Metal dusting Carbon activity

The differences between these redox-polymers and the now much discussed "polymeric metals" is the additional property being conductive in the ion-radical state. This is a feature most welcome in the construction of batteries but as such not a necessary requirement. The possibilities offered by the redox polymers were of course realized many years ago and have been described by Cassidy.Batteries in which one or both electrodes were a redox polymer in nature have been patented.Due to the insulating nature of these early redox polymers the electrodes were prepared from a mixture of the finely divided polymer and a conductor such as graphite, metal dust or activated carbon. Additionally or alternatively a redox electrolyte was added to act as the mediator between the insoluble and insulating redox polymer and the metal electrode. Since the oxidized or reduced form of the redox polymer can be regenerated and the process recycled, the resulting unit is a secondary cell. 

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). 

The metal dusting of pure metals, especially Fe, was studied extensively by Hochman2. The Hochman mechanism for the metal dusting of iron involves three steps. The first step is the formation of metastable iron carbide, FejC, on the surface of iron. This reaction requires carbon activities higher than unity. 

CARBONIC ACID GAS (124-38-9) COj Reacts violently with strong bases and alkali metals. Violent ignition or explosive reaction may occur when dusts of chemically active metals such as aluminum, chromium, lithium, manganese, magnesium, potassium, sodium, titanium, zirconiiun, and some magnesium-aluminum alloys are suspended and heated in carbon dioxide. The presence of strong oxidizers will increase the potential for ignition or explosions of metal dusts. 

Iron-aluminium-chromium alloys should, however, not be applied in conditions where carbon activities above 1 are encountered. In C0-C02-H2 gas atmospheres, which were oversaturated with carbon (ac 1), rapid material wastage by so-called metal dusting was observed [23,24], As long as the carbon activity is below 1, however, excellent resistance of iron-aluminium-chromium alloys to carburisation can be expected even in oxygen deficient atmospheres. 

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. 

Suspensions of oxidizable particles (e.g., flour, coal dust, magnesium powder, zinc dust, carbon powder, and flowers of sulfur) in the air can constitute a powerful explosive mixture. These materials should be used with adequate ventilation and should not be exposed to ignition sources. Some solid materials, when finely divided, are spontaneously combustible if allowed to dry while exposed to air. These materials include zirconium, titanium, Raney nickel, finely divided lead (such as prepared by p5trolysis of lead tartrate), and catalysts such as activated carhon containing active metals and hydrogen. 

As noted before, metal dusting is to be expected if metallic materials are carburised at carbon activities ac > 1, i.e. under a strong driving force for graphite formation. The carbon from the gas molecules should react to graphite (and, in fact, that is the overall reaction which occurs in metal dusting) and destroy the materials. As yet, two different reaction paths have been observed. For iron and Fe-based alloys, the reaction sequence is as follows (see Fig. 1.7) ... 

Metal dusting occurs in strongly carburising atmospheres at carbon activities ac > 1, which condition certainly was present in the furnace feed consisting of hydrocarbons and a few percent of hydrogen. The sulfur content of the... 

High-temperature processes may induce several corrosion phenomena on the metallic substrate. High-temperatme oxidation may occm on the other surfaces of the reactors while this phenomenon may compete with carburization and metal dusting (MD) phenomena in the inner walls of the structured cataljdic devices. The latter phenomenon occurs in high-carbon-activity atmospheres as the ones resulting from reformate streams. 

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