Chemical stripping methods for the removal of NiCrAlY thermal spray coatings

The present paper is focused on the "stripping" of serviced blades and vanes for gas turbine, which need the removal of the coating in order to be repaired and refitted with a new coating. The stripping process requires, normally, the separate removal of the top coat (Yttria Partially Stabilized Zirconia, YPSZ) and of the bond coat (MCrAlY where M stays for Co, Ni or both). The present paper examines only chemical procedures able to remove the NiCrAlY bond coat. The blades and vanes surfaces after removal of NiCrAlY coatings were characterized from the point of view of surface morphology, metallurgical structure and chemical composition. The efficiency of the acid solution in NiCrAlY removal has been investigated and the behaviour of two Ni based alloys substrates in aggressive environment has been tested. The HC1 based stripping solution shows good performances in Vacuum Plasma Sprayed NiCrAlY coatings removal from Ni superalloys. The tested stripping procedure is fast and safe because no damage to base materials has been noted.