Oxidation of Metals最新文献

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.

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.

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.

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.

Analysis 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.