Oxidation of Metals最新文献

Super-duplex stainless steels are biphasic microstructure alloys composed of ferrite and austenite, whose microstructural stability, mechanical strength and corrosion behavior are strongly influenced by heat treatment. Intermetallic phases, such as sigma, can occur depending on the aging parameters, negatively impacting the toughness and corrosion performance of the alloy. The present study evaluated the effects of isothermal aging on the microstructure, mechanical and corrosion behavior of super-duplex stainless steel UNS S32750. The samples were heated at 700 °C and 850 °C for 5 min and 2 h. The performance evaluation included microstructural analysis, microhardness, potentiodynamic polarization and stress corrosion cracking in synthetic seawater, under 1 MPa CO₂ pressure and 60 °C temperature, for a period of 6 months. The steel aged at 700 °C for 5 min presented electrochemical performance similar to the unaged alloy, maintaining the stress corrosion cracking performance. On the other hand, prolonged aging at 700 °C for 2 h and at 850 °C for 5 min and 2 h allowed the sigma phase transformation and increased microhardness, enabling the brittle fracture and compromising the alloy’s corrosion performance, which is emphasized by the shift of the corrosion potential and the increase in current density under the most severe aging conditions. The study denotes that controlling the aging parameters is essential to ensure the mechanical integrity and corrosion performance of the super-duplex alloy, and that aging the alloy at 700 °C for 5 min is desirable when the intention is to promote the oxide layer that can be beneficial to protect the substrate from other damage mechanisms.

The synergistic oxidative and tellurium-induced (Te-induced) corrosion of the fast-reactor candidate 15–15Ti austenitic stainless steel under high oxygen potentials was investigated in this study. Specimens were exposed at 900 K in both Te-free and Te-containing environments, with the oxygen potentials controlled using Mo/MoO₂ and Ni/NiO redox buffers. Under Te-free conditions, a duplex oxide scale formed on 15–15Ti, comprising an Fe-rich outer oxide layer and a Cr-enriched inner oxide layer. In contrast, in the presence of Te, a markedly thicker multilayer reaction scale developed, consisting of four distinct layers arranged from the surface inward: (i) a Mn–O outer oxide layer; (ii) an Fe–Ni–Te metallic telluride layer; (iii) a Cr-enriched outer telluride reaction layer with Cr₃Te₄ as the dominant phase; and (iv) an inner Te/Cr-enriched reaction layer, in which a phase consistent with Cr₂Te₄O₁₁ was identified. These findings indicate that, under high oxygen potentials, corrosion of 15–15Ti is governed by the coupled action of Te and oxidation. The introduction of Te more strongly promotes outward diffusion of metal cations (especially Fe) and suppresses the formation of a protective Cr-rich oxide layer, thereby markedly accelerating scale thickening and stabilizing the multilayer architecture.

The effect of O(_2) traces on the oxidation behavior of iron and a Fe–Cr steel interconnect material (Crofer 22 APU) in a 5(%) H(_2) + 3(%) H(_2)O (bal. Ar) environment at 550 and 600 (^{circ })C has been investigated. The reaction of O(_2) with H(_2) in the gas at 550 and 600 (^{circ })C is slow, allowing O(_2) traces to reach the samples, despite an excess of H(_2) in the gas. Traces of unreacted O(_2) resulted in increased oxidation rate as well as hematite formation on iron. The rate of iron oxidation increased with the level of O(_2) in the range of 2–550 ppm. The acceleration of oxide growth by O(_2) traces is attributed to a greater oxygen activity gradient across the iron oxide scale. To remove O(_2) from the gas, a nickel component was positioned upstream from the samples which allowed the gas to equilibrate. Consequently, iron oxidized without hematite formation. Moreover, the use of the nickel component greatly improved the reproducibility of results for both iron and Crofer 22 APU. The use of a catalyst for the O(_2) + H(_2) reaction in H(_2)/H(_2)O exposures proved essential because it provides better control of experimental conditions and thereby more reliable experimental outcomes.

Hot corrosion leads to a significant challenge for next-generation concentrating solar power systems, particularly in thermal energy storage and heat transfer applications at elevated temperatures. This study investigated the corrosion behavior of 430 ferritic stainless steel in nitrate salts at 650 °C and proposed a targeted protective strategy. The primary novelty lies in the identification of a critical interfacial oxygen recombination mechanism via in situ ¹⁸O isotopic tracing. This mechanism drives oxide phase transformations, reducing activation energy and generating high-density defects that accelerate oxidant transport. Consequently, the uncoated steel suffered catastrophic failure, characterized by a 12-fold increase in oxide scale thickness and severe Cr-depletion. To mitigate this, a multilayer Si-P-Co-W (SPCW) coating was developed. Mechanistically, the coating blocks the interfacial oxygen recombination and suppresses Cr³⁺ outward diffusion through a dense tungsten-rich barrier. The coating significantly enhances corrosion resistance, reducing the internal oxidation depth from 13 ± 2.18 μm to 4.5 ± 0.9 μm, demonstrating its potential for high-temperature thermal energy storage applications.

