Oxidation of Metals最新文献

Regarding potential high temperature applications of materials based on the ternary Al₂O₃-YAG-ZrO₂ (AYZ) eutectic system for turbine blades, their environmental behaviour has to be evaluated in near-service conditions. CMAS attack is one of the phenomena limiting lifetime of ceramic E/TBC coatings for aircraft engines. The present study reports an assessment of the reactivity of AYZ eutectic material facing with two molten CMAS. The effect of test duration (up to 500 h), CMAS composition (with or without Fe₂O₃), microstructure (interlamellar distance), temperature (1250 and 1350 °C) as well as thermal cycling resistance were investigated. Results evidenced that the interaction of CMAS with AYZ eutectic ceramic was limited regardless of the microstructure. No infiltration of molten CMAS within the eutectic ceramic nor mechanical rupture between CMAS and AYZ was observed. The dissolution of AYZ leads to the rapid formation of anorthite with the CMAS components that hinders further reactions. The inward growth of MgAl₂O₄ spinel in place of the Al₂O₃ network occurs in accordance with the thermodynamic available data and without any effect on AYZ integrity.

Ni–30Cr model alloy was used to study chromia-scale formation behaviour. During its formation the mass variation signal is weak and it is important to take specific care to measure oxidation kinetics by thermogravimetric analysis. Symmetrical design allows the determination of very small mass changes in compensating buoyancy effects and limiting measurement drift. The mass variation measurement comprises noise coming from different sources that affects the electronic signal. It must be minimised to improve accuracy. This involves keeping the room-temperature constant, strictly balancing the beam and minimising buoyancy effects. By this way it was possible to acquire kinetics and to measured rate constants, k_p, in a range from 3.10^–5 to 3.10^–10 mg².cm⁻⁴.s⁻¹ equivalent to 10^–12 down to 10^–17 cm².s⁻¹. Oxygen partial pressure (({P}_{{O}_{2}})) was monitor and revealed an abnormal consumption of oxygen at the beginning of the thermal exposure. Experiments with an inert material showed parasitic reactions identified by mass spectrometry as combustion of impurities. As oxygen consumption is not only due to oxidation of the sample, corrosion kinetics can’t be deduced from it. Hence, to determine whether the oxygen supply from the gas is a limiting parameter, a model, which quantifies oxygen consumption by sample oxidation within the thermobalance, is proposed.

The morphologies of Type-II hot corrosion attacks were investigated at 650 °C for the DS200 + Hf nickel-based superalloy. The relationship between corrosion rate and microstructural features, i.e., grain boundaries, carbides, eutectics, dendrites, was considered at different scales and for three environments. Corrosion tests and related characterizations were carried out on a total of forty-five samples, including different sampling strategies, test conditions (air, air + 150 ppm SO₂ and air+  400 ppm SO₂) and exposure times (24, 50 and 100 h). The amount of SO₂ in the inlet gas was found to be the main factor in the degradation, affecting both the duration of the incubation period and directly the corrosion rate. The proportion of grain boundaries as well as their orientation did not have any influence on the degradation kinetics. On the contrary, MC carbides and γ + γ′ eutectic pools, i.e., the interdendritic area resulting from the solidification stage, underwent deeper attacks for all gaseous atmospheres. The global mechanism can be explained by the SO₃-induced hot corrosion mechanism.

An air-oxidized carbon/carbon (c/c) composite was studied under tensile conditions to examine oxidation temperature’s influence on its behaviour and on the residual mechanical properties. Three oxidation temperatures were tested with oxidation duration chosen to obtain the same weight loss on the samples at each temperature. Notable differences were found in the material’s behaviour depending on the oxidation temperatures. These results show that the weight loss criterion commonly used in the literature to quantify c/c composite oxidation needs to be complete to reflect the actual state of the material after oxidation. Microscopic observations revealed different oxidation patterns that underlined a heterogeneous consumption of the material from one temperature to another. Consequently, a non-destructive-test based on ultrasonic signals was explored in order to complete the weight loss criterion. Variations of weight loss at each exposure oxidation temperature were correctly assessed, and the technique was able to discriminate the oxidation temperatures above 5% of weight loss. The low-temperature oxidation behaviour is discussed and an explanation based on a variation on the attack of the fibre/matrix interfaces that could explain the difference of the tensile behaviour is proposed.

Predicting the gas-surface interactions of solid carbon is necessary for the design of many engineering systems that employ graphite. Experimental determination of the reaction rates improves the fidelity of those predictions. Here, we studied oxidation and nitridation of graphite by thermal and nonthermal plasma assisted processes. Experiments were conducted at a pressure of 2 kPa, higher than previous flow reactor experiments of this kind and closer to the conditions experienced in engineering applications. At these higher pressures, the limitations of mass transport and the interference between oxygen and nitrogen species become important. Reaction rates were determined from mass loss, reaction products were identified with mass spectrometry, and surface roughening was characterized by electron microscopy.

