Improvement of NaCl induced hot corrosion and oxidation resistance of gamma-TiAI alloys by a new gold based coating

摘要

Gamma titanium aluminides are of great interest for use in industrial and aeroengine turbomachines at high temperature because of their high specific strength and stiffness associated with low density. However, corrosion due to NaCl, is a critical phenomenon that reduces drastically the life duration of TiAl parts. As typical coatings as MCrAlY, TiAlCr, siliconization or aluminization don't seem sufficiently efficient, a new coating has been developed. This new coating consists in a surface gold enrichment obtained by a 20 μm gold electrodeposit followed by a diffusion annealing. The present work investigated the hot corrosion behaviour, after exposure to sea salt water fog and heating at 600°C under atmospheric pressure, of this type of coating deposited on two different γ-TiAl alloys, named as GE (nominal composition Ti-48Al-2Cr-2Nb) and G4 (nominal composition Ti-47Al-1W-1Re-0.2Si) alloys.NaCl-corrosion cyclic tests were carried out at 600°C. Microstructures of these systems have been studied on as coated, annealed and corroded specimens.After annealing, the as-coated specimens present a multilayer system. The coating is composed of an external TiAlAu_2 phase and an internal TiAlAu layer. A TiAl zone enriched in gold is present at the interface between the coating and TiAl alloy. After corrosion, these coating layers are differently modified according to the nature of alloying elements.