Oxidation of Metals最新文献

The microstructural behavior of sintered Ni-based composites was evaluated to understand the performance of the composites in corrosive and abrasive environments. Ni–Cr–Si, Cr₃C₂–Ni–Cr, Ni–Cr–Ti, and Ni–Cr–Mo composites were synthesized using powder metallurgy at 1350 °C for 1 h. Field emission scanning electron microscopy (FE-SEM) equipped with electron diffraction (EDS) was utilized to analyze the microstructural evolution on both the surface and cross section after exposure. Phase identification was conducted using X-ray diffraction (XRD). Mechanical and tribological properties were assessed via surface hardness testing and erosion evaluation, respectively. Corrosion testing was performed under salt vapor conditions at 600 °C for 100 h, while erosion testing was conducted at a 90° impingement angle and 40 kPa erodent pressure. Among the composites, Ni–Cr–Mo demonstrated excellent resistance to corrosion and erosion, with values of 5.90 × 10^–5 mm/y and 0.955 mg/g, respectively. It is attributed to dendritic nickel matrix and eutectic micro-Mo₂C, which also enhanced surface hardness to a value of 274 HV. In contrast, chromium carbide phases present in Ni–Cr–Si, Cr₃C₂-Ni–Cr, and Ni–Cr–Ti contributed to localized fracture and cracking. These results highlight Ni–Cr–Mo as a promising candidate for high-performance applications in harsh environments.

A series of results obtained with different types of Ni-based alloys for turbine discs and blades and exposed to various (Type I and Type II) hot corrosion conditions (air, air + 150–400 ppm SO₂) is reported. Both the continuous mass change measurements and the characterizations of alloys after a brief exposure at high temperature with sulphate deposits, in air or in air + SO₂ (g) atmospheres, clearly demonstrate fast oxidation rates from the earliest time of exposure in the presence of sulphate deposits, i.e. accelerated oxidation. In situ SEM observations also support these findings. It clearly appears that hot corrosion often starts with a direct reaction involving the metallic substrate, the gaseous species and the sulphate deposits, i.e. it occurs in most cases without any incubation period. The results can therefore be of interest for the development of complementary selection procedures for alloys and coatings.

Various instrumental methods for analyzing high-temperature corrosion of boiler materials were explored and compared. These methods were applied to gain deeper insights into corrosion due to two salt mixtures containing Na, K, SO₄, and Cl below and above the mixtures’ first melting points. Stainless steel AISI316 and high-alloyed Sanicro28, typically used in heat exchangers in power plants, were exposed to salt mixtures in a laboratory tube furnace for 168 h. The extent of the metal corrosion following exposure was measured through mass loss, changes in the surface topography using optical 3D imaging, and dimensional metrology. Additionally, the morphology, thickness, and composition of the formed oxide scales were characterized using SEM–EDX. The information gathered from each method confirmed the impact of the synthetic salt deposit and temperature on the metal corrosion. Combining several methods enables detailed studies of changes taking place on the metal surface after exposure to challenging environments. The results also suggested that partial melting of the deposit had a higher impact on the corrosion than its chloride content.

The aim of the present work is to establish a kinetic law for the oxidation of Ti-6Al-4V (Ti64) spherical powder at high temperatures miming the possible oxidation of such powder within a laser powder bed fusion process. The oxidation experiments were followed by isothermal and isobaric thermogravimetry between 700 and 750 °C, under a controlled partial pressure of O₂ in the range of 0.1 to 0.75 atm. Beside the duplex structure of the oxide layers formed, namely an inner layer composed mainly of TiO₂ and an outer one composed of Al₂O₃, it was found that the oxidation rate is limited by one rate-determining step occurring in a single reaction zone: the Al₂O₃ layer. The study of how the growth rate varies with the partial pressure of O₂ highlighted that the rate-determining step is the diffusion of interstitial oxygen as a dumbbell in this Al₂O₃ layer. Based on physico-geometrical description of the reaction, a complete reaction rate equation is then proposed by taking into account the spherical geometry and the dimensions of the Ti64 particles as well as a dependence of the reaction rate with temperature and partial pressure of O₂. The rate law is very satisfactorily confronted to the experimental data.

Superalloys are high-performing alloys and serve as an important class of structural material for utility in gas turbine key components where high temperatures and pressures are involved. However, prolonged exposure to severe oxidation causes material degradation, which eventually affects the mechanical properties of alloys and results in component failure. Therefore, the material failure at high temperatures can be minimized by surface treatments such as the provision of coatings. On account of protecting the metal components such as gas turbine blades and combustion chamber structures that are subjected to high temperatures, the method of provision of thermal barrier coatings (TBCs) becomes mandatory. The coating extends the life of the component by lowering the oxidation and thermal fatigue, at the same time enhancing the substrate durability by providing excellent thermal insulation to the gas turbine components, to make them operate at higher temperatures. Investigations were conducted on several coating methods, including plasma spray, electron-beam physical vapor deposition with bond coat, and topcoat materials, on the superalloy substrate materials. This review focuses on thermal barrier coating processes, the new coating materials, their property at high-temperature conditions, and subsequent failure mechanisms during their utility in gas turbine applications.

