Oxidation Of Alloys Research Articles

Comparing the oxidation of FeCrAl alloys in isothermal and cyclic high-temperature steam environments

This study investigates the oxidation behavior of FeCrAl (APMT and C26M) alloys in high-temperature steam at 1000 °C using isothermal and cyclic conditions. The FeCrAl alloys exhibited a fully ferritic body-centered cubic structure with no significant phase transformation, and the manufacturing procedures of the alloys resulted in varying grain sizes and porosity. In the 5 h isothermal test, the protective alumina film provided corrosion resistance without cracking in the high-temperature steam environment. However, thermal cycling seemed to compromise passivity; expansion and contraction during heating and cooling allowed oxygen penetration into the underlying metal in local regions, leading to transgranular crack-like features. Spallation of oxides was also observed. Post-exposure analysis of the oxide film formed on the FeCrAl alloys revealed two distinct oxide layers on the alloy: an outer Al-rich oxide film and an inner mixed oxide enriched in Cr and Fe.

Interfacial synergy between Pt3Co nanoparticles and CoO for efficient ammonia borane hydrolysis

Ammonia borane (AB, NH3BH3) with the high H2 content has been regarded as a promising material for chemical hydrogen storage. AB can hydrolyze to release H2 under the catalysis of noble-metal catalysts with high activity, but the high cost of noble-metals and their poor long-term stability prohibit the large-scale applications. Developing more cost-effective and stable catalysts by introducing cheap transition metal elements has been a promising strategy. Herein, we adopted a step-by-step precipitation method to synthesize bimetallic Pt-Co catalysts supported on rutile TiO2, which show significantly improved performance in AB hydrolysis reaction. The optimized 0.9Pt3.5Co/TiO2 catalyst (with the Pt and Co loadings of 0.9 and 3.5 wt%, respectively) exhibits an extremely high H2 generation rate, showing a turnover frequency value of 2163 molH2 molPt-1 min−1 at 298 K, 9 times higher than that of 0.9Pt/TiO2 catalyst and superior to those of most of the reported Pt-based catalysts. Structure characterizations and theoretical calculations reveal that the promoted AB hydrolysis performance arises from the interfacial synergistic effect between the supported Pt3Co alloy and cobalt oxide CoO, which accelerates the B-H bond cleavage and water activation, respectively. The excellent AB hydrolysis performance of the Pt-Co catalysts indicates the great prospects of alloy-oxide interfaces in the field of liquid phase chemical hydrogen storage.

Structural evolutions under surface oxidation of AgPd Alloy: From Orientation, composition and strain effects to catalytic application

The surface oxidation of AgPd alloys can significantly impact their catalytic activity, however, the detailed understanding of such dynamic mechanism and the prediction of catalytic activity is still lacking. Herein, a comprehensive investigation combining ab initio molecular dynamics simulations with experimental observations is performed. The surface oxidation of AgPd alloys is highly dependent on oxygen concentration, where AgOx forms in oxygen-poor environment and PdOx forms in oxygen-rich environment, and shows highly reliance on the orientation, composition and strain state. The oxidation level of Ag, Pd and AgPd alloys follows the trend of (111) < (100) < (110) under 1.0 ML of O coverage. Both Pd single-atom and Ag matrix atoms in PdAg single-atom alloys are susceptible to oxidation under 0.5 ML of O coverage, with the increase of Pd atom number, only AgOx forms. Tensile strain promotes the oxidation of Ag(111) surface by facilitating the inward diffusion of O atoms, while compressive strain promotes the oxidation of Pd(111) and AgPd(111) surfaces by facilitating the outward migration of Ag/Pd atoms. The oxidized AgPd(111) surface with tensile strain under 0.5 ML of O coverage exhibits the optimum electrocatalytic activity toward formate oxidation reaction.

Ternary alloy and metal oxides embedded in yolk–shell polyhedrons as bifunctional oxygen electrocatalyst

Ternary alloy and metal oxides embedded in yolk–shell polyhedrons as bifunctional oxygen electrocatalyst

Phase transformation and diffusion in high-temperature oxidation of FeCrNi medium entropy alloy

The oxidation of the FeCrNi medium entropy alloy is investigated from 700 to 1000 °C by combining experimental observations with density functional theory (DFT) calculations. The sole oxidation product formed is Cr2O3. At 900 °C, the layer exhibits desirable density and continuity, attributed to the rapid diffusion of Cr facilitated by the body-centered cubic phase transformation. There is a significant increase in mass gain rates at 1000 °C, accompanied by the formation of a discontinuous Cr2O3 layer and penetration of oxide. These observations emphasize the significant roles of phase structure in promoting the formation of protective oxide scales and influencing oxidation resistance.

