Project 7: High-temperature Stability in Harsh Environments – Hot Corrosion of Refractory Silicides

M.Sc. Katharina Beck

Supervisors: Prof. Mathias Galetz, Prof. Hans-Jürgen Seifert

Industrial high-temperature environments are characterized by a mixture of different reactive species, causing complex corrosion attacks. A particularly severe attack is induced by deposits of solid or liquid salts on the surface of compounds. 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 and the occurring impairment. These mechanisms have barely been researched for silica-formers so far. It has been generally assumed that SiO2 layers protect the subjacent material against hot corrosion. However experiments have found protective oxide layers formed on SiC to form predominantly non-protective silica scales and sodium silicate in the presence of sodium sulphate.

Project 7 aims to systematically research the corrosion behaviour of Mo-Si-Ti-(B) substrates in order to develop a fundamental understanding of the underlying mechanisms. To this end, refractory bulk materials will be exposed first without and then with deposits consisting of Na2SO4 or CaSO4/CaO in controlled SO2-containing gas atmospheres. Building upon this, protective layer concepts such as Environmental Barrier Coatings (EBCs) using polymer-derived ceramic nanocomposites (PDC-NCs) will be evaluated. The new findings will enable the improvement of the corrosion behaviour of these bulk-materials by applying corrosion-protective coatings.

The project will be carried out in cooperation with other subprojects; Projects 2 (PDC-NCs) and Project 8 (alloys) supply the bulk materials, utilizing the results of the corrosion experiments for further improvement. The corrosion attacks will be microstructurally evaluated (using TEM and APT) jointly with Project 3. The hot corrosion results complement the results from the pure oxidizing experiments carried out in Project 5. To develop a comprehensive understanding of the underlying corrosion mechanisms, the experimental work is supplemented with thermodynamic calculations from Project 11.