  The service lives of nickel superalloys are often limited by environmental
degradation. The present study compares oxidation, sulfidation and hot
corrosion at 750C of three variants of a polycrystalline superalloy: a baseline
alloy, a variant containing 1wt% Mn and one containing 0.5wt% Si. Mn reduced
the oxidation rate without changing the scale morphology. The MnCr2O4 scale
formed proved more protective against sulfidation and hot corrosion, but
internal sulfides extended the damage depth. Si modified the oxide morphology
to a continuous Cr2O3-Al2O3 dual layer. This provided improved protection,
reducing the sulfidation depth by 2/3 and the hot corrosion depth by 1/2.
