Oxidation of Metals最新文献

If a rate of metal oxidation is diffusion-controlled, then a thickness of a scale emerging on its surface at constant temperature is typically a parabolic function of time. If temperature changes, then this inherently parabolic temporal evolution of the thickness may transform into differently shaped functions. By using a concept of equivalent times introduced and elaborated in this work, it is shown how the oxide thickness versus time dependence L(t) can be established for any time–temperature profile T(t). Then, it is explored whether there exists a unique T(t) regime for which a growth rate is constant, i.e., for which L(t) is linear. It is proven that it is possible to design a time–temperature scheme for which the same holding times cause identical thickness changes. Such a growth mode, however, cannot be sustained indefinitely; there is a time threshold beyond which the linear growth of the oxide layer cannot be maintained any longer. Although oxidation is frequently a thermally activated process, mathematical expressions and conclusions remain the same for non-Arrhenius kinetics.

The oxidation behavior of AISI 5140 low-alloy steel was investigated under air at oxidation temperatures ranging from 800 to 1100 °C and oxidation times ranging from 5 to 120 min. The microstructure and chemical composition of the oxide layer were characterized, and an oxidation kinetic model was constructed. The oxide layer exhibited a multi-layered structure, with the outer layer consisting primarily of Fe₂O₃, the intermediate layer of Fe₃O₄, and the inner layer comprising FeO and pre-eutectic Fe₃O₄. At the scale-substrate interface, a Si and Cr enriched layer was present. This study employed vertical rolling to descale the surface of oxidized samples; the effect of secondary oxidation on descaling performance was investigated. The results indicated that after the specimens were oxidized at 1200 °C in air, they were subjected to descaling and rolling processes, with a large amount of residual oxides remaining on their surfaces and poor flatness. When the specimens were oxidized at 1200 °C in air and descaled, followed by secondary oxidation at 1000 °C in air, and then subjected to descaling and rolling processes, their surfaces exhibited reduced residual oxides and improved flatness.

The high-temperature corrosion behavior of Ni-35Fe and Ni-65Fe alloys embedded in sand containing 0%, 0.1%, or 1% of NaCl-KCl-CaCl₂ salt mixture at 480 °C in air was investigated. The growth rate of the iron oxide scale was found to be controlled by diffusion within the Ni-35Fe and Ni-65Fe alloys matrix. Internal oxidation and chlorination of Fe were observed in sand with salt contents of 0.1% and 1%. Chloride salts increased the growth rate of the iron oxide scale on the Ni-35Fe and Ni-65Fe alloys.

The oxidation of UO₂ in Ar–O₂ atmospheres was studied at temperatures between 350 °C and 600 °C and for oxygen partial pressures (pO₂) between 0.20 atm and 0.70 atm. The experiments were carried out on both powder samples and discs cut from sintered pellets. Oxidation kinetics were monitored by TGA and the oxides formed were characterised by XRD, SEM and specific surface area (SSA) measurements. Whatever the temperature or pO₂ tested, total oxidation of UO₂ to U₃O₈ was systematically observed. At all temperatures studied, the conversion of UO₂ to U₃O₈ seemed to involve the formation of an intermediate oxide, which could be U₃O₇. Experiments carried out on powders with different SSA appeared to show that the effect of solid texture predominates over the effect of temperature in the formation (or non-formation) of intermediate oxides. An increase in pO₂ systematically led to an increase in the UO₂ oxidation rate. On the other hand, an increase in the oxidation temperature seemed to cause a decrease in the density of cracks propagating through the samples, probably due to an increase in the plasticity of U₃O₈. The inert marker experiment also showed an inward growth of U₃O₈ involving anionic sublattice point defects.

To understand the influence of Mo, Si, and Ti during aluminizing pack cementation processes of Mo-Si-Ti alloys, two ternary Mo-Si-Ti alloys (eutectic Mo-20.0Si-52.8Ti and eutectoid Mo-21.0Si-34.0Ti) were investigated and compared with pure Ti and Mo-40Ti (all in at.%). The coating formation mechanisms, phase composition, and microstructures of the different substrates were compared. Subsequently, the effect of the different elements on the oxidation behavior was evaluated, using thermogravimetric analysis at ({700},^{circ }text {C}) and ({900},^{circ }text {C}) for 100 h in synthetic air. In addition, the type I (({900},^{circ }text {C})) and type II (({700},^{circ }text {C})) hot corrosion behavior of the Al-coated Mo-Si-Ti alloys was investigated for 24 h and 100 h in synthetic air + 0.1% SO(_2). While the initial Al-rich coating phase was consumed or transformed at ({700},^{circ }text {C}), it successfully facilitated the formation of a protective Al(_2)O(_3) scale on the surface, even if the underlying reservoir was diminished. At ({900},^{circ }text {C}), the Al coatings on both substrates failed, and a hot corrosion-induced pesting dominated. While Si generally has a positive effect on oxidation and hot corrosion resistance, the main impact of Mo is dictated by its evaporation, and Ti can lead to the formation of TiO(_2) as a mixed oxide with Al(_2)O(_3).

