Emma Marie Hamilton White

Vanadate corrosion occurs in devices powered by heavy fuel oil, such as stationary turbines. To avoid this aggressive attack from molten V ₂ O ₅ , inhibitors like fuel‐dissolvable Ca‐species are usually added beforehand to the fuel to bind V ₂ O ₅ as higher‐melting vanadates during combustion. However, this requires a constant addition of inhibitors and increased maintenance due to the accumulation of slag on the turbines, resulting in higher costs. As an alternative strategy, here the inhibitor is directly incorporated into a protective coating to be applied and available exactly where it is necessary. In this study, three Ca‐based coatings were investigated for their potential to mitigate vanadate corrosion at 600°C and 700°C. The various coatings showed promising protective properties for up to ~500 h, demonstrating suppression of the salt fluxing mechanism by the formation of higher melting vanadates, especially CaV ₂ O ₆ . Additionally, during the sintering of the coatings, a Ca‐Si‐Ni intermetallic phase formed, that was identified as a potentially highly protective phase against molten V ₂ O ₅ .

Cr-Mo-Si alloys are promising high-temperature structural materials due to their low density, high melting point, and excellent strength. Their long-term environmental stability, however, remains insufficiently understood. This study investigated the quasi-isothermal oxidation and nitridation behavior of Cr-xMo-Si alloys (x = 0–40at.%) during long-term exposure in air at 800 °C (up to 700 h) and 1200 °C (up to 1000 h). At 800 °C, all alloys showed slow oxidation without catastrophic pesting. Mo additions reduced mass gain and improved nitridation resistance. Complete suppression of nitridation required at least 20–25at.%Mo, while ≥ 30at.%Mo promoted increased oxide growth and MoO ₃ volatilization. At 1200 °C, Mo additions up to 25at.% improved oxidation resistance, whereas ≥ 30at.%Mo caused severe degradation due to accelerated oxide growth, cracking, and volatilization. Long-term exposure revealed additional degradation mechanisms not observed in short-term studies, such as oxide-scale cracking and depletion of the protective Mo-enriched subsurface zone due to the intermediate stops, and thus, thermal cycling. The intermediate stops from the quasi-isothermal exposure resulted in localized nitridation along A2/A15 phase boundaries. In contrast, isothermal exposure largely preserved the Mo-enriched region and maintained nitridation resistance. Amongst the compositions investigated, Cr-25Mo-8at.%Si showed the best overall long-term performance.

In this study SiCNO coatings with tunable carbon content were derived from a single preceramic polysilazane and deposited on AISI 441 steel. The carbon content was controlled through the annealing atmosphere. The coatings were annealed in argon, in argon followed by cooling in air, and entirely in air. As a result three coatings with different carbon concentrations were obtained. XPS together with Raman spectroscopy confirmed that the annealing atmosphere altered both the amount and nature of the free carbon phase. ToF-SIMS depth profiles revealed the outward diffusion of manganese and chromium notwithstanding the annealing conditions. During oxidation at 800°C for 100 h in air, the coatings inhibited the mass gain of steel but they did not act as perfect diffusion barriers, as evidenced by the formation of oxide scales above the coatings. The area specific resistance (ASR) of the coatings was too high for the application in SOFC interconnects, despite the presence of the free carbon phase. This is due to the fact the free carbon phase forming a discontinuous, non-percolating network. The annealing atmosphere modified the relative contributions of silica, and SiON/SiOC/SiOx environments within the coating, thereby affecting metal transport and the resulting oxide-scale composition. The coating annealed in argon developed a scale containing mainly (Mn,Cr)3O4 and Cr2O3, whereas the coatings exposed to air during annealing or cooling formed manganese-rich oxides and silicates together with smaller amounts of (Mn,Cr)3O4 and Cr2O3. Overall, the annealing atmosphere strongly controlled carbon structure, metal transport, and oxidation-product formation.