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This study investigates the oxidation behavior of a binary Cr-20 at.%Ta alloy in a nitrogen-free, reduced oxygen partial pressure environment at 1000 °C for 48 h, aiming to clarify the intrinsic formation and growth mechanisms of protective (Cr, Ta)O₂ oxides. Chromium outward diffusion primarily governs oxidation, leading to a duplex scale with an outer Cr₂O₃ layer and an inner (Cr,Ta)O₂ subscale. Two distinct (Cr, Ta)O₂ phases were identified: CrTaO₄ (rutile structure) in outer regions and CrTa₂O₆ (trirutile structure) closer to the substrate, with CrTa₂O₆ confirmed as thermodynamically more stable through post-oxidation heat treatment and calculations. Thermogravimetric analysis revealed the parabolic oxidation constant of Cr-20 at.%Ta for fine-grained samples was eight times lower than pure chromium, highlighting the beneficial effect of the (Cr, Ta)O₂ layer. The microstructure significantly influences the protectiveness: fine-grained alloys promoted a continuous (Cr, Ta)O₂ layer, leading to enhanced oxidation resistance, particularly after a transient period required for the protective subscale to establish. This research underscores the critical role of (Cr, Ta)O₂ and microstructure in developing advanced oxidation-resistant refractory alloys.

This study presents an advanced cellular automata model for simulating high-temperature corrosion processes in nickel-based superalloys, with a particular focus on internal oxidation phenomena. Building upon previous work, this enhanced model addresses the limitations of Wagner’s theory by incorporating three-dimensional diffusion, precipitation growth and dissolution, and the influence of grain boundary diffusion. Integrating thermodynamic data, the model simulates the formation of oxide layers and precipitates, predicting oxidation behavior under various conditions. The final model shows good agreement with experimental data and was used to investigate the influence of aluminum on the oxidation behavior of Ni–Cr alloys. The results clearly show that an Al content of 2.5 wt% leads to a discontinuous alumina layer, whereas a continuous barrier with correspondingly improved corrosion properties forms at 5 wt% aluminum and above. Through these insights and the possibility of broad application to a wide range of alloy systems and corrosion conditions, the advanced cellular automata approach enables a more comprehensive understanding of corrosion mechanisms and contributes to the development of corrosion-resistant materials for high-temperature applications.

The behavior of several materials in molten NaCl–MgCl₂ at 600 °C during 168 h was studied. In order to ensure the comparison of their respective corrosion resistance, a dedicated experimental setup and protocol were designed both for the control of the impurity level in the salt and for the corrosion testing. IN625 was used as a reference sample in every test, leading to a qualitative comparison between the different alloys. In these conditions, the SiC showed the best resistance, while TA6V and 316L suffered the most. For Ni-based alloys, Hastelloy G35 seemed to provide the best compromise despite its high chromium content. Thermodynamic calculations unveiled a strong link between the corrosion resistance of the alloy and a high dµCr/dnr leading to consider this as a new criterion for alloy design or selection.

A precise methodology for determining the growth mode of oxide layers on metallic materials at high temperatures is proposed. The approach combines sequential isotopic oxidation tests (using ¹⁶O and ¹⁸O isotopes) with secondary ion mass spectrometry (SIMS and nanoSIMS) analyses. NanoSIMS provides high-resolution localisation of oxygen diffusion pathways and oxide growth zones. However, its limited accessibility and specialised instrumentation can pose practical constraints. In contrast, dynamic SIMS offers broader accessibility and the ability to directly quantify oxygen isotope ratios across depth profiles. The detection of both conventional atomic (O⁻) and diatomic (O₂⁻) oxygen signals in dynamic SIMS analysis proved highly effective in offering insights on oxide growth mode, closely replicating nanoSIMS results. The diatomic signal analysis complements the atomic signal data by improving the understanding of oxidant transport within the oxide layer. The methodology was validated through its application to a Co-10Cr alloy oxidised at 900 °C in O₂, under sequential exposures to ¹⁶O and ¹⁸O isotopes. Both SIMS and nanoSIMS revealed the formation of a duplex oxide layer, consisting of an outer layer formed by outward Co cation diffusion and an inner layer growing by inward oxygen penetration, particularly in the grain-boundary regions of the outer oxide layer. The alloy is proposed to oxidise according to the Available Space Model.

Isothermal oxidation of Laser Powder Bed-Fused (L-PBF) IN625 was investigated in air at 950 ℃ for 1000 h, across multiple heat-treated and surface conditions. Surface finish was the dominant factor influencing oxidation kinetics, with microstructure exerting a measurable secondary effect. Ground L-PBF IN625 exhibited oxidation behaviour comparable to wrought Ni-based superalloys, regardless of microstructural anisotropy. Additive surface roughness and Centrifugal High Energy Finished surfaces exhibited accelerated kinetics and a departure from sub-parabolic to cubic behaviour, showing spallation, severe void formation and intergranular oxidation. Segmented kinetic analysis revealed that additive surfaces exhibited an oxidation rate index (n > 2) during early exposure, followed by a transition to slower growth. Mn-spinel was detected throughout the external scale, but its mechanistic contribution to this transition remains uncertain. Growth of the near-continuous Ni₃(Nb, Mo) δ-phase varied by condition and is proposed to improve the adherence of chromia by increasing the critical temperature change for spallation.

Graphical Abstract

The high-temperature oxidation behavior of Haynes 214 alloy with different yttrium contents (0.0016 ~ 0.089 wt.%) at 950 °C was investigated. The results revealed a non-monotonic dependence of oxidation resistance on Y content, with an optimum at approximately 0.012 wt.%. At this optimal composition, Y increases the alloy lattice constant, which enhances the diffusion of Al to the surface. This ensures the formation of a continuous and dense Al₂O₃-based protective scale. In contrast, at Y contents below this optimum, insufficient Al diffusion leads to a non-protective oxide scale prone to spallation. When Y exceeds its solid solubility limit, excess Y precipitates as Ni₅Y phases. These precipitates act as fixed short-circuit paths for oxygen during oxidation, initiating internal oxidation and ultimately degrading the protective scale. Therefore, the optimal Y window for Haynes 214 is defined by a balance between enhanced Al diffusion and the avoidance of detrimental Ni₅Y precipitation.