Studies on corrosion behavior of a single-crystal superalloy and its sputtered nanocrystalline coatings with solid NaCl deposit in O2 + 38 vol.% H2O environment at 700 °C

Corrosion behavior of the N5 superalloy (Ni-7.0 Cr-6.2 Al-7.5 Co-5.0 W-1.5 Mo-6.5 Ta-3.0 Re) and two nanocrystalline coatings prepared by magnetron sputtering is investigated in chlorine and water vapor environment at 700 °C. The alloy shows low corrosion resistance. Its oxide scale spalls off readily and includes porous NiO outer layer and NiCr(Al)2O4 inner one. The nanocrystalline coating performs better. Its oxide scale consists of Al2O3, NiO and Ta2O5. Chlorination and/or oxidation of Ta ruined compactness of scale. The yttrium modified nanocrystalline coating provides the highest corrosion resistance. Its scale is exclusively alumina. Yttrium inhibits the outward diffusion of Ta.