Microstructural aspects of low-pressure plasma-sprayed CoNiCrAlY coating on Hastelloy X

A Co-32Ni-21Cr-8Al-0.5Y alloy coating was plasma sprayed on Hastelloy X. The microstructure of the coating layer consists of γ phase solid solution, γ′ phase, and Y-rich intermetallic phase. This coating exhibits excellent oxidation and sulfidation resistance after exposure in air and in sodium sulfate at 1,000°C for 60 h, due to the formation of α-Al₂O₃ oxide scale. However, the presence of chloride in the sodium sulfate leads to rupture of the aluminium oxide scale, and this results in the precipitation of chlorides and sulfides within the coating layer. © 1989.