Comparison of isothermal and cyclic oxidation behavior of twenty-five commercial sheet alloys at 1150°C

Twenty-five commercial nickel-, iron-, and cobalt-base sheet alloys incorporating chromium or chromium and aluminum additions for oxidation resistance were tested at 1150°C in air for 100 hr in both isothermal and 1-hr cyclic furnace exposures. The alloys were evaluated by sample specific weight change, by type of scale formed, by amount and type of spall, and by sample thickness change and microstructure. In isothermal steady-state oxidation, four types of controlling oxides were observed depending on alloy composition: NiO, Cr₂O₃-chromite spinel, ThO₂-blocked Cr₂O₃, and Al₂O₃-aluminate spinel. The latter three types are considered protective. In the Cr₂O₃-forming alloys, however, scale vaporization is a critical factor in determining the parabolic scaling rate based on paralinear oxidation. In cyclic oxidation the alloys which form Cr₂O₃-chromite spinel scales were degraded severely when sufficient chromite spinel developed to trigger spalling. The cyclic behavior of the other three types of alloys does not differ greatly from their isothermal behavior. If chromite spinel formation is minimal, the thinner the oxide formed, the less the tendency to spall. Factors contributing to a thin scale are low isothermal scaling rates; reactive element additions, such as thorium, lanthanum, and silicon; and scale vaporization. Scale vaporization may, however, lead to catastrophic oxidation at high gas velocities or low pressures or both. A tentative mass-balance approach to scale buildup, scale vaporization, and scale spalling was used to calculate the critical oxidation parameter—the effective metal thickness change. In general, this calculated thickness change agrees with the measured change to within a factor of 3 if a correction is made for grain boundary oxidation. The calculated thickness change parameter was used to rate the oxidation resistance of the various alloys under isothermal or cyclic conditions. The best alloys in cyclic furnace oxidation tests were either Al₂O₃-aluminate spinel formers or Cr₂O₃ formers with ThO₂ blockage.     

Effects of sulfur upon Fe and Cr oxide thermal stability in 304 stainless steel

X-ray photoelectron spectroscopy measurements show that exposure of an Fe–Cr–Ni alloy to H₂S and O₂ under ultrahigh vacuum (UHV) conditions leads to the thermal instability of the Fe oxide without affecting the Cr oxide. An atomically clean metal surface, predominantly Fe in composition and free of internal sulfur, was exposed to H₂S under controlled conditions in order to form a sulfided monolayer. Adsorbed S inhibited surface oxidation at temperatures between 325 K and 825 K. The presence of adsorbed S did not, however, inhibit the surface segregation of Cr for temperatures >600 K, compared to a S-free sample. Upon annealing of a sulfided sample to 900 K in UHV, the Fe oxide largely disappears, while no change is observed in the Cr oxide. The S-free sample shows no significant change in either the Fe or Cr oxides upon annealing. The results presented in this paper show that sulfidation of an alloy surface prior to oxidation and surface segregation can adversely affect the thermal stability of the Fe oxide without affecting the Cr oxide overlayer resulting from surface segregation and preferential oxidation.     

Improvement of cyclic oxidation resistance of a NiAl-based alloy modified by Dy

The effect of the rare-earth-element dysprosium on the cyclic oxidation behavior of NiAl-31Cr-3Mo alloy at 1300 and 1400 K in static air atmosphere was investigated in this paper. The results revealed that doping 0.1.% atom fraction of Dy greatly improved the cyclic oxidation resistance at 1300 and 1400 K. The microstructure and composition of the oxidation layer were studied by means of X-ray diffractometer (XRD), scanning electron microscopy (SEM) with EDAX unit. The oxidation scale, which is located on the surface of NiAl-31Cr-3Mo alloy, mainly consisted of α-Al₂O₃ and a little Cr₂O₃. While the continuous and sole α-Al₂O₃ scale formed on the surface of the alloys doped with the dysprosium. The adherency of scale improved due to a Cr-rich phase decreased the internal stress by thermal cycling and transformation of Al₂O₃ or transverse growing induced for the testing alloy modified by Dy. The oxidation mechanism was discussed from phase constitution.     

Laser processing of titanium aluminides

Using a Nd-YAG laser, laser processing of a series of Ti-Al alloys including pure Ti and Ti-Al intermetallic compounds has been studied. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and optical microscopy were used to determine the surface morphological, chemical, and compositional characteristics of the laser-processed samples. Analysis of results showed that cracks along grain boundaries caused by rapid heating and cooling of laser processing were the dominant characteristics of the surface morphologies of the laser-processed samples. The Al content in the Ti-Al alloys played a very important role in crack initiation and/or development. The more Al content in the samples, the more severe the cracks that developed after laser processing under the same conditions. The experiments were conducted at ambient conditions, resulting in surface oxidation layers being observed on the processed samples. The XPS results indicated that the oxidation layer consisted of adsorbed O₂, Al₂O₃, TiO₂, and TiO. In addition, Al enrichment was found in the oxide film of TiAl as well as in the oxidation layers formed on the surfaces of TiAl and Ti₃Al intermetallics that were processed by the laser; this differs from the reported results for traditional oxidation of TiAl at elevated temperature.     

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe-30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al₂O₃ and Cr₂O₃ films doped with Sm₂O₃ or Nd₂O₃ were prepared via the metal-organic chemical vapor deposition technique. Single Cr₂O₃, Al₂O₃, Nd₂O₃ and codeposited Cr₂O₃-Nd₂O₃, Al₂O₃-Nd₂O₃, Al₂O₃-Sm₂O₃ coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11-18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO₃ or NdTi₂₁O₃₈ and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.     

