphere and 37 at.% in humid atmosphere. Thus, water vapor significantly affected the transition from internal to external oxidation of austenitic alloys. Two oxides of Cr₂O₃ and FeCr₂O₄ precipitated in the Fe–5Cr–30Ni (at.%) alloy and solid state reaction for the formation of FeCr₂O₄ may be influenced by water vapor. Oxygen permeability, which was estimated by considering the effective stoichiometric ratio, was also enhanced by water vapor.     

Short Term Oxidation Behavior of Si-Free Low-Cr Alloy Sputtered Coating

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe₂O₃ and (Fe,Co,Cr)₃O₄ spinel with a small amount of Cr. However, (Fe,Co,Cr)₃O₄ spinel and Fe₂O₃ were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co₃O₄ and (Fe,Co)₃O₄ with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)₃O₄ spinel layer and an inner Cr₂O₃ layer was developed on the sputtered coating.     

Oxidation Behavior of Fe–25Cr–20Ni–2.8Si During Isothermal Oxidation at 1,286 K; Life-time Prediction

The oxidation behavior of an austenitic steel, type 1.4841, with a Cr content of 25 wt% and a high-Si content of 2.8 wt% was studied during isothermal oxidation at 1,286 K in air. A thick, crystalline Cr₂O₃ layer, on top of a much thinner, amorphous SiO₂ layer, developed on the alloy substrate. After formation of a closed Cr₂O₃ scale, parabolic growth kinetics prevailed as long as the associated constant, steady-state Cr concentration in the alloy at the substrate/oxide interface of about 13 ± 1 wt% was maintained. Upon prolonged oxidation, successive cracking and spallation of the thickening oxide scale eventually led to breakaway oxidation, because the “bulk-”Cr concentration in the interior of the alloy dropped below the critical value required to ‘heal’ the protective oxide layer after oxide spallation. Application of a lifetime prediction model of the alloy substrate under isothermal oxidation conditions allowed determination of the breakaway-oxidation time as a function of alloy-sheet thickness, by employing the Cr volume-diffusion coefficient in the alloy and the parabolic growth-rate constant, both determined in the present study by fitting calculated to experimental Cr-depletion profiles for various oxidation times.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

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.     

Kinetics and Microstructural Investigation of High-Temperature Oxidation of IN-738LC Super Alloy

The present study was carried out to investigate the kinetics and the surface chemistry of the oxide layers formed on the IN-738LC super alloy during high-temperature oxidation at 950 °C in air from 1 to 260 h. Oxidation kinetics were studied by mass gain measurement. The oxide layers were characterized by field emission scanning electron microscope, elemental distribution map, energy-dispersive spectroscopy as well as x-ray diffractometry (XRD). The oxidation kinetics followed the parabolic law. The XRD analysis revealed that the oxide scale contained mainly NiO, Ni (Cr, Al)₂O₄, Al₂O₃, TiO₂ and Cr₂O₃. The oxide structure, from the top surface down to the substrate, was clarified by elemental map distribution studies as Ni-Ti oxides, Cr-Ti oxides, Cr₂O₃ oxide band, Ni-Co-Cr-W oxide and finally a blocky Al₂O₃ region. The oxidation scales were composed of three distinct layers of the outer and mid layers enriched by TiO₂ and Cr₂O₃, NiCr₂O₄ oxide, respectively, and the innermost layer was composed of Al₂O₃ and matrix alloy. The depleted gamma prime layer was formed under the oxidation scales due to the impoverishment of Al and Ti which were induced by the formation of Al₂O₃ and TiO₂.     

Influence of Water Vapor on the Cyclic-Oxidation Behavior of a Low-Pressure Plasma-Sprayed NiCrAlY Coating

The oxidation of a low-pressure plasma-sprayed (LPPS) NiCrAlY coating on a nickel-base superalloy was studied at 1050 °C in flows of O₂, and mixture of O₂ and 5% H₂O under atmospheric pressure. Water vapor has an obvious effect on the cyclic oxidation of the NiCrAlY coating. There is more decrease in weight gain when exposure to O₂ is replaced by exposure to O₂ + 5% H₂O. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in pure oxygen is composed mainly of Cr₂O₃, and a thin Al₂O₃-rich layer is formed at the interface between the Cr₂O₃-rich layer and the coating. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in a mixture of O₂ + H₂O is composed of NiCr₂O₄, NiO and Cr₂O₃. The effect of water vapor on the oxidation of the NiCrAlY coating may be attributed to an increase in Ni and Cr cation transport, stress-corrosion cracking of Al₂O₃ and moisture-enhanced volatility of the Cr₂O₃ scale.     

Gas Phase Initial Oxidation of Incoloy 800 Surfaces

The surface oxidation of Incoloy 800 was studied using dilute O₂ gas at temperatures of 300 °C. Samples with two differing grain sizes were studied using time-of flight secondary ion mass spectrometry (ToF–SIMS) and X-ray photoelectron spectroscopy (XPS) as primary analysis tools. A multi-layered oxide film was detected and was composed of an exterior gamma-Fe₂O₃ with a Cr₂O₃ layer at the oxide–metal interface also containing significant concentrations of NiCr₂O₄. Minor concentrations of another spinel oxide, NiFe₂O₄ were distributed throughout the film. The kinetics of oxidation growth was found to follow a direct logarithmic relationship for both grain sizes, suggesting that the oxide would be a suitably protective. Very small oxide nodules formed at later stages, particularly for the small grained samples. A protocol for assessment of XPS spectral envelopes is advanced. The method measures the percent residual intensities remaining after spectral subtraction of reference spectra and appears to be an effective means for screening of possible components.     

