Type II Hot Corrosion of Two Mo‐Si‐Ti Alloys and Mo‐Si‐B With a Salt Deposit of Mixed Na$_2$SO$_4$ and NiSO$_4$

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.