Oxidation of Metals最新文献

Ti–6Al–4V alloys manufactured by laser or electron powder bed fusion (L-PBF and E-PBF) with or without hipping treatment have different microstructures from foundry alloys. Their oxidation kinetics at high temperatures between 500 and 600 °C for durations up to 2,000 h were compared. The effect of oxidation on their room temperature tensile embrittlement was quantified. It was shown that the growth kinetics of the brittle fracture zone, of the zone with cracks at 1% strain, and of the oxygen diffusion zone were perfectly correlated. Therefore, the embrittlement was confirmed to be due to oxygen ingress below the oxide scale and the kinetics were independent of the microstructure.

Paralinear oxidation models provide a description of parabolic scale growth combined with linear loss, as might occur for scales forming volatile oxide, hydroxide, chloride, or fluoride scales. Classic weight change exhibits an initial parabolic oxygen gain, a maximum (ΔW_max at t_max), then a linear loss. The magnitude of these features is determined by the parabolic growth rate, k_p, the linear volatility rate, k_v, and the stoichiometric constant of the reaction, S (fixed by the atomic weights and stoichiometry of the reaction). Model curves were generated (at constant k_p and k_v) to show that, for typical oxides, increases in S only moderately decrease ΔW_max and t_max, but directly increase the rate of mass loss. Universal oxidative behavior can be produced using normalized ½ k_p/k_v weight and ½ k_p/k_v² time constants. Furthermore, it is shown that, on average, k_p ≈ 4.1 (ΔW_max)²/t_max and k_v ≈ 1.2 (ΔW_max)/t_max. These relations apply for a broad spectrum of scale molecular weights, ranging from low mass SiO₂ to high mass Ta₂O₅ oxides. Oxidation of carbides and nitrides may release C and N elements and thus increase the effective S_eff, with concomitant effects on the paralinear curves.

The need for advanced materials that can meet application requirements at ultra-high temperatures in oxidizing environments is an area of active research. One challenge facing the high temperature materials community is the ability to conduct controlled ultra-high temperature oxidation tests with minimal to no contamination or reaction with the chamber. A unique resistive heating system (RHS) capable of achieving ultra-high temperatures (> 1700 °C) to enable such experimentation is described. A concern of such a system is the potential presence of thermal gradients in directions not reflective of actual material applications, e.g., the hottest region being in the center of the sample. Experimental results from the oxidation of ZrB₂ specimens at nominal temperatures of 1500°, 1700° and 1800 °C in low pO₂ (0.1–1% O₂ in Ar) environments are presented. Specimen thermal gradients generated during oxidation were evaluated using finite element analysis models. Thermal gradients on the order of the uncertainty in temperature measurements were calculated, confirming the RHS suitability for conducting ultra-high temperature oxidation exposures on ultra-high temperature ceramics.

The quest for high-entropy alloys (HEAs) with superior resistance against oxidation at elevated temperatures is one of the urgent problems in materials society, since HEAs are candidates for coating machinery parts operating in aggressive conditions (such as turbine blades, turbojet and jet engines, etc.). In this study, the effect of minor platinum alloying on the microstructure, phase composition and high-temperature oxidation resistance of Al_0.5CoCrFeNiCuPt_0.3 HEA was studied. It was demonstrated that platinum does not precipitate as an intermetallic phases; rather, it dissolves in the solid solution phases. High-temperature oxidation tests were carried out in a muffle furnace at 900 °C and 1000 °C for 50 h in air. It was found out that platinum alloying significantly increases oxidation resistance of Al_0.5CoCrFeNiCuPt_0.3 HEA at elevated temperatures with specific weight change of 0.139 mg/cm² and 0.238 mg/cm² after 50 h of isothermal exposure to 900 °C and 1000 °C, respectively. A dense oxide layer, mainly composed of Al₂O₃, without defects and pores protected the surface of the alloy.

The isothermal and cyclic oxidation behavior of a multi-principal element (MPE) TiNbCr alloy at 800–1000 °C in air was studied and compared to Co-based alloy 188. The phase constitution of the MPE alloy consisted of a Nb-rich body-centered cubic (BCC) matrix and Cr-rich Laves precipitates. While isothermal tests conducted at 800 °C led to the formation of a complex mixture of Nb, Ti and Cr oxides, tests at 900 and 1000 °C resulted in the formation of an innermost Cr₂O₃-rich scale layer which provided improved oxidation resistance. However, for all exposure temperatures, the scaling kinetics of the alloy were linear and therefore deemed non-protective. In contrast, alloy 188 exhibited parabolic scaling kinetics and smaller mass gain per area than the MPE alloy. The similarity between isothermal and cyclic test results for the MPE alloy confirmed that the scale does not offer much protection. Additionally, for all tests, there was extensive internal oxidation and nitridation.