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.     

Oxidation und Aufkohlung hochlegierter Werkstoffe für Crackrohre. Teil 1: Das Oxidationsverhalten in Luft

Oxidation and Carburization of High Alloyed Materials for Cracking Tubes. Part 1: The Oxidation Behaviour in Air The oxidation behaviour has been studied of cast materials (German Materials Numbers 1.4848, 1.4857, 2.4813), an experimental melt of 25/20/5 CrNiSi steel and wrought materials (German Materials Numbers 1.4301 and 1.4841) in dry synthetic air between 800 and 1300°C. Only the oxidation of the materials 1.4848 and 1.4841 follows an approximately parabolic law, with the other materials more complex kinetic laws are found. The oxidation rates of all the cast materials are rather similar; the scale layers successively formed on the base material are Cr₂O₃ and MnCr₂O₄. The latter is characterized by clearly distinguishable iron contents. The Cr₂O₃ layer contains SiO₂ inclusions, in particular near the metal boundary. Above 1000°C internal oxidation of silicon is found. An exception to this is the 25/20/5 CrNiSi experimental melt where, because of the high Si content, a continuous SiO₂ layer is formed consistently between base material and Cr₂O₃ layer. The material 1.4301 exhibits locally increased oxidation at temperatures above 1050°C; in this case oxide nodules are formed. The oxidation rate of material 1.4841 at temperatures above 1100°C is higher for the fine grained than for the coarse grained condition; this phenomenon is attributed to the fact that at these temperatures pronounced grain growth occurs which impairs the formation of a protective layer. The scale layers formed contain considerable amounts of iron and the Cr content in the metal at the metal/oxide boundary decreases at a considerable higher rate with increasing temperature than in the case of the coarse grained material 1.4841 and of the cast materials containing about 25% Cr. Below 1100°C the differences in the behaviour are rather small and the oxidation rate is comparable to that of the cast materials. A transition from internal silicon oxidation to external SiO₂ scale formation is found with materials 1.4848 and 1.4841 at 1100°C in H₂/H₂O mixtures below the partial pressure of oxygen corresponding to the Cr/Cr₂O₃ equilibrium.     

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.     

Effect of Chromium Content on the Oxidation Behaviour of Ferritic Steels for Applications in Steam Atmospheres at High Temperatures

Environments containing water vapour are common in many industrial processes, such as power generation systems. Hence, long-term oxidation (1000 h) of P-91 and AISI 430 was studied at 650 and 800 °C, in 100% H₂O atmosphere. The oxidation resistance of the AISI 430 is better than that of the P-91, due to the formation of protective phases on the surface. At 650 °C, a scale composed of Fe₃O₄, Fe₂O₃ and (Fe,Cr)₃O₄ is formed on P-91, although at 800 °C the scale is mainly composed of Fe₃O₄ and (Fe,Cr)₃O₄. On the other hand, on AISI 430 the scale is composed mainly of (Fe,Cr)₂O₃ at 650 °C, and at 800 °C a layer of Cr₂O₃ is formed and remains owing to the higher diffusion rate of Cr at this temperature than at 650 °C, the latter of which compensates the Cr depletion by the degradation of the chromia scale.     

Oxidation of Fe-8Cr-14Ni-Al-Si-Mn from 700 to 1000°C

This paper reports an investigation into reducing the Cr concentration in commercial-grade stainless steels while maintaining oxidation protection at elevated temperatures. Aluminum and Si were added as partial substitute alloy elements to enhance the reduced operation protection resulting from Cr concentration reduced by approximately 50 pct of that found in stainless steels. The goal of this study was to determine the oxidation mechanism of such an Fe, Al-Si alloy: Fe-8Cr-14Ni-1Al-3.5Si-1Mn. During the initial oxidation period the protection resulted from a thin film of Al₂O₃ over an Fe and Cr spinel. Long-term oxidation protection resulted from the gradual formation of a Cr sesquioxide (Cr₂O₂) inner oxide layer. Eventually an outer oxide layer formed that was a mixed composition spinel of Cr and Mn (MnO · Cr₂O₃). The Al₂O₃, which was part of the original protective layer flaked off early in the oxide testing, and the aluminum oxide that formed later appeared as an internal oxide precipitate.     

Corrosion of Cr2AlC in Ar/1%SO2 Gas Between 900 and 1200?��C

Cr₂AlC compounds were synthesized by a powder metallurgical route and corrosion tested at 900, 1000, 1100 and 1200 °C for up to 150 h under an Ar/1% SO₂ gas atmosphere. The compounds were resistant to corrosion because a thin ??-Al₂O₃ barrier layer quickly formed on the surface which suppressed sulfidation. Virtually no sulfur was detected inside the scale except during the initial corrosion stage. The superior corrosion resistance of Cr₂AlC originated from the high affinity of Al for oxygen to form the thermodynamically stable Al₂O₃. Unlike Al, Cr was not active because Cr was strongly bound to carbon as Cr₂C layers in Cr₂AlC. The small amount of Cr₂O₃ that had formed was dissolved in the Al₂O₃ layer. The corrosion of Cr₂AlC resulted in the formation of an ??-Al₂O₃ layer and an underlying Cr₇C₃ layer.