Oxidation of Metals最新文献

The oxidation of δ-stabilized plutonium alloy was studied under dry oxygen exposures for temperatures varying from 100 up to 300 °C and oxygen partial pressures varying from 10^–4 up to 500 mbar. The coupling of X-ray diffraction, Raman spectroscopy and FIB-SEM has allowed to show that the oxide scale is composed of an outer layer of PuO₂ and an inner mixed layer of α + β-Pu₂O₃ platelets propagating into a metallic zone corresponding to the stable phase of unalloyed Pu. Furthermore, the analysis of Pu oxidation kinetics has displayed first a parabolic growth governed by the diffusion of interstitial oxygen. This step consists of the thickening of the Pu₂O₃ layer with a decrease in α-Pu₂O₃ ratio in favor of β-Pu₂O₃. Then, a second step occurs consisting of a linear growth of the PuO₂-layer with the formation of thick nodules which tend to cover the whole oxide surface. Based on the results of this work, a general oxidation mechanism for δ-Pu alloy is provided.

Understanding of the hot corrosion behavior of each constitutive phase of the new Pt-enriched γ/γ′ nickel-based single-crystal TROPEA superalloy is of significant interest for further optimization. Therefore, single-phase γ and γ′ materials containing a low content of Pt (0.13% at.) were manufactured by inductive melting and their behavior was investigated at 900°C with 1 mg/cm² of Na₂SO₄ deposit using thermogravimetry in air and air + 400 ppm of SO₂. After 24 h in air, the scale grown on the γ′ model alloy consisted of an outer heterogeneous NiO layer and of an internal porous α-Al₂O₃ layer. The addition of 400 ppm of SO₂ to air did not really change the nature and morphology of the oxides formed for γ′, but the measured linear rate was tenfold higher than that recorded in air. The γ alloy underwent an incubation period during which the corrosion rate was limited, followed by a propagation stage. In air, the alloy developed an external continuous NiCr₂O₄ spinel layer and a thin internal Cr₂O₃ subscale. The γ alloy was able to efficiently re-passivate after the propagation period. With 400 ppm SO₂, only a thin protective layer Cr₂O₃ formed on the surface of γ, with large Cr₂O₃ crystals growing within the residual Na₂SO₄. The occurrence of Cr-rich sulfides was also greater in the γ′ model alloy compared to the γ counterpart both in air and in air + 400 ppm of SO₂. Therefore, the results clearly evidenced the effect of SO₂ on the type I hot corrosion.

A stepwise multi-temperature thermogravimetric analysis (SMT-TGA) method is a rapid and time- and material-efficient measurement procedure for oxidation kinetics over a wide range of temperatures. It is suitable for alloy design and material selection procedures. It involves subjecting a sample to a series of steps at increasing temperatures, followed by steps at decreasing temperatures to identify possible effects on the evolution of oxide layer microstructures on oxidation kinetics. This method has been tested for a wide range of metallic alloys in the present work, allowing for the mapping of possible ranges of parabolic oxidation kinetics of industrial alloys between 600 and 1300°C. Two examples of effects of thermal history have also been described in this publication.

Titanium and titanium alloys are extensively used in the aerospace, automotive, and medical industries due to their high chemical and mechanical stability. In a previous study, the influence of water vapor on the growth of the oxide scale and the formation of the oxygen diffusion zone (ODZ) for Ti-6Al-4V was investigated using a 6-zone furnace. To elucidate the effect of water vapor on the oxide scale growth and ODZ, without the effect of alloying elements on diffusion, a systematic comparative study at 500, 600, and 700 °C for up to 500 h was carried out on pure Ti. Inert marker experiments showed that outward scale growth and diffusion of Ti⁴⁺ were promoted by water vapor. Additionally, the extent of oxygen enrichment in the subsurface zone (ODZ) as a function of temperature and time was determined for pure Ti by nanoindentation profiles and compared with results obtained for Ti-6Al-4V. The thickness of the ODZ increased with increasing temperature and time for dry air and humid air. The diffusion of oxygen ions within pure Ti and Ti-6Al-4V was not significantly affected by the presence of water vapor in the oxidizing environment. The effect of water vapor on the oxide scale spallation was found to be less critical for pure Ti when compared to Ti-6Al-4V.

