Oxidation of Metals最新文献

Blast furnace ironmaking produces a reducing and carburizing atmosphere in the blast furnace which may lead to metal dusting. However, there is limited research on metal dusting under this particular atmosphere. This paper investigated metal dusting behaviors of ferritic Fe-(10, 20, 25)Cr and austenitic Fe-20Cr-(20, 33)Ni, Fe-25Cr-20Ni (wt.%) alloys at 550 and 650 °C in an Ar-33CO-7H₂-7CO₂-2H₂O (vol.%) gas simulating a blast furnace operating condition. A relatively short reaction time, up to 100 h, was used to evaluate the initial stage of metal dusting of these alloys in the blast furnace gas condition. Severe metal dusting and coke deposition were observed at 550 °C, while no significant dusting but internal oxidation and carburization were observed at 650 °C. Higher Cr and Ni contents in the alloy lowered kinetics of metal dusting at 550 °C and of carburization at both temperatures. Austenitic alloys were more susceptible to metal dusting and coke deposition than ferritic alloys with the same Cr content at 550 °C. At 650 °C, however, ferritic alloys experienced higher extents of carburization and oxidation than austenitic alloys with the same Cr concentration. The effects of alloy composition and reaction temperature on metal dusting in this simulated blast furnace operating condition were discussed.

The eutectoid reaction in FeO scale was examined by thermogravimetric analysis. The oxidized specimens were held isothermally from 100 s to 50,000 s in the temperature range from 300 to 550 ℃. Under a variety of temperature and temporal circumstances, experiments were conducted and the results analyzed using an electron probe micro-analyzer. The findings demonstrate that FeO forms an eutectoid consisting of Fe₃O₄ and Fe at 300–500 ℃. An experimentally derived TTT diagram of the evolution of the FeO phase change shows eutectoid C-curves with nose temperatures of roughly 425 ℃. The primary factor in the formation of eutectoid product is the local position of Fe supersaturation, which is not directly related to the Fe₃O₄ seam layer that forms. There exists a “white area” in the front of eutectoid phase transformation before stable eutectoid structure layered formed. Rods of Fe can be formed in eutectoid transformation. The lamellar spacing in the eutectoid product at different temperatures was obtained by calculation and verified by experimental results.

To enhance the efficiency and durability of the gas turbine and aircraft components, thermal barrier coatings (TBCs) are the most considerable protection techniques available. The present work is mainly concentrating on improving the high-temperature corrosion characteristics of uncoated and plasma-spray coated alloy A-286 under Na₂SO₄-5%NaCl-7.5%NaVO₃ deposit at 900°C. By using plasma spray process NiCrAlY and YSZ (bond coat-NiCrAlY and top coat-YSZ) thermal barrier coating was deposited on the alloy A-286. Coating characterization such as roughness was analysed on the coating surface and coating thickness and hardness analysis were performed on the coating region at the cross section. X-ray diffraction (XRD) and scanning electron microscope (SEM) were employed to study the phases and coating microstructure before and after being subjected to corrosion. The rate of corrosion was derived by thermo-gravimetric analysis. Using the elemental mapping technique and EDS compositional analysis, the scale thickness and corrosion attack were identified. During the corrosion experiment, sputtering and spallation of oxide were noticed on the uncoated specimen. The oxide layer failure was not found in both the coated specimens. The results indicated that YSZ coating exhibits a better corrosion resistance than NiCrAlY. The reaction products such as monoclinic ZrO₂ and YVO₄ were found in the corroded YSZ coating. There was no significant failure, but degradation was found on the coating due to molten salt.

The kinetics of the thermal oxidation of white metal and Cu₂S have been studied by thermogravimetric analysis (TG), which was carried out under atmospheric oxidative conditions (O₂ 100%) with heating rates of 5, 10, 15 and 20 °C min⁻¹. Each experiment was performed three times, the indicated values being the average of the three experiments. The experimental data were evaluated using isoconversional models based on the Arrhenius equation. The models are constructed using first-order mechanisms in the reactions and therefore, since most of them present adequate regression coefficients, it can be verified that order 1 is the most predominant order among the reactions found. In addition, the Kissinger–Akahira–Sunose model, which has the highest regression coefficient, is considered to be the most optimal. Similar behavior was recorded between Cu₂S and white metal, as well as a similar regular increase in the apparent activation energy (Ea) of 10–30 kJ mol⁻¹ for both materials. The reactions that took place during the oxidation of white metal and Cu₂S were determined using a computational model based on thermodynamics developed in this work. The identified phases include CuSO₄, Cu₂SO₄, Cu₂O, CuO, CuO·CuSO₄.

