Low-activity aluminide coatings for superalloys using a slurry process free of halide activators and chromates

Abstract

Slurry aluminide coatings have been well known for a number of years and are widely used to protect metallic surfaces from oxidation and corrosion. A variety of commercial slurries is available to aluminize the surfaces of nickel-based superalloys, however, they have several disadvantages. They contain environmentally harmful substances such as chromates or halides which are used as binders or to activate the diffusion species. Additionally, up to now slurry coatings were only used to produce precipitate-rich coatings, with a microstructure comparable to high aluminum activity coatings produced by chemical vapor deposition (CVD) like pack cementation.In this work these limitations have been overcome by carefully designing the powder composition of the slurry. Three different nickel-aluminide coatings were developed and applied onto CM-247 nickel-based alloy and Pt-diffused CM-247. By addition of chromium we achieved coatings with a similar microstructure and Al-content (of around 40at.%), comparable to the state of the art low-activity coatings produced by CVD. Co-deposition of silicon was also achieved in a single step, maintaining the low-activity structure by combining Al–Si powder and chromium. Furthermore, this coating procedure was combined with a platinum electrodeposition step in order to produce single phase Pt-modified low-activity aluminide coatings.