Abstract The nitridation of three austenitic high-temperature alloys in 95% N2 + 5% H2 environment at 1100 °C was evaluated in terms of gravimetry and investigated by SEM–EDS, EPMA and STEM. Samples made from Alloy 600, 253 MA and 353 MA were exposed for 1 day, 1 week and 3 weeks. Alloy 600 underwent very little nitridation, while 253 MA and especially 353 MA, were heavily affected by nitride precipitation. The nitridation of all three alloys had reached equilibrium after three weeks; the extent of nitridation depending on the chromium activity in the alloy. The kinetics of nitrogen ingress into the alloy depends on nickel concentration, while the rate-determining step in the nitridation process is the nucleation and growth of the nitride precipitates.

Abstract Nowadays the production of steel from scrap in electric arc furnaces is the most common bridging technology to reduce CO2 emissions. Depending on scrap quality, a non-negligible content of tramp elements such as Cu, Sn, or Ni is introduced into the steel. As their affinity to oxygen is lower than that of iron, they typically enrich at the steel/scale interface area and along grain boundaries during oxidation, which may result in quality problems. Oxidation processes are unavoidable in solid steel processing, and therefore, a deeper understanding of the occurring phenomena, such as intergranular oxidation and liquid metal infiltration of grain boundaries, is essential to continuously improve the product quality. In this study, oxidation experiments for slab reheating were performed by simultaneous thermal analysis under near-process conditions. For a clear statement on the role of tramp elements during oxidation, steel grades with and without tramp elements were investigated. The addition of the expected future contents of Cu and Sn does not affect external oxidation, but at the interface the presence of Cu and Sn leads to the formation of liquid Cu phases and infiltration of grain boundaries. The additional presence of Ni counteracts this formation, but due to its huge impact on iron activity it favors the formation of a rough steel/scale interface. In contrast with Ni, Cu and Sn hardly have any influence on iron activity. Numerical calculations based on a diffusion model and results of the well-known thermochemical software FactSage confirm these effects.

The influence of trace levels of O2 and H2O, contamination in an inert gas heat treatment atmosphere on the oxidation behvaiour of IN718 was investigated. Heat treatments consisted of holding IN718 at 1050 °C for 2 h in a combined thermogravimetric balance and gas chromatography-mass spectrometer (GCMS). Furnace atmospheres explored included 22–703 ppm O2 and H2O concentrations of 23–387 ppm. The GCMS measurements were able to quantify the O2 and H2O concentrations during heat treatment and revealed that oxidation became measurable at approximately 800 °C. The oxidation rate was parabolic during the 1050 °C isotherm, increasing linearly with an increase in either O2 or H2O concentration up to a value of 480 ppm. Beyond 480 ppm the oxidation remained constant and equivalent to that reported in air. A two layer surface oxide structure consisting of Cr2O3 and TiNbO4 formed when the O2, and H2O content increased beyond 33 and 23 ppm respectively. Dry O2 conditions (i.e. H2O of approximately 25 ppm), caused spalling of the Cr2O3 oxide surface during cooling when the O2 ppm was 124 ppm or above. In higher H2O concentrations the Cr2O3 layer showed good adherence to the base metal and no cracking during cooling. The use of a He–5% H2 carrier gas did not alter the oxidation rate significantly, but did increase the H2O concentration, thus preventing oxide spalling during cooling.

In most cases, platinum-based alloys are mainly used in high-temperature oxidation environments, and mastering their oxidation behavior and the impact of oxidation on performance is crucial. Two new platinum-based high-temperature alloys, Pt-10Rh-0.5Zr and Pt-10Rh-0.5Zr-0.2Y, were designed and prepared in this study. The research focuses on the high-temperature oxidation behavior of the alloys in air and the influence of oxidation on the room temperature mechanical properties of the alloys. The results show that the relationship between oxidation weight loss and temperature of these two platinum-based alloys conforms to the Arrhenius equation within the temperature range of 1400–1600 ℃, and the oxidation resistance of Pt-10Rh-0.5Zr-0.2Y alloy is better than that of Pt-10Rh.0.5Zr alloy. Examination of the surface and fracture morphology of these oxidized platinum-based alloys revealed that zirconium and yttrium oxide particles, such as ZrO2 and Y2O3, with different morphologies and structures were formed. The study also found that adding a small amount of zirconium and yttrium can significantly improve the room temperature ultimate tensile strength of Pt-10Rh alloy. However, after 20 h of high-temperature oxidation treatment at 1400 and 1500 °C, the tensile strength and plasticity at room temperature of both alloys showed a significant downward trend. Especially, the room temperature plasticity of Pt-10Rh-0.5Zr-0.2Y alloy decreased by more than 80% and exhibited a brittle fracture mode. Our research will contribute to the design and development of new high-temperature platinum-based alloys.