Effect of Cr content on synergistic effect of Cr-Al during oxidation of high-entropy AlCoCrxNiTi alloys

AlCoCrxNiTi high-entropy alloy coatings, mainly composed of the BCC and B2 phases, were synthesized on Ti-6Al-4V substrates by laser cladding. The incorporation of Cr-refined grains enhanced the hardness of the alloy. Oxidation kinetic curves of the alloys at 800 °C in the air followed the parabolic law, and AlCoCr0.5NiTi alloy exhibited excellent oxidation resistance. The incorporation of an appropriate amount of Cr provided a structural template for the formation of Al2O3 and increased the diffusion rate of Al in the stable oxidation stage, leading to a reduction in the concentration required to form a continuous Al2O3 oxide film.

Effect of grain refinement on high temperature steam oxidation of an FeCrAl alloy

FeCrAl alloys are promising candidates to replace Zr alloys as fuel cladding materials in nuclear light-water reactors. Grain refinement has been indicated to improve irradiation resistance. To enhance corrosion resistance as well, the effects of grain refinement on steam corrosion behavior were investigated in this work. Samples of Kanthal D alloy (Fe-21Cr-5Al) with two different grain sizes (coarse-grained and ultrafine-grained) were exposed to steam at 1200 °C for 2 hrs. Results indicate improved steam corrosion resistance in ultrafine-grained Kanthal D with formation of a thinner protective Al oxide layer and the presence of a thin underlying Cr oxide layer.

Comparative early stage high temperature oxidation of equimolar NiTi and high entropy Ti16Hf17Zr17Ni16Cu17Co17 shape memory alloy

Isothermal oxidation experiments were conducted on equimolar NiTi and high entropy Ti16Hf17Zr17Ni16Cu17Co17 shape memory alloy from 1 to 276 min at 1000 °C in air. After oxidation, the thicknesses and structure features of the oxide scale were examined by scanning electron microscopy with a backscattered electron detector. X-ray diffraction was used to characterize the phase compositions of the oxide scale. For the oxide scale of NiTi, a three-layer structure was confirmed, whereas, a four or five-layer structure was observed in Ti16Hf17Zr17Ni16Cu17Co17 under different oxidation times. Furthermore, an outermost Cu2O layer was found for Ti16Hf17Zr17Ni16Cu17Co17 excessing 36 min of oxidation. Besides, Ti, Ni, Co, and Cu tended to diffuse outward, while Hf and Zr tended to diffuse inward, which was caused by the different diffusion coefficients of elements. Meanwhile, the oxidation kinetics of the two alloys were investigated, which revealed that the oxide layer thicknesses of both alloys showed a good parabolic relationship with the oxidation time, and the parabolic rate constant of Ti16Hf17Zr17Ni16Cu17Co17 (kP =2.653 ×10−7 cm2/s) was smaller than that of NiTi (kP =8.884 ×10−7 cm2/s), which indicated a better high temperature oxidation resistance of the high entropy shape memory alloy.

First-principles lattice dynamics and thermodynamic properties of α-, θ-, κ- and γ-Al2O3 and solid state temperature-pressure phase diagram

γ-Al2O3 is a commonly observed high temperature alumina phase during Al-based alloy oxidation. Due to the partially ordered, defective nature of the spinel structure arising from the partial occupancy in the Al sites, first-principles prediction of the thermodynamic properties of γ-Al2O3 has been challenging. In this work, employing the first-principles quasiharmonic approach, we obtain the finite temperature thermodynamic properties of γ-Al2O3, including entropy, chemical potential, heat capacity, thermal expansion coefficient, and elastic constants and the results are compared with those calculated for the other three alumina phases, i.e., α-Al2O3, θ-Al2O3, and κ-Al2O3. The calculated lattice constants and the predicted relative phase stability under 0 K and 0 GPa, α>κ>θ>γ, are consistent with experimental values and existing calculations, respectively. Based on the results, we constructed the temperature-pressure phase diagram covering the four Al2O3 phases (α, κ, θ, γ), which could provide for experimentally synthesizing the different alumina phases.

