ABSTRACT

The carburization behavior of nine commercial nickel-base alloys and four iron-nickel-chromiun alloys was investigated at 650 ºC in a carbonizing H2-CO-H2O - gas with a carbon activity of ac >> 1. The iron-nickel-chromium alloys suffered severe metal dusting after a very short teat period. Nickel base alloys were generally less susceptible to metal dusting than iron-base alloys. However, their corrosion behavior was found to depend sensitively on the chromium concentration of the respective alloys. Alloys like alloy 600H, with a chromium concentration of only 16 %, suffered wastage rates which were similar to those of the more resistant iron-base alloys. Nickel-base alloys with chromium concentrations of 25 % and above, on the other hand, showed no significant evidence of metal dusting even after 10000 hours of exposure. It was found that these alloys are protected against metal dusting by the formation of a dense, self-healing chromia scale, which prevents the penetration of carbon into the base metal.

INTRODUCTION

Severe material failures caused by so-called ‘metal dusting’ have been reported during recent years. The reason for these failures were strongly carburizing CO-H2 gas mixtures such as encountered in chemical plants for the synthesis of hydrocarbons, methanol, ammonia etc. as well as in plants for the reduction of iron ores.

Metal dusting is a rapid material wastage, which can occur under any process condition where the carbon activity of the gas atmosphere ac becomes greater than one1-8. It is characterized by the disintegration of iron-, nickel- or cobalt-base materials to a dust which contains metal particles, carbon and sometimes also carbides and oxides. The mechanisms of metal dusting have been elucidated in the recent years for both iron-base alloys 2,3,4,9 and nickel-base alloys 10. Exposed to the supersaturated gas, materials may dissolve high amounts of carbon. If graphite formation starts and deposition of graphite on the metal surface occurs, the carbon activity at the metal surface drops to ac = 1 and the metal matrix, which now becomes supersaturated with carbon, decomposes. In the case of iron-base alloys, metastable carbides (Fe3C) are formed as an intermediate phase which decomposes to graphite and metal particles. In nickel-base alloys, where metastable carbides do not exist, it is the growth of graphite in (and into) the supersaturated solid solution which destroys the metal10,11. A dust of graphite and metal particles is formed as a reaction product with the metal particles serving as catalysts for further carbon deposition. Once started, the reaction can produce vast amounts of ‘coke’.

Materials can be protected against metal dusting by adding sulfur-containing compounds (often H2S) to the process gas. It is widely known that sulfur “poisons” the metal surfaces and prevents both carbon ingress and graphite nucleation12. However, in many catalytic processes sulfur-bearing substances cannot be injected since they deactivate the catalysts. To prevent material wastage under such conditions, an appropriate material with high resistance to metal dusting has to be selected.

Iron-base alloys are generally susceptible to metal dusting because of their tendency to form metastable iron carbides.

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