The formation of soot on a highly alloyed steel retort has led to metal dusting over time. Via industrialheating.com
Metal dusting is a type of corrosion that occurs when the material (most commonly iron, nickel, or cobalt) is exposed to an environment with high carbon activities. When that happens, the bulk metal breaks up to form metal powder.
While the corrosion mechanism is not completely known, it is thought that a graphite layer first deposits onto the surface of the metal from the carbon-rich environment (typically from carbon monoxide). Then, the graphite layer most likely forms the metastable M3C species (M is the metal. Finally, that species migrates away from the metal surface. This mechanism is usually seen in iron and its alloys. This type of metal dusting is typically divided into four regimes over the temperature range it occurs in. In regime I, the rate of corrosion increases gradually with temperature while in regime II, the rate of corrosion rises rapidly to a peak. Regime III begins at the peak, where the rate of corrosion falls rapidly with temperature and is followed by regime IV where the rate of corrosion is relatively constant. Regimes I-III are associated with the formation of the metastable M3C while regime IV is associated with the direct migration of metal into the graphite layer and no metastable M3C forms. However, in some cases, there are no M3C species, meaning that the metal atoms transfer directly into the graphite layer. This case is generally observed for nickel, cobalt, and their alloys.
Metal dusting occurs under high temperatures (300-850 °C) most likely because at lower temperatures, the rate of formation of the M3C species is too low to form a significant amount. On the other hand, at higher temperatures, the graphite layer is too unstable for carbon monoxide deposition to occur in significant amounts.
However, there is hope in the form of several methods to prevent or reduce the amount of metal dusting. Most commonly, aluminide coatings, alloying with copper, and the addition of steam are used. Sulfur can also be used as an additive in the alloy or even present in the environment as gaseous hydrogen sulfide can inhibit or prevent metal dusting because it suppresses the nucleation of graphite. However, this cannot be used in nickel alloys because a low-melting-point eutectic forms.
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