Study on removal of aluminum alloy oxide film by continuous-nanosecond combined laser with different pulse delays

In this paper, the thermal stresses arising from thermal expansion and the adsorption forces between the oxide film and the substrate are calculated from the one-dimensional heat conduction equation. Comparing the magnitude of the two, the laser cleaning conditions are obtained, and relevant models such as plasma shock wave pressure are established. The plasma plume, surface morphology, and elemental composition of aluminium alloy oxide film removed by continuous-nanosecond combined laser with different nanosecond pulse delays were experimentally investigated. At total laser fluence of 3630.47 J/cm2 (continuous laser fluence of 3628.73 J/cm2 and nanosecond laser fluence of 1.74 J/cm2), the removal effect is better with the increase of pulse delay, and the best removal effect is achieved when the pulse delay is 600 ms. Based on the experimental results, the mechanism of oxide film removal by combined lasers with different pulse delays was discussed, and at least two mechanisms of thermal stress against adsorption and plasma shock wave breaking the oxide layer were present to act.

Mechanisms of Al2O3 and Cr2O3 formation during FeCrAl alloy Oxidation: A First-Principles study

FeCrAl alloys are the best candidates for nuclear fuel cladding materials due to their excellent corrosion resistance and high-temperature oxidation resistance. However, the oxidation mechanisms of them have not been fully explained. In this study, we investigate the oxygen adsorption on the surface of FeCrAl alloys using density functional theory. Our results reveal that Cr are prone to interact with O. Furthermore, an increasing number of Cr atoms in the adsorption configuration leads to a higher absolute value of the adsorption energy, especially for three Cr (-4.48 eV). To gain further insights, we conduct an analysis of their charge and orbital interactions. We find that Al atoms transfer the most charge to O atoms, resulting in the most stable bonding. This finding explains the experimental phenomenon where Cr2O3 is primarily observed at low temperatures or in the initial oxidation stage, while Al2O3 is observed at high temperatures during prolonged oxidation. This study highlights the variations in the behavior of metal atoms during their interaction with oxygen at atomic level. These findings contribute to the understanding of FeCrAl alloys’ exceptional oxidation resistance. Moreover, this research serves as a valuable scholarly reference for the advancement of FeCrAl alloys for their oxidization resistance.

New Insights into Internal Oxidation of Alloy 690 and Model Alloys in Hydrogenated Steam

The oxidation behavior of Alloy 690 and Ni-xCr-10Fe model alloys exposed to 480°C hydrogenated steam, in the nickel metal stability region, was studied using a variety of techniques, including time-resolved scanning electron microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and electron energy loss spectroscopy. The alloy underwent internal oxidation intragranularly, resulting in the expulsion of metallic Ni nodules to the surface. A compact Cr-rich “healing” layer developed over time at the intragranular internal oxidation front near grain boundaries and significantly retards further internal oxidation. Protective external Cr-rich oxide formed at and very near the grain boundaries, hindering intergranular oxidation. In view of the fact that the protective oxide forms micrometers away from the grain boundaries, one cannot simply say that Cr diffuses up the boundary and forms an oxide that spreads laterally—internal stress relief must play a role, just as it does within the grains, only with different mechanisms at play.

Microstructural and micromechanical characterization of intergranular oxidation in Fe-15Cr alloy

Fe-Cr alloy has great potential as an interconnect material for SOFCs at temperatures up to 800 °C, with Cr providing excellent oxidation resistance through the formation of a protective chromium oxide layer. However, internal brittle oxides formed at grain boundaries (GBs) can lead to severe cracking and cause material failure. Here, we report the formation of internal GBs oxide in an Fe-15 wt%Cr alloy. We carried out microstructural and chemical characterization of the intergranular oxide and performed micromechanical testing and FEM simulation to study the fracture behavior at the GBs. Our findings provide insights for improving the oxidation resistance in the material design.

Integrating high-entropy alloy oxides with porous wood architectures for boosted salt-resistant water evaporation

Integrating high-entropy alloy oxides with porous wood architectures for boosted salt-resistant water evaporation

Effect of structural disorder on the oxidation of Zr-based amorphous alloys: A focused review

Structural disordering leads to outstanding mechanical properties of amorphous alloys. Many properties are strongly related to formation of surface oxide layers. An in-depth understanding of effect of structural disordering of amorphous alloys on oxide evolution is essential for applications. This paper presents a review of recent comparative oxidation studies of Zr-based amorphous and single-phase crystalline counterpart alloys. Three representative model alloy systems are selected: Zr–Cu and Zr–Al representing alloys with components of different and similar oxygen affinities, respectively, and Zr–Cu–Al representing multi-component alloys. In combination with molecular dynamic simulations, the fundamental role of the structural disordering of parent alloys in the oxidation processes is revealed. The oxidation of Zr–Cu–Al–Ni amorphous alloys at the supercooled liquid temperature region is further reviewed to clarify the relationship between oxidation and substrate crystallization.

