Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na2SO4 and Na2SO4/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al2O3 (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.