Project 7: High-temperature stability in harsh environments

Doctoral researchers 

Dr. Katharina Beck (1^st cohort)
M. Sc. Lukas Korell (2^nd cohort) 
M. Sc. Sarah Auler (3^rd cohort) 

Supervisors 

Prof. Mathias Galetz,
Prof. Martin Heilmaier,
Prof. Bronislava Gorr

Project 

Industrial high-temperature environments are characterized by a mixture of different reactive species, usually causing the superimposition of complex corrosion attacks. A particularly severe attack is induced by deposits of solid or liquid salts on the surface of components. These deposits usually are the result of impurities in combustion atmospheres. Oxygen, sulphur, calcium, and sodium are of primary importance due to their ability to form sulphur dioxide, sulphur trioxide or sulphates. Corrosion processes involving salts, such as sulphates, are known as hot corrosion, with a distinction between type I and type II hot corrosion, depending on the temperature range. While being thoroughly investigated for Ni-based alloys, these mechanisms have barely been researched for refractory metal based alloy systems. It has been generally assumed that SiO₂ layers protect the subjacent material against hot corrosion. 

Project 7.1 investigated the oxidation as well as the hot corrosion type I and type II hot corrosion behavior of Mo-Si-Ti-(B) alloys. For the latter, refractory bulk materials with Na₂SO₄ deposits were exposed to controlled SO₂/SO₃-containing gas atmospheres. The alloys showed a combination of MoO₃ evaporation, dissolution and reprecipitation as well as the selective oxidation of Moss over the silicide phases. Building upon this, protective layer concepts such as the application of metallic and intermetallic layers via pack cementation were developed and their protective properties were proven in additional oxidation and hot corrosion experiments. 

In project 7.2 the interesting fundamental hot corrosion mechanism of Mo-based alloys were further developed. The objectives were to observe the development of the corrosion rate over the temperature and to determine the temperatures regimes (similar to type I and type II for Ni-base) as well as temperature ranges which promote degradation mechanisms of pesting and hot corrosion. In addition, tests with varying salts such as NiSO₄ and CaSO₄ were conducted. Diffusion coatings on Mo-Si-Ti showed enhanced resistance in hot corrosion, but also a high dependence on the Mo-content of the alloy underneath. Thus, the influence of the Mo-content on the corrosion was examined by the investigation of different Mo-Si-Ti-(B) alloys. 

Building on the findings of projects 7.1 and 7.2, project 7.3 (the 3^rd cohort) shifts the focus from Mo-based to Cr-based refractory alloys. To mitigate the acidic fluxing associated with Mo-containing systems, a modified A2 + B2 microstructure was developed to suppress the formation of the brittle A15 phase while preserving favorable mechanical properties at elevated temperatures, but compromises oxidation resistance. The primary objective of project 7.3 is therefore to restore the formation of protective oxide scales through targeted alloying additions, while maintaining the improved mechanical performance. In addition, the experiments will assess whether the degradation mechanisms established for Mo-based alloys remain valid for the modified alloy chemistry and the reduced Mo content. 

For project 7, projects 8 and 10 provide the required Mo-base alloys, utilizing the results of the corrosion experiments for further improvement. Projects 3 and 11 help to validate some of the results obtained. Projects 2 and 6 receive aluminized samples for further experiments.