Anti Oxidation Performance Of Incoloy 20 At Medium Temperature

Alloy 20 (Carpenter 20Cb‑3® / UNS N08020 / W.Nr. 2.4660), often informally called "Incoloy 20" in Asian markets, is not​ a high‑temperature creep alloy like Incoloy 800HT (N08811). It is a niobium‑stabilized, Cu‑ and Mo‑bearing austenitic Fe–Ni–Cr acid‑resistant alloy​ whose primary design intent is resistance to sulfuric / phosphoric acid + chlorides. Nevertheless, its 19–21% chromium content does form a Cr₂O₃ scale, giving it medium‑temperature oxidation resistance​ suitable for process heating, acid‑plant air‑preheat coils, and moderately warm exhaust gas exposure. This article defines those limits precisely and contrasts them with true high‑temp alloys so buyers don't over‑ or under‑specify.

Continuous Oxidation Limit in Air / Oxidizing Combustion Gas

The protective mechanism is a continuous Cr₂O₃ (chromia) scale​ formed on the surface when pO₂ is sufficient.

• Continuous oxidation service (stationary air / lean combustion gas):​ Up to 750–815°C (1380–1500°F)​ with acceptable scale growth (< 0.025–0.05 mm/year metal loss in clean air).

• Short‑term / intermittent exposure:​ Can tolerate excursions to 850°C (1560°F)​ for limited cycles, but repeated thermal cycling promotes spallation of the chromia film — re‑formation occurs on re‑heating but cumulative scale build‑up / loss of section must be allowed for.

• Upper practical limit:​ Above ~900°C continuous, Cr₂O₃ growth accelerates and slight volatilization (especially in the presence of water vapour) begins; Alloy 20 is not recommended for sustained > 815°C oxidative service.

• Comparison anchor points:

  • 304/316 SS: ~750–800°C max continuous (rapid scaling >850°C)

  • Alloy 20 (N08020): ≈ same as or slightly better than 316L​ in clean air due to stabilized grain & NbC dispersion, but no Al/Ti γ′like 800HT

  • Incoloy 800HT: ~870°C continuous, 980–1100°C intermittent (superior)

  • Inconel 600/601: >1038–1095°C continuous (true high‑temp oxidation alloys)

Behaviour in Process / Sulfur‑Bearing / Carburizing Atmospheres

• Slightly reducing / mixed combustion gases (low SO₂, normal O₂):​ Cr₂O₃ still stable → oxidation rate similar to air data above.

• High‑sulfur / low‑O₂ gases (e.g., un‑desulfurized syngas with H₂S > 50–100 ppm @ > 650°C):​ Cr₂O₃ may become unstable if pS₂ exceeds threshold for Cr‑sulfide formation → sulfidation attack possible. Alloy 20 has no special sulfidation resistance​ beyond its Cr content — Inconel 601 (23Cr+Al) or higher‑Cr alloys preferred in severe sulfidizing.

• Carburizing atmospheres (CO/CH₄, furnace atms.):​ Alloy 20 has no engineered carburization resistance​ (no coarse grain + Al/Ti control like 800HT). Carburization rates will be higher than 800HT and similar to/a bit better than 316L — not suitable for radiant tubes or cracking furnace service.

• Steam / humid air:​ Up to ~750°C, Cr₂O₃ remains stable; no unusual oxidation acceleration observed.

Thermal Stability of Microstructure vs. Oxidation

• Solution anneal condition (940–1010°C, rapid quench):​ Single‑phase austenite with fine NbC — scale forms normally on first heat‑up.

• Long holds 650–900°C (non‑pressurized):​

  • σ‑phase (Fe–Cr–Mo) canprecipitate in very long exposures (> thousands of hrs) → slight embrittlement but does not​ significantly alter Cr₂O₃‑scale forming ability if enough Cr remains in solution near surface.

  • NbC remains stable, no Cr‑carbide sensitization in properly annealed stock.

• Recommendation:​ If Alloy 20 sees cyclic 700–800°C air andacid exposure in the same equipment (e.g., heated acid reactor with external skin at 750°C), verify post‑service IGC (Straight Strauss A262‑E) if sensitization is a concern — normally not an issue if solution‑annealed and not welded without PWHT in aggressive acid zones.

Practical Engineering Summary – When Alloy 20 Oxidation Resistance Is Adequate