Hot Corrosion Type X: Behavior of Mo‐Si‐B at Different Temperatures

To improve gas turbine efficiencies, temperatures must increase, necessitating new materials that can operate beyond the temperature limits of Ni-based alloys. Due to their high melting points and creep strength, novel material candidates contain refractory elements. Molybdenum (Mo)-based Mo-Si-B alloys have been investigated for more than 20 years. Surprisingly, their hot corrosion behavior was not investigated in detail. For Ni-based alloys, two mechanisms are established: Corrosion due to melting of Na$_2$SO$_4$ (Type I, ~900°C) or due to the formation of lower-melting eutectics (Type II, ~700°C). In this work, the temperature-dependent mechanism for Mo-Si-B is investigated, by studying the hot corrosion resistance of Mo-Si-B at 500°C–900°C. Post-testing analysis via XRD, SEM, Raman and EPMA is used to reveal the degradation mechanisms. Na$_2$MoO$_4$, which forms under Na$_2$SO$_4$ deposits, is responsible for dissolving the surrounding material due to its lower melting point in respect to Na$_2$SO$_4$. This alters the mechanism to a super-acidic Na-Mo-based attack (Type X hot corrosion). Finally, these results can trigger mitigation strategies, such as protective coatings for Mo-based systems.