Thermal stability of nanocrystalline grains is a crucial factor that determines the unique microstructure and properties of the gradient nanostructured (GNS) materials at elevated temperatures. Nevertheless, oxidation is unavoidable for GNS metal materials utilized at high temperatures, potentially impacting the microstructure stability. In this study, we reveal the correlation between the high-temperature selective oxidation and the thermal stability of GNS layer through experiments and phase-field simulations. The improved oxidation resistance of GNS samples was ascribed to the excellent thermal stability of (Cr, Mn)3O4 oxides and a large proportion of low-energy twin boundaries. After prolonged oxidation, the GNS layer exhibited a bimodal microstructure. To analyze the elemental diffusion mechanism and microstructure evolution in the GNS layer, the phase-field simulation technique was employed. Selective oxidation led to the concentration of chromium reduced in the grain-boundary region, thereby diminishing the thermal stability of the grains and causing abnormal grain growth in the surface layer. Particularly, grain growth had a cumulative effect, the topmost grains coarsening will cause grain growth in the underlying layers, and subsequently, the grains in the interior region will also be gradually affected.

To address the considerable interest in LiF-BeF2 (FLiBe) compatibility for fission and fusion reactor applications, static and flowing compatibility experiments were conducted to assess the compatibility with type 316H stainless steel. In static testing at 550° and 650 °C, small mass changes were measured and posttest characterization of the FLiBe showed increased levels of Fe, Cr, Ni and Mn in the salt. Adding Be in the static salt test reduced the dissolution of Fe and Ni. An initial assessment of mass transfer in flowing FLiBe without a Be addition was conducted using a monometallic 316H thermal convection loop (TCL) operated for 1000 h with a peak temperature of 650 °C. Similar to prior results in flowing FLiNaK salt, the 316H specimens exhibited small mass losses in the hot leg. Posttest characterization of the 316H specimens suggested Cr surface depletion in the hot and cold legs and possibly Fe deposition in the cold leg. To further understand this behavior, Cr and Fe dissolution was measured in static FLiBe at 550–650 °C.

Abstract The transition from external to internal oxidation of a binary Fe-10Cr alloy has been investigated in Fe/FeO Rhines pack (RP) and H2/H2O between 850 and 900 °C. Internal oxidation is facilitated by increasing temperature and presence of water vapor. A classical Wagnerian diffusion analysis predicts external oxidation for ferritic (BCC) Fe-10Cr and internal oxidation for austenitic (FCC) Fe-10Cr. The α-to-γ transformation is demonstrated to be the primary factor promoting internal oxidation in Fe–Cr around 900 °C. Water vapor is believed to promote internal oxidation due to a higher reactivity of H2O compared to O2 and higher preferential adsorption of the H2O molecule.

Abstract The goal of this study was to use 18O-enriched water to better understand the role of H2O in high-temperature oxidation. Seven model and three commercial M-Cr and M-Cr-Al alloys were studied in air with 10% of H2O at 800 °C for 5 h. Oxygen from water vapor was more reactive than oxygen from the air and 18O enriched at the outermost layers of the formed Cr- and Al-rich oxides. Alloys with Al and/or Ti additions showed signs of internal oxidation but 18O was not enriched inside the alloy in locations with internal oxidation. Depending on the alloy Al content, the oxide went from Al oxidation beneath a chromia scale to external alumina scale formation.

Abstract The transformation of wüstite (FeO) in the oxide layer formed during high temperature oxidation (600 °C and 700 °C) on hot-worked tool steel was investigated. Wüstite plays an important role in the oxide layer of these steels used for hot working. However, understanding its transformation behavior during cooling is crucial for controlling the final oxide layer structure. Slow cooling rates have a significant influence on the final wüstite content, resulting in inaccurate representations of the composition of the oxide layer at temperatures above 570 °C. The aim of this study was to determine the influence of cooling rate on the wüstite content in the oxide layer after high temperature oxidation. It was found that for hot-worked steel samples oxidized at 700 °C or higher, a cooling rate of more than 1000 °C min−1 is required to suppress the eutectoid transformation and maintain the realistic wüstite content. At lower temperatures (570 °C–600 °C), a cooling rate of more than 100 °C min−1 is required to achieve the wüstite content observed at oxidation temperatures in the oxide layer. Overall, the hematite and magnetite contents also vart with the cooling rate, which is associated with changes in the wüstite content.

