Oxidation of Metals最新文献

High-temperature alloys rely on the formation of a protective oxide scale to resist high-temperature oxidation and corrosion attack, and chromia is the most common oxide to provide this function in commercial alloys. However, certain harsh environments require alloys that utilize the formation of even more protective oxide films to provide improved performance and longer lifetime. In these cases, an alumina scale becomes a viable solution to protect high-temperature alloys. This paper summarizes high-temperature oxidation and corrosion behaviors of several high-temperature Ni- and Co-base alloys tested under various high-temperature conditions, including short-term and long-term oxidation, cyclic oxidation, dynamic (burner rig) oxidation, water vapor oxidation, nitridation, and carburization at temperatures ranging from 871 to 1093 °C (1600–2000°F). The oxidation and corrosion behaviors are compared between the alumina-forming and chromia-forming alloys, and the results show that the alumina-forming alloys were significantly superior to the chromia-forming alloys for high-temperature oxidation and corrosion resistance in terms of oxidation and corrosion rate reduction, scale stability and adhesion, mass penetration suppression, etc. Based on the extensive tests, alumina scales were highly effective in resisting oxidation, nitridation, and carburization attacks, especially under severe oxidation and corrosion conditions. To further demonstrate the benefits of an alumina scale, an alumina-forming alloy with pre-oxidation heat-treatment was also studied in the nitridation test.

The role of nitrogen in the oxidation of Ti-2W, Ti-10Al-2W (at.%) and Ti6242S was investigated using experiments in air and in Ar-20%O₂, and two-stage experiments where the reaction gas was switched from one mixture to the other. When switching from Ar-20%O₂ to air, the oxidation rates first increased during a short period, then decreased. This surge of mass gain following the introduction of air was attributed to N pickup, forming a nitride layer and a N-enriched zone in the alloy, below the oxide layer. The subsequent decrease of oxidation rate was attributed to the formation of nitride and/or N-rich zone, which both act as diffusion barriers for oxygen. Switching from air to Ar-20%O₂ caused an increase in the oxidation rate of the W-containing alloys, which was attributed to the consumption of this barrier. The gas change had no significant effect on the oxidation rate of Ti6242S, which formed a much thinner nitride layer in air. The faster the nitride layer grows, the faster it is consumed when removing N from the reaction gas, probably because of a higher diffusion rate of N in W-doped TiO₂ compared to TiO₂ formed on Ti6242S.

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.