Carburisation and Metal Dusting in Reformer Piping: Mechanism and Material Selection
Metal dusting is a catastrophic high-temperature corrosion mechanism that converts solid metal into a powdery mixture of metal, carbides, and carbon — the pipe wall literally disintegrates. It occurs in CO/H₂ process gas streams at 450–750°C and has caused catastrophic failures in steam reformer transfer lines, synthesis gas coolers, and methanol plant piping. Understanding the mechanism is essential for correct material selection in these services.
Carburisation vs Metal Dusting — Two Related Phenomena
Carburisation is the uptake of carbon from a carbonaceous gas environment into the metal lattice, forming internal carbides. It occurs across a wide temperature range (500–1100°C) and causes gradual embrittlement and loss of ductility — the fitting becomes brittle but retains its shape until it eventually cracks under thermal stress.
Metal dusting is a more severe and faster process. It occurs specifically in the temperature range 450–750°C (the "metal dusting window") when carbon activity in the gas exceeds 1.0 (supersaturated in carbon). The metastable carbide M₃C (cementite) that forms decomposes into a mixture of graphite, metal particles, and carbide dust — the surface spalls progressively and the fitting wall thins rapidly. Metal dusting pits can penetrate a full pipe wall thickness in months.
Process Conditions That Cause Metal Dusting
Metal dusting occurs where carbon activity (a_C) exceeds 1.0 in a CO/H₂/CO₂/H₂O gas mixture. The Boudouard equilibrium (2CO → C + CO₂) and the water-gas shift equilibrium (CO + H₂ → C + H₂O) define the carbon activity. In steam reformer systems, metal dusting risk is highest in:
- Reformer outlet transfer lines at 450–700°C — the gas has just left the reformer tubes at high CO/H₂
- Waste heat boiler and synthesis gas cooler inlet zones — gas cools through the metal dusting window
- Methanol and ammonia synthesis loop piping — CO-rich recycle streams at intermediate temperatures
- Iron and steel DRI (Direct Reduction Iron) process gas headers
Material Resistance to Metal Dusting
Aluminium-forming alloys (those that form an Al₂O₃ surface scale) have the best metal dusting resistance because the Al₂O₃ layer is a carbon diffusion barrier — carbon cannot penetrate through it. This is the key advantage of Inconel 601 (1.0–1.7% Al) and Incoloy 800HT (controlled Al+Ti) over standard austenitic stainless steels in reformer applications.
| Material | Metal Dusting Resistance | Mechanism |
|---|---|---|
| Carbon Steel WPB | None | No protective oxide — rapid destruction |
| 316L / 321 SS | Poor | Cr₂O₃ layer not a carbon barrier |
| Incoloy 800H | Moderate | Al+Ti oxide — partial barrier; still attacked in severe conditions |
| Inconel 601 | Good | Al₂O₃ layer — strong carbon diffusion barrier |
| Inconel 617 | Good | Al₂O₃ + Co strengthening — best balance of creep and dusting resistance |
| Alloy 602CA (2.4633) | Excellent | High Al (1.8–2.4%) — specifically developed for metal dusting |
Sulphur as a Temporary Inhibitor
Small additions of H₂S to the process gas (as low as 10 ppm) effectively suppress metal dusting — sulphur poisons the catalytic decomposition of CO on the metal surface. This is why steam reformers that process desulphurised natural gas (H₂S removed upstream) are more vulnerable to metal dusting than those processing sulphur-containing feedstocks. However, sulphur inhibition is not a design solution — feed composition changes, and the protection disappears when S drops below the inhibitory threshold.
Design and Material Selection Guidance
- For reformer outlet transfer lines: Incoloy 800H minimum; Inconel 601 preferred for severe duty
- For synthesis gas cooler inlet zone (first pass — highest metal dusting risk): Inconel 617 or Alloy 602CA
- Avoid standard austenitic SS (316L, 321) in any CO-rich service above 450°C — they will exhibit carburisation even if metal dusting does not occur
- Coating (aluminising) of 800H fittings can extend life by providing an Al₂O₃ layer on an otherwise Cr₂O₃-forming alloy — widely used in reformer tube exit manifolds
- Design piping to avoid dead-legs and stagnant zones — stagnant gas increases carbon activity locally