Simultaneous Chromizing-Aluminizing Coating of Nickel and Nickel-Base Superalloys
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SIMULTANEOUS CHROMIZING-ALUMINIZING COATING OF NICKEL AND NICKEL-BASE SUPERALLOYS V. A. RAVI* AND R. A. RAPP** '*Ravi Lanxide Corporation, 1 Tralee Industrial Park, Newark, DE 19714-6077 **Rapp Department of Materials Science & Engineering, The Ohio State University, Columbus, OH 43210 ABSTRACT Ni and Ni-base alloys have been simultaneously chromized-aluminized by a halide-activated pack cementation coating process. The computer program SOLGASMIX was used to calculate the equilibrium partial pressures of the gaseous species. Chloride-activated packs are predicted to be chromizingaluminizing while fluoride-activated packs are not.
Codeposition of Cr and
Al occurs in packs containing 90-95 w/o Cr in the masteralloy and either NaCl,
CrCl
2
or NH4 C1 as the halide salt activator at 1273K.
INTRODUCTION Sodium sulfate salt films condensed on turbine blades and vanes,
parti-
cularly in marine environments, lead to a severe form of corrosion called low temperature hot corrosion at the turbine operation temperature (600750°C) [1-3]. Coatings containing Cr and Al are effective against oxidation and low temperature hot corrosion. High temperature protective coatings are applied by several methods [4,5] including pack cementation.
Halide-activated pack cementation is an internally generated chemical vapor deposition process carried out in a closed container at elevated temperatures (usually near 1000°C) under an inert or reducing atmosphere. A 'pack' usually consists of the substrate(s) to be coated and powders of an inert filler, a masteralloy rich in the element(s) to be deposited and an activator salt. The activator salt reacts with the masteralloy to generate volatile metallic halides which diffuse through the porous filler material (usually alumina) to the substrate surface. Several possible surface reactions result in the incorporation and subsequent diffusion of the coating element into the substrate. The kinetics of the coating process are controlled by diffusion of the halide vapor in the pack in series with solid state diffusion of Cr and Al into the substrate. The transport of each halide species through the pack is controlled by the difference in partial pressure between the pack and the substrate surface. The surface concentration and coating thickness are controlled by solid state diffusion of the coating element in the substrate. In the following, the thermodynamics of the codeposition process are presented, followed by experimental observations of the coatings on Ni and Ni-base alloys. THERMODYNAMICS Simultaneous chromizing-aluminizing is critically dependent on the choice of the masteralloy and the activator. For unit Al activity, the partial pressures of the Al halides are many order of magnitude higher than those of the Cr halides at unit Cr actiVity [8]. To achieve codeposition, the Al activity has to be lowered by several orders of magnitude to generate comparable Al and Cr halide partial pressures. Mat. Res. Soc. Symp. Proc. Vol. 133.
1989 Materials Research Society
544
Johnson, Komarek and M
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