Metal Dusting
Metal dusting is a particularly aggressive form of high-temperature (mostly) CO corrosion, which has received particular attention in connection with the manufacture of СО-rich synthesis gases using steam reforming or catalytic partial oxidation.
Although certain details of the mechanism of metal dusting, and particularly the potential for countermeasures, remain the subject of some debate, the essentials have been understood for many years (Hochmann 1972; Grabke, Brach-Troconis, and Miiller-Lorenz 1994). Under gasification conditions, the Boudouard reaction (2-4) oxidizes carbon with C02 to form carbon monoxide. However, during syngas cooling a point is reached where the equilibrium favors the reverse reaction. At sites on the surface of materials containing particularly iron or nickel, the reverse Boudouard reaction is catalyzed. The carbon thus formed destroys the matrix, and the surface metal is lost as a “dust” into the gas phase. The temperature range in which metal dusting can occur is from a lower limit of about 450°C, where the reaction kinetics cause the corrosion to slow down to a rate that may be considered acceptable, up to the Boudouard equilibrium temperature of the syngas (Hohmann 1996). A typical sign of metal dusting during process operation is the appearance of the products of corrosion as a “magnetic soot” in the process condensate.
Metal dusting is unusual in gasification plants, because sulfur is an excellent inhibitor for this type of corrosion (Gommans and Huurdeman 1994) and most gasifier feeds contain some sulfur. Where a desulfurized natural gas is used to manufacture syngas for Fischer-Tropsch products, however, the potential for metal dusting is very high, since the CO partial pressures will tend to be considerably higher than in a steam reformer plant. Furthermore, the corrosion product, which is very fine and has a large surface area, provides an ideal catalyst for the very exothermic methanation of the CO in the syngas.
There is still a consensus that although careful choice of metallurgy can slow down the effects of metal dusting, there is no real metallurgical solution. The best way to avoid this form of corrosion is to avoid gas-metal contact in the endangered temperature range. For a reformer tube outlet, Hohmann (1996) determined the area of the material at risk and introduced a purge gas at this location with success.
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