Metal dusting corrosion of metals and alloys

T A RAM AN ARAYAN AN, Princeton University, USA andC M C H U N, Exxon Mobil, USA 

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

Following this reaction, carbon deposits on the metastable Fe3C by any number of reactions, such as for example  

The immediate consequence of the carbon deposition reaction is that the thermodynamic activity of carbon at the interface between Fc3C and deposited carbon is reduced to unity. Since iron carbide cannot exist thermodynamically at unit carbon activity, it starts to dissociate by the reaction 

Reaction [5.4] generates iron powder or dust - thus the above scheme explains how bulk metal is converted to metallic powder. A pictorial representation of the Hochman mechanism has been given by Grabke.  

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T. A. Ramanarayanan and C. M. Chun, Chapter 6 Metal Dusting Corrosion of Metals and Alloys, New Development in High Temperature Corrosion and Protection of Materials, Ed. W. Gao and Z. Li, Woodhead Publishing Ltd., Cambridge, UK p.80-116 (2008). 

Ramanarayanan TA, Chun CM. Metal dusting corrosion of metals and alloys. In Gao W, Li Z, editors. Developments in high temperature corrosion and protection of materials. Cambridge - UK Woodhead Publishing 2008. p. 80-116 [chapter 5]. 

C. M. Chun, G. Bhargava and T. A. Ramanarayanan, Metal Dusting Corrosion of Nickel Based Alloys,/. Electrochem. Soc., 154 C231-C240(2007). 

The two corrosion phenomena caused by carbonaceous gases, carburisation and metal dusting, are both a consequence of carbon transfer into the metal matrix of steels and alloys. In hydrocarbons, the carbon is transferred by the reaction... 

Metal dusting corrosion of Inconel 600 at 750°C for 160 hours showing localized corrosive attack, regions protected by surface oxide, metal dust within carbon deposit, graphite intrusion into alloy, and M3C2 carbide precipitates in the alloy subsurface region (adapted from ref. 16). 

Metal dusting corrosion of 304 SS at 550°C for 160 hours showing filamentous carbon bundles on the surface, regions of localized corrosion filled with carbon deposit, regions protected by Cr-rich surface oxide, and M7C3 carbides in the alloy subsurface (adapted from ref. 16). 

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). 

E. Pippel, J. Woltersdorf and R. Schneider, Micromechanisms of Metal Dusting on Fe-base and Ni-base Alloys, Materials and Corrosion, 49 309-316 (1998). 

J. Klower, H. J. Grabke and E. M. Muller-Lorenz, Metal Dusting of Ni-based Alloys, Materials and Corrosion, 49 328-329 (1998). 

S. Strauss, R. Krajak and H. J. Grabke, Coking by Metal Dusting of Ni-base Alloys, Materials and Corrosion, 50 622-627 (1999). 

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