Three cast alloys based on a HEA composition were exposed to air at 1100°C for 50 h, a simple equimolar CoNiFeMnCr alloy, an equimolar CoNiFeMnCr alloy added with 3.7 wt.%Hf and 0.25 wt.%C, and an equimolar CoNiFeMnCr alloy added with 3.7 wt.%Ta and 0.25 wt.%C. The CoNiFeMnCr alloy is single-phase, and the two others are two-phase with a HEA matrix and either HfC or TaC interdendritic carbides. The three alloys resisted isothermal oxidation, with the formation of a M₂O₃ scale comprised of Cr and Mn. In this scale, the relative proportions of Cr and Mn varied from the alloy/scale interface (much more Cr than Mn) to the scale/atmosphere interface (much more Mn than Cr). Internal oxidation took place too, with locally noticeable deep oxidation penetrations in the simple equimolar alloy. The carbides-containing alloys were also affected by internal oxidation (M₂O₃, HfO₂ and CrTaO₄). Cr and Mn obviously diffused outward. Quantification of Cr and Mn lost by the alloys allowed for estimating the total oxide masses formed. The Cr and Mn losses were rather great, and equivalent values of parabolic constants were estimated to allow comparison with a model chromia-forming Ni-based binary alloy. The calculated oxidation kinetics were deduced to be faster than in the case of a pure chromia-forming behavior.

Spray parameters were fixed to depositing a MCrAlY coating by HVOF process on IN-708 substrate. The deposited coatings exhibited a microstructure with characteristic splats and β and γ phases. An MOCVD process was subsequently carried out to deposit a thin alumina layer on the sprayed MCrAlY coating. Cyclic oxidation tests were conducted at high temperature on the alumina/MCrAlY coating system. Both SEM and XRD analyses were used to characterize the thermally grown oxide after cyclic oxidation. A kinetic analysis was done to determinate the behaviour of the alumina/MCrAlY coating system during oxidation.

The cyclic oxidation behavior of HK30Nb heat-resistant steel processed by laser powder bed fusion (LPBF) was compared to its cast counterpart during exposures in air and air + 10% H₂O at 800 °C. The specific finer microstructure and lower Mn of the LPBF alloy resulted in lower oxidation rates in dry air and faster establishment of a continuous Cr₂O₃ scale in air + 10% H₂O compared to coarse-grained cast HK30Nb with higher Mn. Differences in alloy mechanical strength and therefore their ability to accommodate high temperature and oxidation-induced stresses as well as differences in thermal expansion coefficients between the alloy and the formed oxides (Cr₂O₃ only for the LPBF and Cr₂O₃ and MnCr₂O₄ for the cast specimens) during temperature cycling were found to result in a greater extent of spallation for the LPBF than for the cast alloy in dry air at 800 °C.

An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al₂O₃ scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al₂O₃ scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.

Improved oxidation kinetics for a polycrystalline Ni-based superalloy used in turbine disc applications has been shown to be possible by controlling the heating rate of the first thermal exposure to 5 °C min⁻¹. The beneficial effect arises from the formation of a protective layer of NiCr₂O₄, instead of the more usually formed doped Cr₂O₃. This study shows that it was possible to form the NiCr₂O₄ at temperatures up to 725 °C, within the operational conditions for this alloy, and that at higher temperatures Cr₂O₃ formed. The improvements in alloy performance extended to the internal oxidation processes where reduced depths of degradation were observed. It is demonstrated here that Al₂O₃ formation is less thermodynamically stable when the highly protective NiCr₂O₄ oxide is present at the alloy surface compared to the doped Cr₂O₃. Synchrotron XRD was performed on samples removed during the heating stage and provided evidence of the oxidation sequence occurring, enabling refinement in the thermodynamic calculations and suggesting an additional route to the formation of the NiCr₂O₄.

In this study, the effect of the surface state on the behaviour of Inconel 718 alloy exposed to 640 (^{circ })C and 700 (^{circ })C environments for times varying between one and one hundred hours was investigated. In particular, the focus was set on the evolution of oxidation and precipitation phenomena during thermal exposure. Three surface states were considered: two generated through shot peening treatments featuring different coverage levels, while the third condition is a non-peened one. Shot peening treatments modify the surface condition and introduce higher residual stresses and microhardness values than in the non-treated condition. The morphology of the oxides appears to be different depending on the condition observed. Regarding the kinetics, over time the oxidation process follows a parabolic trend and appears to be influenced by the surface state; in particular, severe shot peening treatment is characterized by the highest intensity of the phenomenon. However, the order of magnitude of the weight gains measured suggests that the observed variations can be neglected, and that the positive effect of shot peening can be exploited without introducing oxidation problems. From the point of view of the microstructural evolution, an increase in the coarsening kinetics of (gamma)” phase was observed in the shot peened layer.