The oxidation behavior of two ternary Mo-Si-Ti alloys (eutectic Mo-20.0Si-52.8Ti and eutectoid Mo-21.0Si-34.0Ti) was investigated using thermogravimetric analysis at 700 °C, 900 °C, and 1300 °C for 100 h. The eutectic alloy formed a protective SiO₂/TiO₂ oxide scale, whereas the eutectoid alloy showed catastrophic oxidation. To improve the oxidation behavior of these alloys, chromium diffusion coatings deposited via pack cementation on the surface were investigated. Cr coatings were found to be suitable for improving oxidation resistance at 700 °C and 900 °C but failed at 1300 °C due to the evaporation of thin scales. At 700 °C and 900 °C, the formation of a Cr₂O₃ scale was proven on the Ti-rich/Mo-lean eutectic composition. After exposure, the Ti-lean/Mo-rich eutectoid composition, which is intrinsically more prone to oxidation, was found to form a continuous Cr₂O₃ scale only at 700 °C.

The hot corrosion behavior of two ternary Mo–Si–Ti alloys (eutectic Mo–20.0Si–52.8Ti (at.%) and eutectoid Mo–21.0Si–34.0Ti (at.%)) was investigated at C and C for up to 100 h. Both Mo‐based alloys evidenced a hot corrosion attack, resulting in a uniform attack of the substrate surface. The higher Ti content of the eutectic alloy significantly reduced the formation of solid and volatile Mo oxides compared with the eutectoid alloy. After 24 h at C, the corrosion products formed on the eutectic alloy were only one‐tenth the thickness of the layers formed on the eutectoid alloy. The corrosion products were examined with optical and electron microscopy. Semiquantitative electron probe micro analysis and X‐ray diffraction measurements were used to identify the formed phases. The underlying corrosion mechanisms comprising not only hot corrosion‐induced fluxing but also oxidation‐induced pesting are discussed in detail.

In recent years, interest in Mo‐based alloys has increased significantly due to their promising high‐temperature behavior and their possible uses for industrial high‐temperature applications such as within turbines. An important corrosion mechanism in turbines is the sulfate salt‐induced attack of the structural materials, known as hot corrosion. Therefore, the Type I and II hot corrosion behavior of Mo–9.0Si–8.0B was investigated at 700°C and 900°C for 24 and 100 h. The Mo‐based alloy showed severe degradation, but not by sulfates. Thus, the underlying hot corrosion mechanism through molybdate fluxing, which is accompanied by Mo oxide evaporation, is discussed in detail. At the higher temperature, the MoO₃ evaporation becomes predominant, leaving behind a Si‐glass with molybdate inclusions.

To understand the influence of Mo, Si, and Ti during aluminizing pack cementation processes of Mo-Si-Ti alloys, two ternary Mo-Si-Ti alloys (eutectic Mo-20.0Si-52.8Ti and eutectoid Mo-21.0Si-34.0Ti) were investigated and compared with pure Ti and Mo-40Ti (all in at.%). The coating formation mechanisms, phase composition, and microstructures of the different substrates were compared. Subsequently, the effect of the different elements on the oxidation behavior was evaluated, using thermogravimetric analysis at $${700}\,^{\circ }\text {C}$$ 700 ∘ C and $${900}\,^{\circ }\text {C}$$ 900 ∘ C for 100 h in synthetic air. In addition, the type I ( $${900}\,^{\circ }\text {C}$$ 900 ∘ C ) and type II ( $${700}\,^{\circ }\text {C}$$ 700 ∘ C ) hot corrosion behavior of the Al-coated Mo-Si-Ti alloys was investigated for 24 h and 100 h in synthetic air + 0.1% SO $$_2$$ 2 . While the initial Al-rich coating phase was consumed or transformed at $${700}\,^{\circ }\text {C}$$ 700 ∘ C , it successfully facilitated the formation of a protective Al $$_2$$ 2 O $$_3$$ 3 scale on the surface, even if the underlying reservoir was diminished. At $${900}\,^{\circ }\text {C}$$ 900 ∘ C , the Al coatings on both substrates failed, and a hot corrosion-induced pesting dominated. While Si generally has a positive effect on oxidation and hot corrosion resistance, the main impact of Mo is dictated by its evaporation, and Ti can lead to the formation of TiO $$_2$$ 2 as a mixed oxide with Al $$_2$$ 2 O $$_3$$ 3 .

The oxidation resistance of novelγ/γ’-strengthened Co-base superalloys is clearly outmatched by their Ni-base counterparts within the high-temperature regime. Therefore, surface modification strategies to foster protective alumina growth seem auspicious. This study elucidates the impact of fluorination and shot-peening on protective alumina formation at 900 °C for a quaternary Co-base model alloy (Co-Al-W-Ta system) which is well known for an exceptionally low inherent oxidation resistance. Time-resolved isothermal gravimetric analysis (TGA) in synthetic air, detailed electron microscopic analysis, and X-ray diffraction (XRD) were used. For polished samples, no pronounced enhancement of oxidation resistance could be obtained by halogenation. However, in case of shot-peened samples (halogen-free), an increased tendency for alumina formation is found compared to polished surfaces. The very early stages of oxidation were identified to be especially crucial with respect to sustainable protective scale growth. Most noteworthy is the observation of a strong synergistic effect derived by a combination of halogenation and shot-peening, leading to significantly increased oxidation resistance.