The use of ternary molten carbonate mixtures Li₂CO₃—K₂CO₃—Na₂CO₃ as heat transfer systems for the CSP technology as well as for heat storage for the chemical industry is widely under consideration. Experiments were carried out on austenitic steels DMV310N compared to nickel alloys in order to evaluate the corrosion properties in a molten 33 wt.% Li₂CO₃—33 wt.% K₂CO₃—34 wt.% Na₂CO₃ mixture at 700 and 750 °C for 1000 h in closed crucibles. The austenitic steel DMV 310N passivates by the formation of an outer LiFeO₂ scale due to its iron content. If the iron content is low (< 5 wt.%), as in Alloy 625 the alloy forms NiO, which obviously does not passivate the material and leads to a strong internal corrosive attack. It has been shown by short-term experiments (3, 30, 300 and 1000 h) that a quick formation of LiFeO₂ is necessary to avoid chromium dissolution and NiO formation. If LiFeO₂ is formed quickly, the growth of the internal corrosion front by chromium dissolution is retarded.

The present study describes the corrosion behavior of alloy 625 fabricated by laser metal deposition-powder in the presence of a solid NaCl deposit in laboratory air at 650 and 800 °C. The results showed that at both temperatures, the presence of the deposit had a catastrophic effect on the corrosion resistance of the alloy. The active corrosion mechanism resulted in a very thick and non-protective oxide scale and in a highly damaged metal zone beneath the oxide scale. Although the mechanism involved was the active corrosion mechanism at both 650 and 800 °C, differences were observed between these two temperatures. At 800 °C, the corrosion products were thicker, and the substrate was significantly more damaged, especially due to the formation of an interconnected network of voids. At 650 °C, the thick and continuous Cr₂O₃ layer, predominantly observed at 800 °C, was not present. The use of thermodynamic data helped to suggest possible explanations for the observed differences. Overall, the increase of temperature accelerated the degradation of the alloy and it was enhanced by a radical change of the main reactions of the active corrosion mechanism between 650 and 800 °C.

This article focuses on the fine characterization of steels commonly used in the petrochemical industry damaged by the phenomenon of high temperature hydrogen attack (HTHA). The study was conducted in two steps. To begin with, a damaged 0.5-Mo pearlitic steel from the petroleum refineries, submitted to HTHA for decades, was characterized in detail using multiscale electron microscopy techniques. As part of an upstream study to better understand the onset and the growth of cavities, a brand new SA516 grade 60 low carbon–manganese steel was subsequently exposed to accelerated HTHA conditions through interrupted cycles carried out in autoclaves and then examined. Numerous cavities, plausibly filled with methane, were noticed in both materials. These cavities were mostly located at ferrite–pearlite grain boundaries along carbides and at triple grain boundaries near large carbides. The 0.5-Mo pearlitic steel showed cavities reaching significant sizes, up to 1 µm, but surprisingly no cracks were observed in the depth of the pipe. The major outcome is that 3D focused ion beam–scanning electron microscopy combined with transmission electron microscopy (TEM) analyses unveiled different natures of precipitates as well as in and nearby HTHA cavities for both 0.5-Mo and low carbon–manganese steels. Inclusions, likely AlN, but also Mo- and Cu-rich precipitates were observed in cavities of the industrial steel. These results confirmed a previous study performed on a similar industrial steel that drew a possible correlation between cavities nucleation and the intersection of transgranular inclusion-enriched plane with a grain boundary or carbides in pearlite grains (Flament in Microscopy and Microanalysis 28:1602–1604, 2022).

Due to their higher thermal and chemical stability than other high-temperature materials, chromium-silicon-base (Cr-Si-base) alloys are promising materials for future gas turbines and other high-temperature applications operating under harsh conditions. To enable near-net-shape casting of Cr-Si-base alloys, a compatibility of the alloy melt with the ceramic crucibles and molds is necessary. Additionally, a metal-ceramic contact exists at the interface between thermal barrier coating (TBC) and alloy, where metallic may melts play a role in the case of coating failure and overheating. In this study, molten Cr₉₂Si₈ (in at. %) alloy is brought into contact with powders of ceramics commonly used for casting molds or crucibles (e.g. ZrSiO₄, Al₂O₃, 3YSZ), to investigate liquid metal corrosion, interdiffusion, and stabilities. Additionally, the high entropy oxide (Sm_0.2Gd_0.2Dy_0.2Er_0.2Yb_0.2)₂Zr₂O₇ (HEO), a potential future TBC material, is investigated. Before melting using an electric arc furnace, the powders of the investigated ceramics were mixed with pulverized Cr₉₂Si₈ and pressed into alloy-ceramic pairs, to maximize the contact area between molten metal and ceramic. For microstructural investigations and phase analysis, the materials were assessed using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The widely used mold material ZrSiO₄ and the coating BN were found to decompose, while reaction products of SiO₂ and CoAl₂O₄ with the melt were detected. Al₂O₃, 3YSZ, and the HEO did not show decomposition or corrosion by the melt. Al₂O₃, 3YSZ, and the HEO are therefore considered as promising crucible, mold, and TBC materials for Cr-Si-base alloys.