Oxidation of Metals最新文献

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.

Conventional NiCrAlY coatings primarily consist of α, β, and γ phases. Interdiffusion between the coating and substrate during high-temperature oxidation, along with the associated issue of oxide spallation, has long been a significant concern. In this study, a novel α/β-free γ’ coating was developed, and its oxidation and interdiffusion behavior at 1100 °C was compared with that of a conventional NiCrAlY coating. The oxidation rate constant of the α/β/γ NiCrAlY coating was measured to be 2.4 times that of the γ’ coating, which may be attributed to the doping of reactive elements in the oxide scale promoting oxide growth, while oxide scale spallation further accelerates this kinetic process. The Ta-rich γ’ phase formed at the γ’ coating/substrate interface effectively suppresses the diffusion of substrate elements such as Hf into the coating. Simultaneously, the inherent suppression of α and β phase formation in the γ’ coating is expected to prevent oxide scale spallation induced by phase transformations. Furthermore, a second reaction zone (SRZ) over 50 μm thick was observed beneath the α/β/γ NiCrAlY coating after only 20 h of oxidation, while no SRZ was observed beneath the γ’ coating during oxidation. This significant difference indicates that variations in phase composition directly affect the chemical compatibility between the coating and substrate, thereby playing a crucial role in SRZ formation.

This study mainly focuses on the effect of small amount additions of Eu³⁺ in substitution for Y³⁺ on the thermal conductivity of YSZ TBCs, and the evaluation of coatings resistance to severe thermal shock cycles. Eu³⁺ doped and undoped YSZ coatings were prepared by APS method. Thermal conductivity of the two coatings was measured and both types of coatings were subjected to thermal shock condition under a steady propane flame at 1100 °C. Results showed that thermal conductivity of the YSZ:Eu coating is 10–19% lower than that of the YSZ coating at the same temperature, indicating that only 2 mol% Eu³⁺ doping can effectively reduce the thermal conductivity of the YSZ TBCs. For the same treatment conditions, the interfacial fracture toughness of the YSZ:Eu coating is slightly higher than that of the YSZ coating and the TGO thickness is thinner, indicating that 2 mol% Eu³⁺ doping is prone to inhibit the growth of TGO and improve the coating’s interfacial properties. Comparison between flame thermal shock and furnace isothermal / cyclic oxidation treatment confirmed that flame thermal shock is a more severe and destructive treatment as it results in higher densities of micro-cracks and pores within the TGO, thus inducing a lower interfacial fracture toughness value.

The oxidation reactions of monocrystalline iron (mono-1), bicrystal iron (poly-14), and four-grain iron (poly-1234) were assessed by ReaxFF MD simulations to systematically investigate grain boundary effects on iron oxidation. For both monocrystalline and polycrystalline irons, the surface of the oxide film was characterized by a rough and island-like texture. A transformation in the crystal structure of iron was observed after oxidation, and the disordered structure of grain boundaries would expand into the grains in polycrystalline irons. The oxidation process consists of a linear and rapid stage controlled by the chemical reaction and a parabolic and slow stage governed by the inward diffusion of oxygen and outward movement of iron. Atomic stress analysis showed that surface iron atoms and inner oxygen atoms exhibited a tensile stress state while inner iron atoms and surface oxygen atoms demonstrated a compressive stress state. Such a stress state could facilitate the atom diffusion and thus promote the oxidation process. The grain boundary is found to enhance the oxidation process, as evidenced by the greater oxide film thickness and phase transformation, accelerated oxidation rate and atom diffusion, higher O/Fe ratio and atomic stress in polycrystallines.

Various chromia-forming alloys were exposed to atmospheres of Ar-10%H₂O and Ar-10%H₂O-2.5%NH₃ at 800 °C. A Cr₂O₃ scale formed on SUS310S and Inconel 718 in both atmospheres, and no nitride formation was observed. However, the high-temperature corrosion resistance of Alloy 800H substantially decreased in Ar-10%H₂O-2.5%NH₃ compared with in Ar-10%H₂O, and internal oxide and thick internal nitride layers formed, despite the initial formation of a Cr₂O₃ scale. These results suggest that the nitrogen permeability through Cr₂O₃ scale may depend on the alloy substrate. In Alloy 800H, because the initially-formed Cr₂O₃ scale did not suppress the nitrogen diffusion, an internal nitride layer formed beneath the Cr₂O₃ scale, resulting in the breakaway of the Cr₂O₃ scale and severe internal oxidation and nitridation.

Turbine blades of aerogas turbines can be at risk of stress corrosion cracking (SCC) below 700 °C due to the effects of stress, sulphur-containing gases and deposits that are ingested into the turbine. Therefore, understanding the effect of different deposits on the SCC susceptibility of single-crystal nickel-based superalloys at different temperatures is crucial. This study investigated the effect of NaCl, sea salt and 80/20 mol% Na₂SO₄/K₂SO₄ on the SCC susceptibility of CMSX-10 at 550 °C and 700 °C. The results suggest that chlorine-containing salts play an important role in accelerating stress corrosion cracking at 550 °C, where the formation of HCl leads to the breaking down of the oxide and exposing the base alloy to a sulphidation environment. At 700 °C stress corrosion cracking is accelerated by the mix of sulphates that lead to reduced melting points, where the 80/20 mol% Na₂SO₄/K₂SO₄ has shown the highest susceptibility to SCC.