Enamel coatings with different thicknesses of 5 µm, 15 µm and 30 µm were deposited on 316L stainless steel. The coated steels were exposed to 600, 700, and 805 ℃ in an environment of NaCl + water vapor + air. The effect of coating thickness on the hot corrosion behavior was investigated. Results demonstrated that the corrosion resistance of the enamel coating declines with reducing thickness. The 5 µm thick coating exhibits minor cracks at 600 ℃ and peels off at higher temperatures. Small amounts of spalling pits emerge on the surface of the 15 µm thick coating at 805 ℃. The 30 µm thick coating effectively protects the steel substrate at the three temperatures studied.

This research investigates the boiler tube failure that was exposed to high-temperature service in the Arjo Didessa sugar factory, Ethiopia. The study aims to analyze the root cause and failure mechanisms of the water tube boiler exposed to high-temperature service for over eight years. Using a field-emission scanning electron microscope fitted with an energy-dispersive X-ray spectroscope and X-ray diffraction, the surface morphology and chemical composition of service-exposed boiler tube surface layers have been characterized. In addition, the research included the investigation of microstructural and micro hardness testing for both high-temperature service-exposed and virgin (new) tube samples. The composition analysis indicated the presence of calcium, magnesium, silicon, sodium, chlorine, and manganese, and deposit layers such as calcite and hematite (Fe₂O₃) were identified. This result revealed that the surface corrosion emerged from the interaction between the boiler tube surface wall materials and the boiler working environment. The metallurgical analysis result also confirmed the microstructural degradation in a boiler tube sample that was due to high-temperature service exposure for a long period. These microstructure changes brought on by exposure to high temperatures with surface oxidation during service deteriorate the mechanical strength of the material.

A new Pt-modified NiSiAlY coating was prepared in this study. Oxidation and hot corrosion behaviors were investigated at 750 ℃. The coating exhibited good resistance against oxidation, attributed to the formation of a dense and continuous Al₂O₃ scale. Pt could promote the selective oxidation of Al by increasing the Al/Ni ratio and suppressing the formation of NiO. During the hot corrosion, the coating caused more intense attack when exposed to NaCl. NaCl could cause oxychlorination reaction in the initial stage, leading to cyclic corrosion. The corrosion mechanism, including the role of NaCl in initiating and accelerating degradation, was discussed, providing insights into performance of the Pt-modified NiSiAlY coating in harsh environments.

In this study, the oxidation behavior of a 0.3 mm thick GH5188 alloy foil in air at 800–1150 ℃ and its effect on tensile properties were investigated. The results showed that the oxidation kinetics of the GH5188 alloy foil followed a parabolic law and exhibited a characteristic a two-stage oxidation process. A protective oxide scale primarily composed of Cr₂O₃ and MnCr₂O₄ formed on the foil surface. The consumption of chromium led to the formation and deepening of a carbide-free precipitation zone within the foil. However, thermal stress and growth stress led to stress concentration within the Cr₂O₃ layer, resulting in spallation of the oxide scale. After oxidation, strip-like precipitates interconnected at grain boundaries, forming a network structure, while fine granular precipitates were diffusely distributed within the grains. The room temperature tensile strength and plasticity of the alloy foil decreased significantly. The tensile strength and elongation of the initial state alloy foil are 1045.7 MPa and 66.4%, respectively. After oxidation at 1150 °C for 50 h, the tensile strength and elongation of the specimen decrease to 888.7 MPa and 35.6%. The fracture exhibited brittle fracture features due to the network structure within the matrix. Additionally, the tensile properties at high temperatures further deteriorated. After oxidation at 1150 °C for 50 h, the high-temperature tensile strength and elongation of the specimen decrease to 516.2 MPa and 30.4%. The deterioration of high-temperature tensile properties was related to stress concentration at oxidation-induced voids.

An experimentally validated and self-standing CFD (computational fluid dynamics) model was used to analyze chromia volatilization under high-velocity conditions close to industrial applications range, in wet air and pure O₂ environment at 800 and 900 ℃. A rig of complex geometry with several lined-up samples was designed. The simulations revealed the combined and non-trivial influences of gas-phase enrichment from upstream samples, local gas velocity and ratio between sample surface and corresponding free volume for gas flow. The CFD results also validated the often-used analytical approach for single sample planar geometries. For more complex situations, the CFD route is necessary.