Metal Dusting

Metal dusting is a catastrophic form of carburization that can result in rapid metal wastage in both ferritic and austenitic alloys. This damage mechanism typically has the appearance of localized pitting, or grooving, along the inner walls of pipe and tubes.

Environments with high carbon activity (greater than 1) and low oxygen partial pressures can be prone to this type of damage if temperatures become high enough for carbon diffusion to occur in the base metal. Depending on the type of alloy, this temperature may be as low as 800°F (427°C) and as high as 1400°F (760°C). In iron-based alloys, this mechanism initiates with the saturation of the alloy matrix with carbon, usually in a very localized manner, followed by the formation of metastable Fe3C, or cementite. The cementite will decompose, as the carbon activity increases and approaches unity, to form iron particles and powdery carbon. With nickel-based alloys, there is no intermediate formation of a metastable carbide. Instead, carbon will diffuse into the matrix material and then decomposes into graphite and metal particles.

Common process streams where metal dusting has been known to occur include; methanol production, where the production of a synthetic hydrogen gas results in ideal conditions for this to occur; hydroforming units, where the 9% Cr material used in many of the fired heaters have been found with this type of damage and waste heat boilers and where high metal temperatures and high activity of carbon lead to initiation of this damage.

Protection of an alloy against metal dusting requires the presence of an adherent, protective, self-healing oxidation layer on the surface of the material. In general, nickel-based alloys perform much better in a metal dusting environment than do iron based alloys. Alloy 800H is one of the most susceptible austenitic alloys to this mechanism, with a fast initiation rate and high wastage rates. Similarly, alloys with 20% – 40% nickel are also strongly attacked.