Type II Hot Corrosion of Two Mo‐Si‐Ti Alloys and Mo‐Si‐B With a Salt Deposit of Mixed Na₂SO₄ and NiSO₄

To enhance turbine efficiencies by increasing gas inlet temperatures, new materials must be developed which can withstand harsh corrosive environments and provide sufficient mechanical properties at elevated temperatures. The hot corrosion behavior of the promising alloy candidates Mo‐20Si‐52.8Ti (at.%), Mo‐21Si‐34Ti (at.%) and Mo‐9Si‐8B (at.%) under mixed 60mol.% Na 2 SO 4 and 40mol.% NiSO 4 deposit in dry synthetic air plus 0.1vol.% SO 2 were investigated at 700°C. Optical and electron microscopy, EPMA and XRD were employed for analysis. Comparing the results to tests with pure Na 2 SO 4 reveals that the Mo content significantly impacts the corrosion attack if Ni is present. Na 2 MoO 4 is observed during corrosion besides the initial eutectic phase between Na 2 SO 4 and NiSO 4 . With melting points below 700°C, both compounds are capable of dissolving the surrounding material. However, the formation of NiMoO 4 is energetically favored, transforming the initial liquid eutectic sulfate mixtures. Therefore the corrosion rate is lowered by the addition of NiSO 4 to the Na 2 SO 4 deposit.