Iron-based alloys metal dusting

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 thickness of the enamel layer varies with the type of use and metal. However, typical thicknesses are from 75 to 150 Jim (3 to 6 mils) for sheet steel, 175 to 359 im (7 to 14 mils) for hot-rolled steel, 100 to 125 pm (4 to 5 mils) for each coat of wet process cast iron (760 to 788°C fire), 15 to 25 pm (0.5 to 1 mil) for dry process cast-iron base coats and 750 pm (average) to nearly 2250 pm (30 mils to 90 mils) for the dusted cover coats (898 to 955°C fires), and 25 to 50 pm (1 to 2 mils) for aluminum alloys. Stainless steel and copper may have enamel coatings from 40 pm to 175 pm (1.5 to 7 mils) thick. 

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) ... 

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