The codeposition of two or more elementli in II halide-activated cementation pack is inherently difficult because of large differences in the thermodynamic stabilities for tlreir volatile halides. However, through II computer-assisted analysis of the pack equilibria, combiniltions of suitable masteralloys and activator salts can be identified. The codepositwn of chromium plusaluminum or chromium plus silicon by pack cementation has yielded diffusion coatings with excellent resistance to higll-tempemtureoxidation and corrosion fora wide range of alloy substrates.

A single-step chloride-activated pack cementation process has been developed for growing a Cr/RE(Y, Zr) -modified or a RE(Y, Zr, Hf)-deped aluminide diffusion coating on commercial Ni-base superalloys. The coatings consisted of an outward-grown hypostoichiometric ~-NiA1 layer with a substantial enrichment in Cr and a low reactive element content, The Cr was both dissolved in the ~-NiA1 matrix and precipitated as ~-Cr second-phase particles. Aluminide coatings grown outwards in a pack permitting powder contact with the substrate were embedded with A1203 and reactive element (RE) oxide pack particles. Physical separation of the substrates from the powder mixture eliminated powder entrapment but reduced the growth rate and the amount of Cr enrichment in the coating. Isolation of the substrate from the pack introduced partial rate control by gaseous diffusion.

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The isothermal oxidation of reactive element (RE)-doped aluminide coatings on IN 713LC alloy substrates at ll00~ in air formed a continuous slow-growing ~-A1203 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 Ren~ 80 and IN 713LC alloy substrates, and of RE-doped aluminide coatings on IN 713LC alloy substrates, at ll00~ 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 A1203 scales compared to undeped aluminide coatings. The hot corrosion behavior of Cr/RE-medified aluminide coatings on Ren~ 80 and Mar-M247 alloy substrates at 900~ in a 0.1% SO2/Q 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 (A1, Cr)203 scale better resists attack by the molten salt.