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.

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Microstructural and microchemical investigations were carried out on nickel and Inconel 600 after exposure to strongly carburizing atmospheres at temperatures of about 600 to 650°C to study their metal dusting behaviour. Contrary to iron and low‐alloy steels, where metal dusting proceeds via the formation and disintegration of a metastable carbide M3C, both nickel and Inconel 600 directly disintegrate. Inside the metal this disintegration proceeds by formation of thin graphite filaments of nearly 10 nm in diameter, the atomic basal planes of which are oriented perpendicular to the surface thus effecting a high reactivity at the growth front. Subsequently, fine particles or larger parts of the metal surface layer are released, which are buried in the deposited graphite layer on the surface. In case of Inconel 600, containing Cr with mass contents of about 15%, the disintegration can be delayed by the formation of a chromium oxide layer, but no safe protection against metal dusting is obtained.

Metal dusting of Fe‐Ni‐Cr alloys has been observed in industrial processes in strongly carburizing atmospheres at temperatures from 450°C to 800°C. At temperatures below 650°C the alloys are generally not able to form dense, well adherent oxide layers in spite of relatively high Cr‐contents, therefore, metal dusting can take place. Already a lot of experimental work has been done to elucidate the mechanism and to compare the resistance against metal dusting for high alloy steels [1]. The intention of this study was to obtain additional information concerning the role of alloying elements and the effects of carbide precipitates in austenitic high alloy steels such as Alloy 800. The susceptibility to metal dusting was determined by measuring the metal loss under metal dusting conditions of Fe‐20%Cr‐32%Ni alloys modified with additions of different carbide formers (W, Mo, Nb) or oxide formers (Si, Al). The samples were exposed at 600°C in a CO‐H2‐H2O‐gas mixture for repeated periods up to 500 – 1500 h. The attack by the oxidizing and carburizing atmosphere leads to the precipitation of internal carbides and metal dusting and more or less to formation of an oxide layer. In comparison to the undoped material, the addition of carbide formers retards the initiation of metal dusting attack. The additions of Si and Al seem to prevent metal dusting under the given laboratory conditions. When carbides are present at the metal surface, they affect the initial oxide growth and have a negative effect on the protectivity of scales. Very striking is the effect of Ce, this rare earth element is generally known to favour Cr‐oxide formation and to improve the adherence of the oxide layer [2], but in the case of metal dusting it clearly enhances metal dusting and metal wastage.

In strongly carburizing atmospheres iron and steels are subject to 'metal dusting', a catastrophic carburization, leading to disintegration into a dust of carbon and metal particles. TEM investigations of iron foils in different states of this corrosion process have clearly shown now that unstable cementite is an intermediate in the reaction. The carbide is formed at the iron surface after supersaturation (ac > 1), its disintegration is started by graphite deposition on its surface (ac = 1). The carbon atoms from Fe3C disintegration are attached to the graphite planes which are vertically oriented to the cementite surface, the iron atoms diffuse through the graphite and agglomerate to small particles (~20 nm) which act as catalysts for further carbon deposition from the atmosphere, till they are densely covered with graphite. The TEM investigations are in good agreement with preceding studies concerning kinetics and mechanisms of metal dusting.