Metal subsurface

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. 

Figure 15 Layer-resolved density of states (DOS) for hydrogen-covered metal slabs of palladium and rhodium. Top DOS on the hydrogen layer middle on the metal surface layer bottom on the metal subsurface layer. (From Ref. 82.)...

Fig. 4. a)TEM micrograph of the metal/oxide interface and metal subsurface layer ofTi36Al after oxidation at 900 °C in air for 0.5 h. b) SAD pattern of the new cubic phase. 

Fig. 5. BSE image (a) and element distribution (b oxygen, c aluminium, d titanium) of the oxide scale and metal subsurface layer of Ti36Al after oxidation at 900°C in air for 4 h.

In Fig. 12 a TEM micrograph and a schematic illustration of the oxide scale and of the metal subsurface zone ofTi35AI5Nb after 4 h of oxidation at 900°C is shown. Beneath an outer oxide mixture of A1203 and TiOz a layer of coarse-grained Ti02 is observed. 

In contradiction to the binary phase diagram of TiAl the depletion layer in the metal subsurface zone after a 0.5 h oxidation at 900°C consists of a phase with a composition between 7-TiAl und arTi3Al. The analysis of this phase by electron diffraction reveals that this phase is equal to the recently reported new cubic phase (NCP) which is similar in compositions toTi3Al2,Ti2Al orTi5Al302 [17,18,19,20]. 

The striking features after oxidation of Ti35A15Nb for 4h at 900°C are the slight enrichment of aluminium in the metal subsurface zone instead of aluminium depletion, the preferred formation of AlON in wide parts of the metal/oxide interface and the development of a rather dense, coarse-grained partial layer consisting of titania in the oxide scale. Several reasons for the beneficial effect of niobium addition on the oxidation behaviour are discussed in the literature [5,10,11]. Beside the influence of niobium on the aTl/aAI ratio and expansion of the 7-TiAl phase field the effect of doping of titania by niobium is often discussed. By doping of titania with niobium the concentra-... 

In the niobium containing alloy which shows a better oxidation resistance the doping of titania with niobium may reduce the dissolution of AlON. By this means a thin layer AlON is formed at the interface leading to a reduced oxidation rate. Thus it is assumed that the oxidation behaviour of titanium aluminides could be improved by stabilizing the aluminium oxide at the metal/oxide interface either by prevention of aluminium depletion of the metal subsurface zone or by reduction of A1203 dissolution in Ti02. 

Meanshile, surface segregation was found on very many hydride-froming intermetallic compounds (Jacob and Polak, 1981 Schlapbach, 1982 Smith and Wallace, 1986), On most compounds it already occurs at room temperature. The compound FeTi is an exception in the sense that it has to be activated at 700 K for H absorption. Indeed, surface segregation is very weak at room temperature and becomes strong above 600 K. In addition to TiO and Fe other near surface species can be formed according to temperature and partial pressure of oxygen and H. The reaction H 2H can proceed on the near-surface precipitates of Fe or on the metallic subsurface of FeTi (Schlapbach and Riesterer, 1983 Khatamian and Manchester, 1985). Pederson et al. (1983) conclude from volumetric adsorption measurements that at 80 K dissociation occurs on a non-oxidized Ti surface. 

Figure 2-14. Silicon and chromium concentration in the metal subsurface zone of the 9Cr-steel P91 after oxidation at 650 °C as a function of oxidation time (Vossen et al., 1997).

Formation of local physical defects in the scale (microcracks, cracks, pores, channels, etc.) which allow carbon transfer from the carburizing environment to the metal/oxide interface and dissolution in the metal subsurface zone (Fig. 2-36 a). 

Figure 2-48. Oxide scale and metal subsurface zone of Alloy 800H after 200 h in air +2% Clj at 800 "C (Glaser, 1994).

Schematic of alloy depletion by the corrosion process (in this example by oxidation, i.e., chromium-rich surface oxide scale formation). The dark areas are those of high (original) Cr content of the alloy. By Cr consumption due to scale growth also chromium carbides in the metal subsurface zone become dissolved (cross section of the surface area).

In particular, for alloys consisting of several alloying elements, internal corrosion is also possible, which means that instead of forming a continuous surface layer there are discrete corrosion product particles surrounded by metal in the metal subsurface zone, which result from the corrosion process. In particular, this situation is encountered if a protective surface scale shows cracks or other types of channels for direct gaseous transport through the scale to the metal subsurface zone or if no surface scale exists at all acting as a barrier to keep the reactive species away from the metal. Internal corrosion... 

Metal dusting is a special form of catastrophic corrosion that takes place in carbon-containing environments where the carbon activity exceeds a value of 1 [26,27]. For Fe-based materials the principles are shown schematically in Figure 13.17. The catalytic effect of the metal or cement-ite surface leads to a dissociation of the carbon-containing gaseous species (CO , C ,Hy) with the effect of carbon uptake into the metal subsurface zone and carbon deposition (amorphous coke) on the surface. This mechanism is evidently influenced by the crystallographic situation of the metal grains on the material surface [28]. For carbon monoxide (CO) adsorbed on metal... 

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