AbstractAnalysis of scanning electron microscope (SEM) images is crucial for characterising aluminide diffusion coatings deposited via the slurry route on steels, yet challenging due to various factors like imaging artefacts, noise, and overlapping features such as resin, precipitates, cracks, and pores. This study focuses on determining the thicknesses of the coating layers Fe2Al5 and, if present, FeAl, pore characteristics, and chromium precipitate fractions after the heat treatment that forms the diffusion coating. A deep learning SEM image segmentation model utilising U-Net architecture is proposed. Ground truth data were generated using the trainable Weka segmentation plugin in ImageJ, manually refined for accuracy, and supplemented with synthetic data from Blender 3D software for data augmentation of a limited number of SEM label images. The deep learning model trained on a combination of synthetic and real SEM data achieved mean dice scores of 98.7% ± 0.2 for the Fe2Al5 layer, 82.6% ± 8.1 for pores, and 81.48% ± 3.6 for precipitates when evaluated on manually labelled SEM data. The deep learning procedure was applied to evaluate a series of SEM images of diffusion coatings obtained with three different slurry compositions. The evaluation revealed that using a slurry without a rheology modifier may lead to a thicker partial Fe2Al5 layer that is formed by inward diffusion. The relation between the outward and inward diffusion Fe2Al5 layers was not affected by the coating thickness. The thinner diffusion coating presents lower pores and chromium precipitate fractions independently of the slurry selected.

The diffusion properties of polycrystalline materials depend on their grain shape and size, which determine the spatial distribution of grain boundaries. These morphological characteristics are of interest when evaluating an alloy ability to form a protective oxide scale by selective oxidation at high temperature. The composition changes induced by selective oxidation in 2D polycrystals were studied by finite element simulations. We examined the effect of the grain boundary orientation in lamellar polycrystals, and the effects of the grain size distribution in random equiaxed polycrystals. Fine-grained polycrystals were found to behave as uniform media. The effective diffusivity of fine lamellar polycrystals depends on the grain boundary orientation and is bounded by the upper and lower composite diffusivities, while the effective diffusivity of fine equiaxed polycrystals can be estimated by a modified Hart equation. The behavior of coarser equiaxed polycrystal was shown to vary according to the local grain size: the concentration at the alloy-scale interface is fully determined by the local grain size in larger grains, while it is affected by the surrounding grains in finer grains. Increasing the grain size dispersion led to a more scattered response and shifted the minimum interface concentrations toward lower values, which is expected to have a detrimental effect on the oxidation resistance.

AbstractHeat treatments and oxidation tests were performed on systems composed of a single-crystal AM3 superalloy with a NiCoCrAlYTa+Pt coating to investigate the effect of Pt on Ta and Ti diffusion at high temperature. Experimental results were compared to an AM3 superalloy directly coated with Pt. For all the studied systems, the effect of Pt on elemental activities was evaluated through thermodynamic calculations. Pt diffusion was found to be faster in the NiCoCrAlYTa coating than in AM3. High Ta contents were measured in the Pt-rich γ′ phase below the surface and significant Al and Cr transport toward the surface was observed, in agreement with thermodynamic calculations which predicted an important decrease in their activities in the presence of Pt. An outward diffusion of Ti was also noticed, whereas calculations did not show a decrease in Ti activity due to Pt. Other discrepancies between experiments and thermodynamic calculations were noted and are discussed in this work.

High entropy alloys (HEAs) challenge conventional alloy design by incorporating five or more principal elements in near-equal atomic proportions, forming random solid solutions with simple phases. HEAs exhibit exceptional properties such as high phase stability, mechanical strength, corrosion, oxidation, wear, fatigue resistance, and notable thermal stability. While traditional methods like arc melting and casting are often used for HEA preparation, they pose limitations due to cost and processing challenges. Additive manufacturing has emerged as a transformative technique, enabling the cost-effective fabrication of complex structures with customized properties. Here, we summarized the following “state-of-the-art” additively manufactured alloy systems: AlCrCoNiX (X = Fe, Si, Ti, etc.) HEAs, CoCrFeMnNi HEAs, and refractory HEAs. This review focused on elucidating their oxidation properties, emphasizing key findings, challenges, and opportunities. It also discussed the potential strategies for enhancing oxidation resistance. Additionally, it highlighted research gaps and underscored the urgent need for further exploration to meet the demands for high-temperature applications.