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This study provides a comprehensive investigation of the formation and behavior of a layer enriched in fission products encountered between the (U,Pu)O₂ fuel pellet and the cladding, designated as JOG (“Joint Oxyde Gaine” in French). Employing a multifaceted approach that combined thermodynamic calculations, experimental synthesis, and advanced characterization techniques, simulated JOG has been synthetized (without radioactive fission products). Using thermodynamic calculations with the TAF-ID database, the phase compositions in the JOG was assessed for various temperatures, pressures, and oxygen potential conditions, revealing insights into the environmental factors influencing JOG formation. Experimental simulation of the JOG composition, exposed to controlled conditions, confirmed the presence of key compounds such as Cs₂MoO₄, CsI, and PdTe, as evidenced by SEM, EDS, and XRD analyses. The results of the calculations highlighted notable differences in the nature of the phases constituting the JOG under varying pressure and oxygen potential conditions. At 873 K and oxygen partial pressure of 10^–4 bar, Cs₂MoO₄, Pd–Te, and a gas phase rich in tellurium and CsI were predominant, contrasting with the emergence of liquid phases at 70 bar. This study offered a comprehensive understanding of JOG microstructure, and highlighting the importance of accurate characterization for reactor safety. This information lays the foundations for future studies on the chemical interaction between the JOG and the steel cladding.

Ni-based superalloys are commonly used in gas turbines because of their exceptional high-temperature mechanical properties. To secure a long service life, the materials must also have sufficient corrosion resistance. Therefore, diffusion coatings are widely used to enrich the surface in protective oxide scale-forming elements. For temperatures between 650 and 950 °C, where hot corrosion occurs, Cr-based coatings are advantageous. These are commonly applied via the laborious pack cementation process. Recently, a novel cost-effective Cr/Si slurry coating process has been developed which demonstrated resistance to oxidative high-temperature environments. Here, the protection of the slurry coatings against hot corrosion type I at 900 °C on the Ni-based superalloy Rene 80 is investigated and compared to coatings produced by pack cementation. Prior to the 300-h exposures in air containing 0.1% SO₂ at 900 °C, 4 mg/cm² of Na₂SO₄ was deposited on the material surfaces. The uncoated Rene 80 exhibited rapid dissolution of the initial oxide scale followed by catastrophic break away oxidation. In comparison, the slurry coatings showed significantly improved hot corrosion resistance compared to the uncoated alloy and a better protection than a Cr pack cementation coating. The Cr pack cemented Rene 80 showed improved hot corrosion resistance, but Cr depletion in the subsurface zone occurred with increasing exposure time, associated with the propagation of Al internal oxidation and increasing sulfidation. In contrast, the slurry coatings formed an external Cr₂O₃ scale coupled with an agglomeration of SiO₂ underneath and a continuous Al₂O₃ subscale which offered a better diffusion barrier and leading to superior long-term protection against hot corrosion.

The impact of Cl on alkali-induced high-temperature corrosion of stainless steels/FeCrAl alloys after breakaway oxidation was investigated in a simulated biomass- and waste-fired boiler environment at 600 °C. For this investigation, three alloys were exposed to low Cl load environment (H₂O+KCl) and to high Cl load (H₂O+KCl+HCl). Post-exposure analysis showed that the stainless steel SVM12 experiences fast oxidation and forms thick double-layered Fe-rich oxide scales. The corrosion attack is further accelerated with addition of HCl for this material with the effect being more pronounced in the inward-growing scale. The FeCrAl and FeCrNi alloys exhibit slower oxidation kinetics after the breakaway corrosion compared to SVM12 in the H₂O+KCl exposure. Furthermore, in contrast with SVM12, the addition of HCl did not accelerate the corrosion attack on these alloys. It is argued that the properties of the secondary oxide layer formed after breakaway corrosion are important in the continued corrosion resistance against chlorine-induced corrosion attack. Especially, the Cr content in the inner scales is suggested to be important in corrosion mitigation.

The mechanism of oxide scale exfoliation from P92 steel during formation in CO₂ under over-temperature conditions at 650 ℃ was studied by the experiments, thermodynamics, and molecular dynamics. A duplex oxide scale with an inner FeCr₂O₄ scale and an outer Fe₃O₄ scale was formed on P92 steel, in which honeycomb pores were observed in Fe₃O₄ oxides. The honeycomb pore size and CO gas generation rate on P92 steel showed maximum values after 80 min. The exfoliation of Fe₃O₄ oxide scale was divided into two stages based on the deposition of carbon in the honeycomb pores.

High temperature corrosion and slag deposition significantly reduce the thermal efficiency and lifespan of biomass-fired boilers. Surface modification with protective coatings can enhance boiler performance and prevent commercial losses due to maintenance and damage. This review focuses on the development of corrosion-resistant coatings (CRCs) and anti-slagging coatings (ASCs) over the past decade. CRCs are explored through thermal spray processes that include arc spray, atmospheric plasma spray (APS), high-velocity oxygen fuel (HVOF), detonation gun (D-gun™), and cold spray. Studies on alloys, ceramics, and ceramic–metal composites are summarised, highlighting the high temperature corrosion prevention mechanisms and discussing new coating materials. ASCs are reviewed in the context of advancements via thermal spray and slurry spray methods. The mechanisms for slag reduction, testing methods to evaluate ASC effectiveness, and the necessary architecture for preventing slag deposition are examined. A lab-based rig simulating fly ash deposition onto water-cooled coating coupons for anti-slagging investigations is also presented. Further research is needed to develop and evaluate materials for ASCs effectively.

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