Hot corrosion of nanostructured Cr/Co-Al coatings

Nanostructured Cr/Co-Al coatings on Superni-718 superalloy substrate are deposited by direct current/radio frequency magnetron sputtering. The microstructure and hot corrosion behaviour of sputtered Cr/Co-Al coatings on Superni-718 superalloy have been investigated in molten salt of 40 wt-%Na2SO4+60 wt-%V2O5 at 900 degrees C. The results showed that a dense scale formed on the coated samples during thermal cycling. The spinel phases of CoCr2O4, CoAl2O4 and NaAlO2 are present in the scale of the corroded coating, resulting in an effective inhibition of O and S diffusion. The sputtered coatings showed better performance of hot corrosion due to its possession of beta-CoAl phases (as Al reservoir) in the outer layer of the coating and Cr as an intermediate layer. The relevant corrosion mechanisms are discussed.