High temperature oxidation behaviour of a TiAl-Al2O3 intermetallic matrix composite

A TiAl-based intermetallic matrix composite has been produced through sintering of mechanically milled Al/TiO₂ composite powder. The composite contains 42-50 vol.% of α-Al₂O₃ as the particulate reinforcement phase. Oxidation experiments were carried out at 800-900 °C in air up to 500 h to evaluate its oxidation and scale spallation resistance. A cast Ti-50at.%Al alloy was also tested for comparison. The composite samples showed much lower oxidation mass gain than the cast alloy under all testing conditions. Moreover, the composite samples exhibited extremely strong scale spallation resistance. Spallation could never be recorded and observed even under long-time intensive cyclic oxidation exposure. Based on the kinetic and microstructural studies, the mechanisms for the improved oxidation and spallation resistance are discussed.     

Si对625合金激光熔覆层高温氧化特性的影响

利用循环氧化法,研究了不同Si含量（0%,1%,3%,质量分数）的625合金熔覆层在700、800、900 ℃下氧化144 h后的高温氧化行为。用XRD分析了氧化物相。通过SEM/EDS研究了氧化物表面和截面的形貌、元素组成和氧化膜的厚度。结果表明,不同温度下试样的氧化动力学都保持抛物线规律,随着温度的升高,氧化增重逐渐增加。通过观察,在900 ℃时,0% Si含量的625合金熔覆层出现了氧化膜大面积剥落的情况,3% Si含量的合金熔覆层氧化膜保持完整。在700 ℃时,随着Si含量增加,氧化膜表面的氧化颗粒尺寸减小且更加致密,同时促进了Cr₂O₃氧化物的生成。在700 ℃下,0 % Si含量的试样出现了大片的内氧化区域;1% Si含量的试样基体部分出现了2处条状的含Ni,Cr,Mo的氧化物相区;而3% Si含量的试样氧化后由于生成了富Si的内氧化层,这阻止了内氧化的发生。外层Cr₂O₃氧化膜和内层SiO₂的联合作用既阻止了O阴离子的渗入也抑制了Fe等金属离子的扩散,提高了合金熔覆层的抗氧化性。     

Characterisation and oxidation of LVOF sprayed Al2O3–40%TiO2 coating on superni 601 and superni 718 superalloys at 800 and 900°C

Failure of components due to high temperature oxidation is the major degradation mechanism in boiler and gas turbine industries. Superalloys having superior mechanical properties and creep resistance are used in these applications but lack resistance to oxidation under aggressive environments. Protective coatings are used to improve their oxidation resistance in such applications. In the present investigation, Al₂O₃–40%TiO₂ coating was deposited on superni 718 and superni 601 superalloys by low velocity oxy fuel process. The as sprayed coating was characterised for microhardness, surface roughness, scanning electron microscopy and X-ray diffraction analysis. High temperature oxidation behaviour of Al₂O₃–40%TiO₂ coated and uncoated superni 718 and superni 601 superalloys has been evaluated at the elevated temperatures of 800 and 900°C for total duration of 50 cycles under cyclic conditions. Each cycle consisted of keeping the samples for 1 h at the elevated temperature followed by 20 min cooling in ambient air. Al₂O₃–40TiO₂ coating in the as sprayed condition showed the presence of Al₂O₃–TiO₂, α-Al₂O₃, TiO₂ as the main phases. Al₂O₃–40%TiO₂ coating on superni 718 and superni 601 superalloys has shown a lower oxidation rate as compared to those of uncoated superalloys. However, the oxidation rate of the coating was not steady due to the occurrence of spallation/sputtering at various stages. The coating was found adherent on the substrate superalloys throughout the study.     

High-Temperature-Oxidation Behavior of Iron–Aluminide Diffusion Coatings

Aluminide diffusion coatings were oxidized in air under atmospheric pressure under isothermal and cyclic conditions. The high-temperature efficiency of the pack-aluminized alloys was tested by comparing their oxidation behavior in the temperature range 800–1080°C. The k _p values deduced from the parabolic plots of weight-gain curves showed that α-Al₂O₃ composed the major phase of the oxide scale on samples oxidized at T > 1000°C. For lower temperatures, transient-alumina phases were observed. The aluminide materials also exhibited excellent resistance to cyclic oxidation at 1000°C. The second aim of this study was to dope the aluminide compounds obtained by a pack-cementation process with yttria, which was introduced by metal-organic chemical-vapor deposition (MOCVD). The beneficial effect of the reactive-element-oxide coating is strongly dependent on its mode of introduction, since the oxidation resistance is drastically increased when the Y₂O₃ coating was applied prior to the aluminization process. When applied after the aluminization, the reactive element gave negative effects on the high-temperature oxidation behavior of the iron aluminides. The oxide morphologies, X-ray diffraction patterns and two-stage experiments helped to understand the oxide-scale-growth mechanisms.     

Corrosion of MCrAlY: Pt composites prepared by spark plasma sintering

Cyclic oxidation tests of sintered MCrAlY:Pt composites were carried out at temperatures of 1223 and 1273?K. Composite tablets were previously produced by spark plasma sintering. To assess an effect of platinum dispersed, microstructural characterisation was performed on the tablets before and after the cyclic oxidation treatment. Scanning electron microscopy analysis of the oxide layer showed that the presence of typical α-Al₂O₃ and NiAl₂O₄ spinel had structure columnar and equiaxial in both samples. Fine sintered microstructure and addition of Pt into the MCrAlY matrix enhanced the resistance of the material to cyclic oxidation.