Metastable carbides

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

In the systems Co-C and Ni-C and in the other transition metal-carbon systems not mentioned so far, no stable carbide phases are observed. The carbon solnbilities in the metals are of importance for the fabrication and properties of hardmetals (see Section 9.1.1). The phase diagrams are of the entectic type. Metastable carbide phases have been reported in rapidly qnenched Co-C and Ni-C alloys. 

Fernandez Guillermet and Grimvall have also written a series of papers [11-17] devoted to deriving enthalpies of formation and standard entropies for a range of metastable carbide and nitride phases of the 3d, 4d and 5d transition metals. Thermodynamic values were obtained from an analysis of the cohesive properties and vibmtional entropies of the compounds concerned, whereby a quantity with the dimensions of energy, E, was defined as... 

Iron, nickel, cobalt, and their alloys are the most studied metals for the catalytic growth of CNFs or CNTs. The readiness of these metals to produce metal-carbon solid solutions and to form metastable carbides in the appropriate reaction temperature range should be an important factor to take into account for the comprehension of their reactivity. The different carbon species formed depending on the temperature range employed in the steam reforming of hydrocarbons on nickel catalysts have been discussed [29] and consist of ... 

Metal dusting is another carbon interaction that is harmful. In this reaction, the carbon activity in the gas must be greater than unity, and it appears that the carbon from the atmosphere gas species dissolves into the metal faster than it can nucleate as soot on the metal surface. This produces high carbon activities in the metal and, for the case of iron-base alloys, leads to the growth of metastable carbides, which subsequently decompose to a powdery product. These reactions occur typically in the temperature range 450-800 °C. 

The C-Fe phase diagram is mainly accepted fi-om [2006MSIT]. The properties of metastable carbides such as Fe2,2C (Haegg carbide) and Fe2,4C (known as e carbide ) are well discussed by [1972Chi] who proposed also simple expressions for carbon activities in austenite and thermodynamic properties of carbides, which may be usetul because they are in good agreement with more recent and more sophisticated relationships. 

Guillermet and Grimvall (1990) came to a similar conclusion from analysis of the thermodynamic parameters of metastable carbides. 

Some metastable carbides such as MeC were identified, but their occurrence was only confirmed for materials annealed for very long periods ( 60 h). The identification of hyperfine molybdenum carbide-nitrite precipitation (< 0.5 nm) was performed using the APFIM technique. 

Metal dusting of Ni-based alloys starts from carbon absorption, dissolution and further supersaturation on the metal smface. Unlike Fe-based alloys, there is no formation of metastable carbide as an intermediate. The subsequent graphite deposition is generally accepted as the process which produces nanoscale particles for further carbon deposition - forming metal dusting. 

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