Metal dusting

Abstract

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...

First-principles investigations of Ni3Al(111) and NiAl(110) surfaces at metal dusting conditions

“…Operation at low S/C is often accompanied by challenges in the development of materials [1]. The present work deals with a high-temperature corrosion phenomena known as metal dusting, which occurs in steam reforming plants operating at a low steam to carbon ratio (S/C) [1,2,3]. Metal dusting is a catastrophic form of carburisation, which attacks metal hardware, often consisting of Ni alloys.…”

“…Metal dusting is a catastrophic form of carburisation, which attacks metal hardware, often consisting of Ni alloys. The metal is supersaturated with carbon, which forms graphite by inwards growth into the metal, and which finally breaks up the metal into particles and graphite [2,3].…”

“…Sn, Cu and Au also protect Ni surfaces by being less reactive towards carbon, and decrease the carbon solubility in the alloy. Selective poisoning of graphite growth centers on the surface is another option and can be done by adding sulphur or alkali metals in the feed gas [1,2]. A disadvantage of gas additives in steam reforming plants is the need for continuous feed and subsequent cleaning of the process gas.…”

“…This gives a more convenient relation to the carbon activity, a key parameter in the field of metal dusting corrosion [2]. In the following, it is demonstrated, how the free energy of adsorbed carbon is calculated, and how the chemical potential of carbon in a gas environment relative to the chemical potential of carbon in graphite influences this.…”

Growth of massive cementite layers; thermodynamic parameters and kinetics

“…This dearth of fundamental knowledge on cementite is mainly due to the difficulty in preparing single-phase (single-or polycrystalline) cementite specimens. In the past, several works dealt with the production of bulk cementite specimens applying different methods [1][2][3][4][5]. Yet, only recently, it was reported for the first time that massive cementite layers can be grown on ferrite substrates by a dedicated gaseous nitrocarburising process [6].…”