TABLE OF CONTENTS Diffusion coatings (Goward and Seigle, 1994; Goward, 1998) consist of a substrate alloy surface layer enriched with the oxide scale formers Al, Cr, Si, or their combination to a depth of 10 to 100 ?m. These elements combine with the primary constituents of the substrate alloy to form intermetallics with significant levels of the oxide scale formers. For example, in Ni base superal-loys, surface enrichment with aluminum forms nickel aluminide, NiAl (the ? phase in the Ni-Al system), which is the predominant constituent of the coating. The substrate alloy participates in the formation of diffusion coatings. For oxidation protection, the diffusion coatings of choice are the aluminides, which form a protective alumina scale on high-temperature exposure in air. For protection against hot corrosion, incorporation of platinum in the aluminide, chromizing, and siliconizing are more beneficial. Because the ? phase field in the Ni-Al system is quite broad in composition range, the Al content in diffusion aluminides can vary within a wide range with typical aluminides having Al ?30wt %. Higher Al content results in hyperstoichiometric composition. Such aluminides exhibit a bluish tint and are called "blue beta". Lower aluminum content, on the other hand, results in the hypo-stoichiometric composition. Compositions of diffusion coatings based on Cr and Si also may vary over a wide range. These coatings can be applied to components of complex shapes. The coating process involves exposure to high temperature. Additional postcoating heat treatment may be required to restore substrate...
