Metal dusting mechanisms

Z. Zeng and K. Natesan, Relationship of Carbon Crystallization to the Metal-dusting Mechanism of Nickel, Chem. Mater., 15 872-878 (2003). 

Zeng Z, Natesan K, Relationship of carbon crystallization to the metal-dusting mechanism of nickel Chem. Mater., 2003 15 872-878... 

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. 

H. J. Grabke, Thermodynamics, Mechanisms and Kinetics of Metal Dusting, Materials and Corrosion, 49 303-308 (1998). 

J. Zhang and D. J. Young, Kinetics and Mechanisms of Nickel Metal Dusting 1. Kinetics and Morphology, Corrosion Science, 49 1496-1512(2007). 

Dry Powder A tenn applied to the extinguishing agent suitable for use on combustible metals. Dusts Solid particles generated by mechanical processes such as crushing and grinding, without any chemical change from the paren material. Their size range is typically between 0.1 and 100 microns. 

As the name implies, the affected material disintegrates into fine metal and metal oxide particles mixed with carbon. Depending on the defects in a protective oxide film on the metal surface and the ability of the material to sustain this film, an induction period may be observed until metal dusting manifests itself as pitting or general attack. A possible mechanism was proposed by Grabke [1250] and Hochmann [1251]. 

Figure 9.3 Mechanism of metal dusting (a) oversaturation of the metal (b) formation of surface carbides (c) graphite deposition on the cementite layer (d) graphite precipitation.

This reaction is in particular fast for low alloy steals. For high alloy steals, a protective layer of chromium oxide. However, even in this case, small defects in the metal surface can lead to metal dusting of the metal. Another protective measure, which helps to reduce metal dusting, is the presence of sulfur compounds such as H2S, dimethylsulfide (DMS), or dimehyldisulfide (DMDS) (55). These sulfur compounds intervene in the mechanism of metal dusting and suppress some of the steps in this mechanism. 

Mechanism of metal dusting on iron and low-alloy steels [9-15]. 

Adsorbed sulfur effectively hinders step (iii), the nucleation of graphite, and in this way interferes with the mechanism. The adsorbed sulfur stabilizes the cementite and one can grow thick layers of cementite on iron and steels [14], The continuous presence of only a few ppm H2S in the atmosphere is an effective remedy against metal dusting of steels [14] (see Fig. 1.8). The data on the effect of sulfur on metal dusting can be readily combined with the data on its effect on carburisation [35-37]. 

Low alloy steels which are not protected by a dense oxide layer may be attached by metal dusting. Carbon which is formed via the whisker mechanism reacts with alloy components to carbides resulting in a disintegration of the alloy (Figure 5.13) into dust, which is blown away in the gas flow. 

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