AbstractThe influence of cobalt and cobalt–manganese oxide coating thickness on its ability to be a good diffusion barrier against Cr outward diffusion was investigated for stainless steel interconnects (AISI 441) of a solid oxide cell (SOC). The coatings were all synthesized using a DLI-MOCVD (Direct Liquid Injection-Metal Oxide Chemical Vapor Deposition) hot wall reactor. The study shows that a minimum cobalt oxide thickness of 300 nm was needed to be a good diffusion barrier against Cr for the 500-h exposure test. This observation was linked to the Mn concentration reached in the cobalt spinel during exposure. Indeed, during exposure at high temperature, Mn diffused from the substrate into the cobalt coating and transformed cobalt spinel into Co-Mn spinel. Whereas pure cobalt spinel was a good Cr diffusion barrier, cobalt-manganese spinel, Co3-xMnxO4, was not when x > 2. The thickness of the cobalt coatings must be chosen so that the Mn quantity coming into it from diffusion from the substrate does not degrade the protectiveness of the coating.

The addition of Si is known to improve the oxidation resistance of pure titanium and titanium alloys. However, most past studies concern the influence of relatively large Si contents (0.35–8 wt%), which is often incompatible with a good ductility because of the presence of Ti5Si3 precipitates. This study focuses on the influence of the Si content on the high-temperature resistance of near-alpha alloys from the family of Ti6244 alloy (6% Al, 2% Sn, 4% Zr 4% Mo–wt%) with small proportions of Si. The silicon addition was found to improve the high-temperature oxidation resistance of near-alpha Ti-Al-Sn-Zr-Mo family alloys by decreasing the oxidation rate and the oxide scale thickness, as well by producing a denser oxide scale. Rutile and alumina were detected within the oxide scale by XRD and Raman spectrometry. The presence of Si is associated with the presence of larger quantities of N at the oxide/metal interface.

AbstractDegradation of coatings and structural materials due to high temperature corrosion in the presence of molten salt environment is a major concern for critical infrastructure applications to meet its commercial viability. The choice of high value coatings and structural (construction parts) materials comes with challenges, and therefore data centric approach may accelerate change in discovery and data practices. This research aims to use machine learning (ML) approach to estimate corrosion rates of materials when operated at high temperatures conditions (e.g., nuclear, geothermal, oxidation (dry/wet), solar applications) but geared towards nuclear thermochemical cycles. Published data related to materials (structural and coatings materials), their composition and manufacturing, including corrosion environment were gathered and analysed. Analysis demonstrated that random forest regression model is highly precise compared to other models. Assessment indicates that very limited sets of materials are likely to survive high temperature corrosive environment for extended period of exposure. While a higher quality and larger dataset are required to accurately predict the corrosion rate, the findings demonstrated the value of ML’s regression and data mining capabilities for corrosion data analysis. With the research gap in material selection strategies, proposed research will be critical to advancing data analytics approach exploiting their properties for high temperature corrosion applications. Graphical Abstract

Aluminide coatings were prepared by chemical vapor deposition (CVD) method on Ni-based superalloy Mar-M247 to improve the corrosion resistance. The Na2SO4-induced hot corrosion behavior of Mar-M247 with and without aluminide coating was investigated at varying temperatures. The results revealed that the substrate underwent relatively mild corrosion attack at temperatures below the Na2SO4 melting point, but extremely severe corrosion attack above it. The aluminide coating significantly improved the corrosion resistance of the substrate, with the formation of Al2O3 scale during corrosion. The effects of both solid and molten Na2SO4 on hot corrosion resistance of Mar-M247 alloy and its aluminide coating was discussed, as well as the detrimental effect of tungsten on the substrate in ‘type I’ hot corrosion.

The microstructural evolution of T91 steel by pre-steam oxidation during liquid lead-bismuth eutectic (LBE) dissolution corrosion was investigated. A bi-layered pre-oxide film with Fe-rich outer layer and Cr-rich inner layer was formed on T91 steel, which was similar to the oxide film of T91 after oxidation corrosion in LBE. The pre-oxide film effectively protects the matrix from LBE corrosion at 620 °C. However, the composition and microstructure of the pre-oxide film changed dramatically. Unlike the original duplex structure, the pre-oxide film exposed to LBE undergoes a process of reduction of the outer layer and oxidative growth of the inner layer and changes into five layers, the loose and easily peeling outer Fe layer, the dense and intact inner layers successively consisting of Fe–Cr spinel layer, a transition layer of matrix and Cr-rich oxide, continuous Cr-rich oxide layer with tetragonal distorted spinel structure and amorphous SiO2 layer. The evolution mechanism of the pre-oxide film during LBE dissolution corrosion is discussed.

