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This introductory review paper summarizes shortly the research on metal dusting, conducted in the MPI for Iron Research during the last dozen years. Metal dusting is a disintegration of metals and alloys to a dust of graphite and metal particles, occurring in carburizing atmospheres at a C > 1 and caused by the tendency to graphite formation. The cause of destruction is inward growth of graphite planes into the metal phase, or in the case of iron and low alloy steels into cementite formed as an intermediate. The kinetics of metal dusting on iron and steels was elucidated concerning dependencies on time, temperature and partial pressures. High alloy steels and Ni-base alloys are attacked through defects in the oxide scale which leads to pitting and outgrowth of coke protrusions, after initial internal formation of stable carbides M 23 C 6 , M 7 C 3 and MC. A dense oxide layer prevents metal dusting, but formation of a protective Cr-rich scale must be favored by a fine-grain microstructure and/or surface deformation, providing fast diffusion paths for Cr. Additional protection is possible by sulfur from the atmosphere, since sulfur adsorbs on metal surfaces and suppresses carburization. Sulfur also interrupts the metal dusting mechanism on iron and steels, causing slow cementite growth. Under conditions where no sulfur addition is possible, the use of high Cr Nickelbase-alloys is recommended, they are largely protected by an oxide scale and if metal dusting takes place, its rate is much slower than on steels. Definition of metal dustingMetal dusting is a disintegration of metallic materials into a dust of fine metal particles and graphitic carbon. In the case of chromium steels and Ni-base alloys the corrosion product (coke) may also contain carbides and oxides. This corrosion phenomenon occurs in carburizing atmospheres, containing CO and/or hydrocarbons, at carbon activities a C > 1 which means that a tendency for graphite formation prevails (in equilibrium with graphite a C ¼ 1). Susceptible are metals and alloys which dissolve carbon, i.e. Fe, Ni and Co and their alloys. The carbon is transferred from the atmosphere and dissolved into the metal phase, at a C > 1 to oversaturation, leading to growth of graphite which destroys the materials. In the case of iron and steels, cementite is formed as an intermediate into which the graphite grows [1 -5]. Nickel and Ni-base alloys are disintegrated by direct inward growth of graphite [6 -9].Fe and Ni and low alloyed materials are attacked uniformly. In the most critical temperature range 400 -800 8C mostly chromia forming steels and Ni-based alloys are used, on these materials metal dusting starts locally, where the oxide scale fails, and leads to pitting and hole formation [3, 8 -12], see Fig. 1a. The corrosion product "coke" is growing out from the pits and holes, and mostly carried away in the fast flowing process gases, but observed in laboratory studies as outgrowing protrusions (Fig. 1b). Typical for Cr-steels and alloys is the zone with internal carbides (Fig. 1c...

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Metal dusting, the disintegration of metallic materials into fine metal particles and graphite was studied on nicke1,Fe-Ni alloys and commercial Ni-base alloys in CO-Hz-H20 mixtures at temperatures between 450-750"C.At carbon activities a, > 1 all metals can be destroyed into which carbon ingress is possible, high nickel alloys directly by graphite growth into and in the material, steels via the intermediate formation of instable carbide M,C. Protection is possible only by preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of the alloy and by a surface treatment which generates fast diffusion paths for the supply of chromium to the surface.The metal dusting behaviour 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°C and 750°C.Metal dusting, der Zerfall metallischer Materialien in feinste metallische Partikel und Graphit, wurde fur Nickel, Eisen-Nickel-Legierungen und kommerzielle Nickel-Basis-Legierungen in CO-Hz-H20-Gemischen bei Temperaturen zwischen 450-750°C untersucht. Bei Kohlenstoffaktivitaten a, > 1 konnen alle Metalle, in die Kohlenstoff gelost eindringen kann, durch metal dusting zerstort werden, hoch nickelhaltige Legierungen direkt durch Graphitwachstum in das Material und in dem Material, Stahle iiber die Bildung eines instabilen Karbides M,C als Zwischenprodukt. Schutz ist moglich durch Verhinderung des Eindringens von Kohlenstoff. Die Bildung von Chromoxid ist die beste Moglichkeit des Schutzes, dieses wird begiinstigt durch hohe Chromkonzentrationen in den Legierungen und durch Oberflachenbehandlungen, die schnelle Diffusionswege fur die Anlieferung von Chrom an die Oberflache erzeugen. Das metal dusting-Verhalten von Alloy 600 wird im Detail beschrieben. Weiterhin wurde durch Auslagerungen bei 650°C und 750°C fur verschiedene kommerzielle Nickel-Basis-Legierungen eine Rangfolge beziiglich des Widerstands gegen metal dusting erhalten.