METAL DUSTING OF NICKEL-BASE ALLOYS

Metal dusting, the disintegration of metallic materials into fine meta l particles and graphite was studied on nickel, Fe-Ni alloys and comme rcial Ni-base alloys in CO-H-2-H2O mixtures at temperatures between 45 0-750 degrees C. At carbon activities a(c) > 1 all metals can be destr oyed into which carbon ingress is possible, high nickel alloys directl y by graphite growth into and in the material, steels via the intermed iate formation of instable carbide M(3)C Protection is possible only b y preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of th e alloy and by a surface treatment which generates fast diffusion path s for the supply of chromium to the surface. The metal dusting behavio ur of Alloy 600 is described in detail. A ranking of the metal dusting resistance of different commercial nickel-base alloys was obtained by exposures at 650 degrees C and 750 degrees C.