Chromium and Reactive Element Modified Aluminide Diffusion Coatings on Superalloys: Environmental Testing

Abstract

The isothermal oxidation of reactive element (RE)‐doped aluminide coatings on IN 713LC alloy substrates at 1100°C in air formed a continuous slow‐growing scale after 44 h of exposure. RE‐free (reactive element‐free) aluminide coatings were characterized by a cracked oxide scale which exposed an underlying voided coating surface. The cyclic oxidation behavior of Cr/RE‐modified aluminide diffusion coatings on René 80 and IN 713LC alloy substrates, and of RE‐doped aluminide coatings on IN 713LC alloy substrates, at 1100°C in static air was determined. Coatings deposited by the above pack (AP) arrangement, as opposed to the powder contacting (PC) arrangement, showed improved resistance to cyclic oxidation attack. RE‐doped and Cr/RE‐modified aluminide coatings exhibited considerably more adherent protective scales compared to undoped aluminide coatings. The hot corrosion behavior of Cr/RE‐modified aluminide coatings on René 80 and Mar‐M247 alloy substrates at 900°C in a 0.1% gas mixture also was determined. The Cr/RE‐modified aluminide coatings provided better resistance to hot corrosion attack (i.e., thin film studies) than a commercial low activity aluminide coating. Coating lifetimes were strongly dependent on the chromium surface composition, since a mixed scale better resists attack by the molten salt.