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°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 β-CoAl phases (as Al reservoir) in the outer layer of the coating and Cr as an intermediate layer. The relevant corrosion mechanisms are discussed.