Pack Cementation Aluminide Coatings on Superalloys: Codeposition of Cr and Reactive Elements

Abstract

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 lement 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 inthe coating. Isolation of the substrate from the pack introduced partial rate control by gaseous diffusion. Alumin ide diffusion coatings commonly are applied by the. pack cementat ion method to Ni-base superalloy tur-bine components to improve their oxidation resistance. However, a luminide coatings lack adequate resistance to hot corrosion (fused salt attack, e.g., by Na2SO4) and to thermal stresses generated during gas turbine service.