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

ABSTRACT 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.