CHROMIUM AND REACTIVE ELEMENT MODIFIED ALUMINIDE DIFFUSION COATINGS ON SUPERALLOYS - ENVIRONMENTAL TESTING

The isothermal oxidation of reactive element (RE)-doped aluminide coatings on IN 713LC alloy substrates at 1100-degrees-C in air formed a continuous slow-growing alpha-Al2O3 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 Rene 80 and IN 713LC alloy substrates, and of RE-doped aluminide coatings on IN 713LC alloy substrates, at 1100-degrees-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 Al2O3 scales compared to undoped aluminide coatings. The hot corrosion behavior of Cr/RE-modified aluminide coatings on Rene 80 and Mar-M247 alloy substrates at 900-degrees-C in a 0.1% SO2/O2 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 (Al, Cr)2O3 scale better resists attack by the molten salt.