Oxidation of Metals最新文献

Samples of the Ni-based superalloy, RR1000, were exposed to 98% Na₂SO₄/2% NaCl salts at 700 °C with a flux of 1.5 µg cm⁻² h⁻¹ in flowing air + 300 ppm SO₂ for a total of 250 h. Three pre-exposure conditions were studied: a bare reference alloy; fast heating to the test temperature followed by a 100 h hold; heating at a rate of 5 °C min⁻¹ to the test temperature following by a 100 h hold. The surface oxide formed under the latter two conditions were Cr₂O₃ or NiCr₂O₄, respectively. The results show corrosion pit formation on the surface of the base, reference sample, and no pits present on the sample with the preformed Cr₂O₃. Some protection was found for the sample heated at 5 °C min⁻¹ with a delay in the progression to accelerated corrosion attack. Additional testing under moisture containing air was also conducted. This showed no obvious difference in surface oxide morphology under the two tested heating rates for the short-term exposures examined but a difference was noted to be dependent on the moisture content of the air.

Intergranular oxidation (IGO) of the Ni-based superalloy Inconel 718 was studied at 650 °C, 700 °C and 900 °C. The oxidized samples were characterized by X-ray diffraction and scanning electron microscopy. For all the studied temperatures, the external scale was mainly composed of Cr₂O₃, while the oxides along the grain boundaries were rich in Al and, to a minor extent, Ti. This was consistent with thermodynamic computations. The time evolution of the maximum depth of IGO was found to be parabolic with an apparent activation energy of 164 kJ/mol. The results of this study confirm with three temperatures that IGO kinetics can be described using an extension of the Wagner’s theory of internal oxidation, as recently suggested in the literature at 850 °C. According to this description, the mechanisms controlling the IGO kinetics of Inconel 718 are the aluminum diffusion in the alloy matrix and the oxygen diffusion along the interface between the alloy matrix and the oxidized grain boundary.

Ni-Cu alloys are promising for application at temperatures between 400–900 °C and reducing atmospheres with high C-contents. Typically, under such conditions, metallic materials in contact with the C-rich atmosphere are degraded by a mechanism called metal dusting (MD). Ni-Cu-alloys do not form protective oxide scales, but their resistance is attributed to Cu, which catalytically inhibits the C-deposition on the surface. Adding other alloying elements, such as Mn or Fe, was found to enhance the MD attack of Ni-Cu alloys again. In this study, the effect of the Mn and Fe is divided into two affected areas: the surface and the bulk. The MD attack on binary Ni-Cu alloys, model alloys with Fe and Mn additions, and commercial Monel Alloy 400 is experimentally demonstrated. The surface electronic structure causing the adsorption and dissociation of C-containing molecules is investigated for model alloys. Analytical methods such as scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, electron probe microanalysis combined with wavelength-dispersive X-ray spectroscopy, X-ray diffraction analysis, and near-edge X-ray absorption fine structure measurements were used. The results are correlated to CALPHAD calculations and atomistic simulations combining density functional theory calculations and machine learning. It is found that the Cu content plays a significant role in the surface reaction. The effect of Mn and Fe is mainly attributed to oxide formation. A mechanism explaining the enhanced attack by adding both Fe and Mn is proposed.

A single thin MnCr₂O₄ spinel layer was synthesized on a Ni–25Cr–1.5Mn alloy by a fine control of oxygen partial pressure using the Rhines-pack method, a technique that utilized an appropriate buffering powder mixture. The spinel was characterized using X-Ray diffraction, Raman spectroscopy and photoelectrochemistry. The cubic spinel MnCr₂O₄ was formed under the oxygen partial pressure close to 5 × 10^–21 atm at 1050 °C controlled by the buffering Ni–25Cr/Cr₂O₃ powder mixture. Raman MnCr₂O₄ spectrum is characterized by five vibrational modes, whereas photoelectrochemical characterization revealed the MnCr₂O₄ band gap measurement at 3.7 eV with an n-type conductivity.

Model alloy, Fe-20Cr (wt%), was oxidized in two gas mixtures Ar-5H₂O-(5H₂) (vol%) at 850 °C. The alloy formed Cr₂O₃ scales in both gases. The Cr₂O₃ scale developed faster in Ar-5H₂O-5H₂ and contained fine pores, whilst that grown in Ar-5H₂O was dense. Experiments with inert SiO₂ marker revealed that the Cr₂O₃ scale growth in Ar-5H₂O-(5H₂) was controlled mainly by outward Cr diffusion. When adjusted for grain boundary diffusion effects, Wagner’s theory was successful in describing the hydrogen effect, provided that the Cr₂O₃ scales are n-type.

