Oxidation of Metals最新文献

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.

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.