Differences in Oxidation Behavior of Conventionally Cast and Additively Manufactured Co-Base Alloy MAR-M-509

AbstractHigh-temperature oxidation behavior of conventionally cast and additively manufactured (AM) Co-base alloy MAR-M-509 was compared in the present study. The specimens were exposed in air at 1000 °C and characterized by means of scanning electron microscopy equipped with energy/wavelength dispersive x-ray spectroscopy (EDX/WDX) and electron backscatter diffraction as well as transmission electron microscopy. Substantial differences in the oxidation processes of two alloy versions were observed. Faster oxidation of the cast alloy was mainly induced by (1) oxidation of coarse primary carbides, (2) internal oxidation and nitridation processes and (3) incorporation of other alloy constituents (e.g., Co, Ni, W) into the Cr-oxide scale. AM specimens, in contrast, formed a more homogeneous, thinner and better adherent external oxide scale. The results are discussed in terms of differences in the chemical composition and alloy microstructure, including the grain size distribution in the material and the morphology of the strengthening precipitate phases.

Thermal stress analysis of laser cleaning ofaluminum alloy oxide film.

In this paper, a theoretical model for laser cleaning of aluminium alloy oxide film is presented from the perspective of thermal stress. Additionally, we developed a two-dimensional axisymmetric finite element model for calculation. Thermal stresses result from thermal expansion. Using thermodynamic equations, numerical calculations enablethe determination of a theoretical cleaning threshold by comparing the thermal stresses to the adhesion between the oxide film and the substrate. Through theory and experiments, it is known that the greater the laser fluence, the better is the cleaning effect. The findings indicate that cleaning of the oxide film on aluminum alloys can be achieved under appropriate parameters. The cleaning threshold for laser cleaning of the oxide film is determined to be 3629.47J/c m 2 (continuous laser fluence is 3628.73J/c m 2; nanosecond laser fluence is 0.74J/c m 2). The thermal stress model of laser cleaning is highly useful for selecting the appropriate laser flux in practical applications. Both a simulation and experimental results can provide an explanation for the mechanism of interaction between the laser and the aluminum alloy oxide film, demonstrating that thermal stress is one of the cleaning mechanisms during the laser cleaning process of the oxide film.

Ferritic layers formation mechanism during oxidation of FeMnSiCrNi alloys: Effect of temperature and influence on oxidation behavior

Ferritic layer formation occurs after oxidation in Mn-rich austenitic alloys. It has significant effects on high-temperature oxidation resistance, but a better understanding of its formation is needed to control these effects. Thus, two FeMnSiCrNi alloys were exposed to six temperatures between 675 and 800 °C for 100 h. A relationship between mass variation and ferrite thickness was revealed, as Mn and Cr depletion occurred due to the Mn2O3/MnCr2O4 formation. Thermodynamic calculated phase diagrams indicated ferrite stability at high temperatures, affecting oxidation resistance. Finally, the Mn concentration profile changes during oxidation were studied with DICTRA analyses to explain the ferritic layer growth. By describing the growth and characteristics of this layer, the study can serve as a starting point for future investigations into designing more oxidation-resistant FeMnSiCrNi alloys.

The oxidation of an equimolar FeCoNiAl medium-entropy alloy at 900 °C in various oxygen pressures

The oxidation of an equimolar FeCoNiAl medium-entropy alloy was studied at 900 °C in various O2-containing atmospheres over the oxygen pressure range of 10–105 Pa. The alloy oxidized parabolically, having its oxidation rates steadily increasing with oxygen pressure. Duplex scales formed on the oxidized substrate, consisting of an outer CoFe2O4 layer and an inner α-Al2O3 layer. The position of the Pt-marker was located at the outer- and inner-layer boundary, demonstrating that the outward cation diffusion controlled the formation of CoFe2O4, while the growth of Al2O3 was contributed by the inward oxygen diffusion.

Strong correlation between high temperature oxidation resistance and nitrogen mass gain during near alpha titanium alloys exposure in air

Nitrogen is known to play a role in the oxidation of titanium and titanium alloys in air. This paper shows the strong correlation between the high temperature oxidation kinetics of eight near-alpha titanium alloys, in the class of Ti6242S, exposed for 3000 h in air at 650 °C. Nuclear reaction analysis (NRA) was used to quantify and locate the nitrogen inside the material. An excellent linear correlation between the total mass gain measured by gravimetry and the nitrogen mass gain measured by NRA is revealed: less the alloys oxidize, more the nitrogen in involved in the oxidation process.