Current nickel-based single crystal superalloys (SX) are mainly designed to increase significant mechanical loading at high temperatures. Therefore, the mechanical resistance is greatly dependant on the microstructure and on the potential metallurgical defects. Corrosion and oxidation at high temperatures may further induce a loss of load-bearing section and lower the overall mechanical performance of such single crystals. While the yet complex mechanical and corrosion mechanisms are relatively well established separately, little is known on their combined effects, let alone on as-cast (AC) versus fully heat-treated (FHT) microstructures. This paper shows that the low cycle fatigue (LCF) at 0.5 Hz, R_σ = 0.05 and 950 °C is lowered when the AM1 nickel-based single crystal superalloy is pre-corroded with 1 mg/cm² Na₂SO₄ at 950 °C. The degradation increases with increasing pre-corrosion time due to the formation of a porous, brittle corrosion layer that favours the number of crack initiation sites, which are subsequently assisted by hot corrosion and oxidation. In addition, AM1 FHT shows better LCF fatigue resistance than AM1 AC, due to a better creep resistance of the FHT microstructure under these conditions.

The isothermal oxidation of Ni-base single-crystal superalloy AM1 was investigated for up to 3600 h at 850 °C and 900 °C. The aim of the study was to test an existing model of oxidation kinetics that considers transitory oxide growth. The samples were characterized at various intervals to correlate the microstructure of the oxide scale with the oxidation kinetics. Transition alumina (θ) was observed among other transition oxides such as spinel, rutile, and chromia, which helped in understanding the nature and kinetics of the transitory stage. After a sufficiently long duration, all samples formed a continuous α-alumina layer at the metal/oxide interface. The previously published model, based on three kinetic parameters, was validated in the temperature range of 800–1200 °C. The duration of the transient regime characterized in this study at 850 °C and 900 °C was consistent with the kinetics model, with a slight increase in the value of the model parameter describing the lateral growth kinetics of α-alumina. This modification resulted in a slight reduction in the duration of the transient regime at low temperatures.

In most chemical and high-temperature processes, metals are exposed to temperature gradients which, in turn, affect the extent of corrosion phenomena. In this study, a long, continuous strip of alloy 625 was exposed to lithium fluoride in a temperature range of 50–600 °C, air environment. The hottest section of this strip was analyzed as a coupon and compared with two other coupons which were exposed isothermally. One of the isothermal exposures was carried out in a tube furnace, and the other one was in a vertical furnace. Oxygen had three different kinds of access to these three coupons, which, in turn, affected the corrosion process. In order to limit the access of oxygen, a long column of lithium fluoride was used in a vertical furnace. The results of the isothermal exposure showed that more access of oxygen in a horizontal tube furnace facilitated the fluoride ingress to a great extent. However, a long sample exposed to a temperature gradient suffered more corrosion attack than the isothermal coupon, under the same LiF load in the vertical furnace. This was associated with the reduction of oxygen at a larger cathode area reaching into colder regions in Inconel 625 strip. Increased oxygen reduction also increases the efficiency of an inner anode at the hottest section, causing the observed rapid intergranular fluoride uptake. The study proposes a mechanism explaining these observations.

Regarding potential high temperature applications of materials based on the ternary Al₂O₃-YAG-ZrO₂ (AYZ) eutectic system for turbine blades, their environmental behaviour has to be evaluated in near-service conditions. CMAS attack is one of the phenomena limiting lifetime of ceramic E/TBC coatings for aircraft engines. The present study reports an assessment of the reactivity of AYZ eutectic material facing with two molten CMAS. The effect of test duration (up to 500 h), CMAS composition (with or without Fe₂O₃), microstructure (interlamellar distance), temperature (1250 and 1350 °C) as well as thermal cycling resistance were investigated. Results evidenced that the interaction of CMAS with AYZ eutectic ceramic was limited regardless of the microstructure. No infiltration of molten CMAS within the eutectic ceramic nor mechanical rupture between CMAS and AYZ was observed. The dissolution of AYZ leads to the rapid formation of anorthite with the CMAS components that hinders further reactions. The inward growth of MgAl₂O₄ spinel in place of the Al₂O₃ network occurs in accordance with the thermodynamic available data and without any effect on AYZ integrity.

Ni–30Cr model alloy was used to study chromia-scale formation behaviour. During its formation the mass variation signal is weak and it is important to take specific care to measure oxidation kinetics by thermogravimetric analysis. Symmetrical design allows the determination of very small mass changes in compensating buoyancy effects and limiting measurement drift. The mass variation measurement comprises noise coming from different sources that affects the electronic signal. It must be minimised to improve accuracy. This involves keeping the room-temperature constant, strictly balancing the beam and minimising buoyancy effects. By this way it was possible to acquire kinetics and to measured rate constants, k_p, in a range from 3.10^–5 to 3.10^–10 mg².cm⁻⁴.s⁻¹ equivalent to 10^–12 down to 10^–17 cm².s⁻¹. Oxygen partial pressure (({P}_{{O}_{2}})) was monitor and revealed an abnormal consumption of oxygen at the beginning of the thermal exposure. Experiments with an inert material showed parasitic reactions identified by mass spectrometry as combustion of impurities. As oxygen consumption is not only due to oxidation of the sample, corrosion kinetics can’t be deduced from it. Hence, to determine whether the oxygen supply from the gas is a limiting parameter, a model, which quantifies oxygen consumption by sample oxidation within the thermobalance, is proposed.