Hangbo Alloy Group
Shanghai · Est. 2012
Metal dusting is catastrophic carburization in carbon-rich gas above 450°C. Learn how high-Cr/Si/Nb nickel alloys resist it in ethylene, syngas, DRI service.
Metal dusting is one of the most destructive forms of high-temperature corrosion a process plant can face. It occurs when a carbon-rich gas — one with a carbon activity greater than one — contacts a metal surface around 450–900°C, driving carbon inward until it precipitates as graphite and brittle metal carbides. The surface then disintegrates into a porous, sooty mass of metal powder, graphite, and carbides that grows until the component fails, sometimes within months. Owners of ethylene furnaces, syngas and methanol trains, and direct-reduced-iron (DRI) units know it as a quiet killer of reformer tubes, outlet manifolds, and heat-exchanger components. As a nickel-alloy manufacturer, Hangbo supplies the high-chromium, silicon- and niobium-bearing grades that form the protective oxide scales able to survive this environment.
The condition is governed by carbon activity, not simply by the presence of carbon. In a gas where carbon activity (aC) exceeds one — as in cracked gas, synthesis gas, or the atmosphere above a DRI shaft — the metal surface is thermodynamically forced to absorb carbon. Carbon diffuses inward, supersaturates the matrix, and precipitates as M7C3, M23C6, and eventually graphite. As graphite nucleates at grain boundaries and interfaces, it pries the metal apart into dust.
Temperature is the gate. Below roughly 450°C the reaction is negligible. Between 450°C and 900°C it accelerates, peaking in the 550–750°C band. Above about 900°C the regime shifts toward conventional oxidation and carburization, but the transition zone remains dangerous. The deciding factor is whether the alloy can hold a continuous, adherent chromium-rich scale that blocks carbon ingress.
Resistance comes from forming a dense, self-healing Cr2O3 scale, reinforced by silicon and aluminium oxides at the scale-metal interface. The practical recipe is high chromium (typically 25–30%), deliberate silicon (1.5–3%), and often niobium to stabilize the matrix and slow carbon transport. Plain austenitic stainless steels and even many standard nickel alloys fall short because their lower Cr and Si budgets cannot sustain the scale under high carbon activity.
The workhorse grades are Inconel 693 (about 29% Cr, 3% Si, ~2% Nb, ~2% Al) and alloy 602CA (25% Cr, 2% Si, 2% Al plus zirconium and titanium). Both were developed specifically for metal-dusting and carburizing environments. Modified HP-type cast alloys with niobium additions also perform well in furnace tubing. Hangbo produces bar, forgings, and tube stock in these grades with full chemistry and high-temperature property documentation.
Metal dusting is rarely a laboratory curiosity; it shows up wherever carbon-rich process gas meets metal at temperature:
Each of these is addressed in more detail in our reformer tube selection guide and refining corrosion guide.
Material choice carries most of the burden, but operating practice reduces risk further. A clean, smooth surface finish and a controlled pre-oxidation (heat-up in oxidizing air) help establish the protective scale before the unit sees process gas. Holding the gas dew point and carbon activity within design limits, avoiding cold spots where condensates accelerate attack, and rotating inspection of high-risk manifolds all extend service life.
When a component is already in a marginal alloy, the practical fix is replacement with a properly specified high-Cr/Si/Nb grade rather than continued repair — welding over dusted metal only delays the inevitable. Our carburization-resistance overview explains how these grades also resist the milder, gradual form of the same mechanism.
Metal dusting is most aggressive between roughly 450°C and 900°C, with a peak around 550–750°C. Below 450°C the reaction is effectively negligible, and above 900°C the failure mode shifts toward oxidation and carburization. The exact onset depends on carbon activity and alloy composition.
The strongest performers are high-chromium grades with deliberate silicon and niobium: Inconel 693 and alloy 602CA. Both are engineered specifically for carbon-rich, high-temperature service and are supplied by Hangbo as bar, forgings, and tubing.
Both involve inward carbon diffusion, but metal dusting is the catastrophic form that occurs when carbon activity exceeds one and the surface disintegrates into metal powder. Conventional carburization is slower and gradual, thickening a carbide zone without destroying the part. See our carburization-resistance guide for the distinction.
Generally no. 300-series austenitic stainless steels lack the chromium, silicon, and niobium budgets needed to hold a protective scale under high carbon activity, and they fail quickly. Resistance requires the specialized high-Cr nickel alloys covered here, as explained in our nickel alloy vs stainless steel guide.
As a dedicated nickel alloy manufacturer serving chemical processing, oil & gas, power generation sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.