La₂Ce₂O₇ (LC) has been identified as a promising thermal barrier coating (TBC) for use up to 1250 °C. In this study, a TBC system was deposited on grit-blasted Inconel 738 using atmospheric plasma spraying (APS) with NiCrAlY as the bond coat, followed by YSZ, and then LC as the top layer. The coatings were exposed to a mixture of molten Na₂SO₄ (45 wt.%) and V₂O₅ (55 wt.%) at 950 °C for hot corrosion. On the top LC layer, LaVO₄, CeVO₄ and CeO_{(1.66–2.00)} formed as hot corrosion products after 4 h exposure. A reaction between YSZ and the corrosion products could not be observed due to the absence of YVO₄. The hot corrosion mechanism of the LC-based TBC is also discussed in this study.

Oxidation behavior of additively manufactured ZrB₂–SiC in air and in CO₂ is reported in the temperature range of 700–1000 °C. Observed scale morphologies in air and in CO₂ were similar, featuring an outer borosilicate layer and an inner porous zirconia layer containing partially oxidized silicon carbide particles and remnant borosilicate products. Oxide scale thicknesses and parabolic scaling constants in air were approximately twice those observed in CO₂ across all studied temperatures. Activation energies for oxidation of 140 ± 20 kJ/mol in air and 110 ± 20 kJ/mol in CO₂ were determined, indicating similar diffusion processes that appear to be rate-limiting. The formation of protective scales across wide temperature ranges both in air and in CO₂ makes additively manufactured ZrB₂–SiC an attractive candidate for high-temperature industrial process applications featuring varied oxidants such as heat exchangers.

Refractory high-entropy alloys (RHEA) are considered as potential candidates for new-generation energy-related high-temperature applications. However, the poor high-temperature oxidation resistance of RHEAs, resulting in phenomena such as significant weight gain, scale spallation, pesting, and even complete oxidation, limits their applications. In this study, the oxidation behavior of Al_xHfNbTiVY_0.05 (x = 0.75; 1; 1.25) high-entropy alloys was investigated at 700–900 °C. The isothermal oxidation tests showed that the oxidation resistance of Al_xHfNbTiVY_0.05 RHEA is strongly influenced by temperature and time. In addition, accelerated oxidation, known as pesting, was observed to occur at 700 °C for all alloys; while, partial spallation was observed at 800 °C for the Al₁ and Al_1.25 alloys. Detailed analyses of oxidation kinetics have been carried out for the oxidation test series at 900 °C. The mechanism behind disintegration was investigated and attributed to accelerated internal oxidation followed by the formation of voluminous Nb₂O₅, TiNb₂O₇, and fast-growing AlNbO₄, and is also thought to be related to the partial evaporation of V₂O₅.

The phase composition of the pre-oxidized film on an alloy surface usually has a great influence on its corrosion resistance. In this work, the surface oxide film growth behavior in low-oxygen atmosphere at 720 °C of two 12Cr heat-resistant steels with different Mn content was studied, and their corrosion resistance in liquid lead–bismuth eutectic (LBE) with saturated oxygen at 600 °C was tested. The results indicated that the pre-oxidized film of 1.6Mn steel is mainly composed of large-size Mn–Cr spinel and Fe–Cr spinel, while that of improved 0Mn steel is mainly composed of continuous and dense Cr₂O₃ and Fe–Cr spinel. This is because Mn has a high diffusion rate in Cr₂O₃, so it can pass through the Cr₂O₃ layer and combine with O to form Mn-rich oxides, and then the Mn-rich oxides react with Cr₂O₃ to form Mn–Cr spinel. However, due to the high solubility of Mn in LBE, the Mn-rich pre-oxidized film of 1.6Mn steel will dissolve and fail quickly, so its long-term corrosion resistance in LBE is lower than that of 0Mn steel.

Hot corrosion is a severe form of industrial corrosion which occurs at high temperatures under the influence of oxidizing gases and can be prominently linked to the formation of a molten salt/ash layer over any metallic substrate. Throughout the years, several types of inhibitors and coatings have successfully been employed to reduce its devastating effects to a certain extent. In this study, two synthesized metal oxides namely Al₂O₃ and MnO₂ were used as corrosion inhibitors and XRD, FTIR analysis were carried out to assess their structural properties. Further, TGA analysis for Al₂O₃ and MnO₂ was carried out to determine thermal stability characteristics. The as-synthesized materials were further deposited as inhibitor coatings (Al₂O₃ with MnO₂ bond coating, MnO₂ coating and Al₂O₃ + 50% MnO₂ coating) on T22 boiler steel specimens. All the specimens (bare T22 and coated T22 steel) were investigated for hot corrosion studies in Na₂SO₄-60%V₂O₅ environment at the temperature of 900 °C for 50 consecutive cycles. Every cycle involved a 1-h heating step in furnace followed by 20-min cooling at room temperature. Weight gain data were collected using a digital balance. XRD and (SEM–EDS) analysis were carried out to characterize the samples after exposure to hot corrosion environment. A better resistance to hot corrosion was observed for all the different types of coatings, with Al₂O₃ + 50% MnO₂ coating representing maximum resistance. A high concentration of protective oxides such as Al₂O₃ and Cr₂O₃ present on the surface and their interaction to form dense layers on the coated samples explains the enhanced hot corrosion inhibition.