The oxidation behaviour of TaMoCrTiAl in steam at 900–1100 °C has been investigated by thermogravimetric analysis of oxidation kinetics and detailed cross-sectional SEM and TEM characterization of the multilayered oxide scales formed during exposure. Experiments point to the significance of both outward diffusion of metal cations and inward transport of oxidizing species, whereas the latter becomes increasingly dominant with increasing exposure times. Based on these findings, a mechanistic oxidation model for TaMoCrTiAl in steam is proposed and compared with the oxidation behaviour in air. In contrast to oxidation in synthetic air, the (Cr, Ta, Ti)O₂ layer typical for TaMoCrTiAl is absent, but formation of (Ti, Ta)O₂ observed, with impact on both formation of external oxide and internal corrosion.

A novel route for the fabrication of core–shell structured V–MgO alloys via in situ MgO formation is developed. Spark plasma sintering is employed to obtain fully dense V– x MgO ( x = 1, 2.5, 5 wt.%) samples. The sintered alloys consist of vanadium grains with finely dispersed MgO particles uniformly distributed along the grain boundaries. An increase in oxidation resistance at 600 °C is achieved for alloys containing more than 2.5 wt.% MgO. XRD analysis shows that during oxidation, V ₂ O ₅ reacts with MgO, leading to the formation of MgO ₆ V ₂ and Mg ₂ O ₇ V ₂ oxides. However, these compounds formed at high temperature are not stable in ambient air with atmospheric CO ₂ since it forms MgCO ₃ . For further development of this concept, other more stable vanadate formers need to be investigated. The room temperature compressive strength increases from 1350 MPa for pure vanadium to 2180 MPa for the V–5MgO alloy, while at 600 °C it increases from 300 to 350 MPa.

Presuming that doping of protective (Cr,Ta,Ti)O ₂ with cations of +5 or higher valence further improves the oxidation resistance of Ta–Mo–Cr–Ti–Al, the effect of 3 at.% of Re or W in otherwise equimolar Ta–Mo–Cr–Ti–Al is investigated in oxidation tests performed at 1000°C in air. Both minor additions indeed decrease the parabolic mass gain (72 h), which for W also manifests in reduced thickness of oxide layers and zone of internal corrosion that persists for at least 100 h. Besides the hypothesised doping, other factors that may explain the effect of Re or W are discussed. A general finding of the analytical work is the identification of (Ti,Ta)O ₂ with Mo inclusions underneath the inward growing (Cr,Ta,Ti)O ₂ layer.

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)O ₂ oxides. Chromium outward diffusion primarily governs oxidation, leading to a duplex scale with an outer Cr ₂ O ₃ layer and an inner (Cr,Ta)O ₂ subscale. Two distinct (Cr, Ta)O ₂ phases were identified: CrTaO ₄ (rutile structure) in outer regions and CrTa ₂ O ₆ (trirutile structure) closer to the substrate, with CrTa ₂ O ₆ 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)O ₂ layer. The microstructure significantly influences the protectiveness: fine-grained alloys promoted a continuous (Cr, Ta)O ₂ 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)O ₂ and microstructure in developing advanced oxidation-resistant refractory alloys.

Refractory high‐entropy alloys (RHEAs) offer high strength at extreme temperatures but suffer from poor oxidation resistance. The Ta‐Mo‐Cr‐Ti‐Al system shows promising oxidation resistance due to protective (Cr,Ti,Ta)O₂ scale formation. This study investigates how varying Cr:Ti ratios (2:1, 1:2, and equimolar) affect microstructure and oxidation behavior in Ar‐5 vol.%H₂O and Ar‐2.5 vol.%O₂ atmospheres at 1000°C. Titanium influences oxide defect structure through its valence state, controlling oxygen transport kinetics. Results demonstrate strong atmosphere‐dependent performance: Cr‐rich alloys excel in dry oxygen due to continuous chromia formation and reduced oxygen diffusion zones, while Ti‐rich alloys perform superior in humid conditions, likely due to hydrogen uptake effects and reduced chromium volatilization. The study reveals that optimal oxidation resistance requires atmosphere‐specific compositions, with microstructural factors (Laves phase distribution) playing equally important roles as oxide defect chemistry in determining overall performance.