This paper highlights that hot corrosion at 750 °C can develop on the surface of AM1 nickel-based single-crystal superalloy (SX) turbine blades, whether in the As-Cast (AC) or Fully Heat-Treated (FHT) states even in the absence of SO3 (g) flow. It was found that the 1 and 3 mg/cm2 Na2SO4 deposits induce sulphidation, oxidation and basic flux at such low temperature like in Type I hot corrosion. Sulphidation is mainly located in the γˈ-depleted zone irrespective the substrate (AC and FHT). The metallurgical segregations in the AC superalloy extend the incubation period in contrast to what is observed upon pure oxidation. The increase in salt content showed a reduction in hot corrosive attack by forming a barrier layer.

AbstractThe effect of water vapor content on the oxidation behavior of In625 at 900 °C in synthetic air was reported. The higher the water vapor content, the greater the oxidation and volatilization rates were. Increasing the water vapor content led to an increase in the proportion of spinel and rutile-type oxides in the oxide scale compared to chromia, and the proportion of Al-rich oxides within the alloy. A kp-kv mass variation model was used to quantify the experimental results, and Fluent Ansys® CFD simulations of the gas phase were used to predict volatilization rates. CFD simulations were used to calculate local gas velocity, temperature and composition along with local volatilization rates at each point on the sample surface. It was possible to explain not only the variations in volatilization between upstream and downstream samples, but also the increased volatilization at sample corners. For longer durations, it was shown experimentally that the rate of volatilization decreases. This was explained by the enrichment of the oxide scale with spinel and rutile-type oxides.

The focus of this study was to compare the isothermal oxidation behavior of IN 939 nickel-based superalloys produced by selective laser melting and casting. Oxidation experiments were performed on both heat-treated and non-heat-treated, as cast and additively manufactured samples, to reveal the role of heat treatment and manufacturing methods on oxidation behavior. As cast samples underwent a two-step aging at 1080 and 843 °C, while a one-step aging was carried out for additively manufactured samples at 845 °C. The microstructure of the as cast IN 939 exhibited a dendritic structure with gamma prime precipitates. Following the heat treatment, primary and secondary gamma prime precipitates were formed. Additively manufactured IN 939 exhibited clearly visible melt pools and no trace of gamma prime precipitates. After heat treatment the melt pools disappeared, and gamma prime precipitates formed. Oxidation experiments were performed at 800, 900 and 1000 °C. All samples exhibited similar weight gain characteristics and obeyed a parabolic rate law. Spallation did not occur at 800 and 900 °C, whereas at 1000 °C all samples experienced spallation. The activation energies of all samples, calculated for three temperatures (800, 900, and 1000 °C), were similar, ranging between 260.99 and 287.51 kJ/mole. XRD and EDS analyses indicated that the oxide scale formed on all IN 939 samples was mainly Cr2O3 and TiO2 in rutile form. The internal oxidation and nitridation zones were investigated using SEM and image analysis. The results showed that at 1000 °C, internal oxidation and nitridation extended deeper into the bulk material for additively manufactured samples due to the finer and columnar grains along the building direction which contained extensive amounts of precipitates compared to cast microstructure.

Durability is still a critical factor that limits solid oxide cell (SOC) technology industrialization. In order to maintain a good level of performance for the overall targeted lifetime of about 40 kh, the oxidation of the interconnects made of ferritic stainless steel and Cr volatilization from this material to the cell electrodes have to be restricted. CeCo-based coatings were applied by PVD HiPIMS on AISI441 alloy. Their ability to reduce the thickness of the poorly conductive formed oxide and improve Cr retention was studied at sample scale by measurements of weight gain and Cr content by ICP-OES after 5000 h of exposure in ambient air at 700 and 800 °C. In the testing conditions, post-test characterization by SEM/EDX showed that oxide scale thickness was reduced when coatings were applied compared to bare AISI441 steel. Moreover, the strong oxide scale spallation observed at 800 °C with bare AISI441 steel was avoided. Cr volatilization was also strongly decreased. Post-test SEM/EDX and ToF–SIMS characterization of a short stack integrating coatings on the air side in some repeat units (RU) confirmed the limited Cr diffusion in the strontium doped lanthanum manganite (LSM) contact layer when the coating is present after 5200 h of solid oxide electrolysis cell operation (SOEC).