The long-term oxidation behavior of the HAYNES^® 282^® superalloy was systematically investigated in air at temperatures ranging from 800 to 950 °C for durations of up to 720 h. The oxide phases that developed on the surface of the alloy were characterized using X-ray diffraction and energy-dispersive X-ray spectroscopy (EDS). The residual stress within the Cr₂O₃ layer was assessed utilizing the average X-ray strain method. The primary oxide phase was identified as rhombohedral Cr₂O₃, with secondary phases including rutile-TiO₂, spinel-MnCr₂O₄, and perovskite CoTiO₃. The thickness of the external oxide layer increased with both oxidation temperature and time, adhering to parabolic kinetics. EDS mapping indicated the dispersion of Al-rich and Ti-rich oxides internally, suggesting the precipitation of Al₂O₃ and TiO₂ beneath the external Cr₂O₃ layer. The activation energy for the long-term oxidation of HAYNES^® 282^® was calculated to be 272.5 ± 15.0 kJ mol⁻¹. The total residual stresses within the Cr₂O₃ phase measured at room temperature were found to be entirely compressive. The calculated intrinsic residual stress associated with Cr₂O₃ growth at 800 °C exhibited a transition from tensile to compressive, whereas at 950 °C, it remained tensile. The evolution of intrinsic stress in relation to oxidation time, temperature, and scale thickness was discussed in the context of the crystallite coalescence model and the Pilling–Bedworth ratio.

The structure, morphology and high-temperature oxidation of composite powders made by high-energy ball milling of powders, mixture of aluminum, activated carbon and graphite in the ratio 95/4/1, were investigated. The ball milling was carried out in AGO-2U laboratory planetary mill with water cooling. The grinding medium consisted mixture of hexane, SPAN-80 (sorbitan monooleate), and paraffin. The content of SPAN-80 was varied within the range of 1 to 7.5% by weight (wt). It was found that oxidation of aluminum in composite powders proceeds in two clearly distinguished stages, and the oxidation rate is extremely high. The reasons for the high reactivity of aluminum in composite powders are discussed.

Graphical Abstract

The oxidation behavior of Ni36 was investigated through high-temperature oxidation experiments under air conditions at oxidation temperatures of 1000 °C and 1200 °C and oxidation times ranging from 0.5–3 h. A kinetic model for internal oxidation was established to elucidate the formation mechanism of the internal oxidation zone. The results indicate that the external scale consists of two layers: outer layer primarily composed of Fe₂O₃ and inner layer primarily comprising a mixture of Fe₃O₄ and NiO. The internal oxidation zone exhibits both intragranular and intergranular oxidation structures, intragranular oxidation primarily composed of Fe₃O₄ and FeO, intergranular oxidation predominantly consists of Fe oxides. At 1200 °C, the depth of the intragranular oxidation approaches that of the intergranular oxidation. Single-pass hot rolling experiments using wedge-shaped samples were conducted to study the effects of heating temperature and reduction ratio on the deformation behavior of the internal oxidation zone. The results show that cracks and holes in the internal oxidation zone decrease as the heating temperature increases. Furthermore, under the influence of tensile stress along the rolling direction, cracks and holes are more severe in the side surface compared to the rolled surface.

The oxidation behaviour of binary Ni-(3, 5)Al and ternary Ni-(3, 4, 5)Al-(15, 20)Cr alloys (all in wt%) was investigated in both Ar-20%O₂ (dry oxygen) and Ar-20%O₂-20%H₂O (wet oxygen) at 750 °C. For Ni-3Al alloy, an external NiO and an internal oxidation zone (IOZ) were formed with an enhanced thickening kinetics in wet oxygen. Increasing Al to 5 wt% led to a partial protection of the surface by a thin alumina scale, together with non-protective NiO and IOZ as that for Ni-3Al. Adding Cr into Ni–Al alloys significantly increased oxidation protection. In dry oxygen, all ternary alloys formed mainly a thin protective alumina scale. This protective effect was significantly reduced in the presence of water vapour. For Ni-(3, 4)Al-15Cr, a complex multi-layered oxide structure was observed, with an external NiO, an inner oxide layer with a chromia band at the bottom, and an IOZ with alumina precipitates. Further increasing Cr to 20 wt% led to a dominant chromia band formation in the inner oxide region. For high Al (5 wt%) ternary alloys, a protective alumina scale was formed in most of the surface area. Oxide characterisation revealed that this thin layer of alumina was α-Al₂O₃. The effects of water vapour and alloy composition on oxide formation were discussed based on classic diffusion theory and oxidation kinetic analysis.

In this study, creep tests were conducted on P92 steel at 650 °C under 110–155 MPa in both air and superheated steam environments to investigate the interaction between creep and oxidation. The combined effects of steam and different applied stresses influenced the structure and compactness of the oxide film, as well as the mechanisms of creep damage and creep crack growth, ultimately affecting creep rupture behavior. High applied stress levels (≥ 130 MPa), due to minimal oxidation and a rapid creep rate, the steam environment has a limited impact on creep life and material toughness. Additionally, crack propagation was hindered by the intact martensitic lath, delaying the creep fracture. In contrast, under low applied stress (≤ 110 MPa) in steam, cracking of grain boundary oxides facilitated creep crack growth along high-angle grain boundaries of recrystallization grains, which formed due to severe deformation during necking process in accelerated creep stage. Simultaneously, the presence of oxide on grain boundaries promoted grain boundaries sliding at the crack tip, thereby accelerate the accumulation of creep damage and reducing the toughness of the material.