Oxidation Resistance of Nimonic 263 in Air to 980°C: Cr₂O₃ Scale Behavior & Limits

Nimonic 263 (UNS N07263 / W.Nr. 2.4650 / China approx. GH263) is a γ′-precipitation hardened Ni–Cr–Co–Mo superalloy​ (nominally Ni–20Cr–20Co–6Mo–0.4Ti–≤0.15Al). It is primarily selected for 650–850°C creep-resistant welded fabrications​ (combustor liners, transition ducts). A prerequisite for such service is adequate protective scale formation in hot gas path air.

This article details the oxidation behavior of Nimonic 263 in static and mildly cyclic air up to 980°C (1800°F), the nature of its Cr₂O₃-based scale, the influence of 6% Mo, the onset of Cr₂O₃ volatilization, and practical service limits versus cobalt-base or higher-Cr nickel alloys.

1. Oxidation Protection Mechanism — Chromia (Cr₂O₃) Scale

Nimonic 263 contains 19–21% Cr​ (typ. 20.0%). Upon exposure to air > 700°C, the alloy forms a continuous, adherent Cr₂O₃ (chromia) scale:

• Formation:​ 2Cr + ³/₂ O₂ → Cr₂O₃

• Protective range:​ Cr₂O₃ is thermodynamically stable (ΔG⁰ negative) in air to ≈ 1090–1120°C (depends on pO₂); below this, it acts as an effective oxygen barrier if dense and continuous.

• Role of 6% Mo:​ Mo is nota primary scale former at 800–980°C in air — Mo tends to oxidize beneath or at edges of Cr₂O₃ if Cr is depleted locally, but Mo improves pitting resistance in condensed aqueous phases​ and contributes minor solid-solution stabilization. It does notsignificantly alter high-T air oxidation rate vs. Cr-only alloys.

• Ti/Al (γ′ formers):​ Remain in the substrate; Al is kept ≤0.15% to minimize η-phase and SAC sensitivity — too low to form a continuous Al₂O₃ (α-Al₂O₃) outer layer (unlike René N5 / CMSX series where Al ≈ 5–6% forces Al₂O₃ scale).

Result: Predominant scale = Cr₂O₃ with trace Co–Ni spinel (CoCr₂O₄ / NiCr₂O₄) islands at longer times / higher T.

2. Quantitative Oxidation / Mass-Gain Data (Typical, Static Air)

Values below are typical for solution-annealed + aged sheet, from producer tests and open literature (cyclic data noted separately):

Parabolic rate constant (approx. 900–980°C):​ kp ≈ (1–5) × 10⁻¹² g²/cm⁴/s — typical of protective Cr₂O₃-forming Ni-base alloys.

3. Cyclic Oxidation (Thermal Shock) Behavior

Test: 980°C × 1 h hold ↔ air cool to RT × 15 min cycles.

→ Nimonic 263 is acceptable for combustor liners experiencing normal engine start/stop cycles​ (dozens–hundreds per campaign). It is not​ the optimal choice for severe continuousthermal cycling > 1000°C ↔ RT repeatedly (select Haynes 188 or coated Inconel 617 for those extremes).

4. Scale Failure Modes & Upper Temperature Limit

• Cr₂O₃ Volatilization (≥ ~975–1000°C):​

  Cr₂O₃(s) + ³/₂ O₂(g) ⇌ CrO₃(g)

  Above ~ 975–1000°C in flowing air, CrO₃ vapor pressure becomes significant → gradual scale thinning. This sets the continuous useful oxidation limit at ≈ 980°C (short excursion to 1050°C allowable).

• Cr Depletion Under Scale:​ Long exposure can form a Cr-depleted zone → if T > 1000°C + O₂, subsurface voids / internal oxidation of Ti/Al may initiate — rarely an issue within recommended T & time.

• Breakaway Oxidation:​ Only if initial Cr₂O₃ fails (mechanical damage, excessive spall in cyclic service, or Cr below ~ 15% due to long-term subsurface depletion + reheating) → base NiO forms underneath → faster attack. Proper design (≥ 0.13–0.25 mm corrosion allowance) avoids in normal campaign life (< 20–30 kh @ ≤ 950°C).

5. Comparison With Related Alloys (Oxidation Context)

6. Practical Design & Maintenance Guidance

• Max. Continuous Oxidizing Air Temp (static/normal cycle):​ 980°C (1800°F)​ — design corrosion allowance 0.13–0.25 mm / 10⁴ h typical for combustor life.

• Short-Term Excursions:​ Up to 1050°C for takeoff / transient overshoot — OK if not sustained.

• Atmosphere Purity:​ In reducing / low-pO₂ + high-pS₂(fuel-rich primary zone with high S-fuel), Cr₂O₃ may be partially replaced by Cr-sulfides → evaluate sulfidation resistance separately (Ni-base less resistant than Co-base here, but acceptable in most aeroderivative combustors).

• Coating:​ For > 950°C gas path T with long dwell or for sea-salt (Type I/II hot corrosion) → consider MCrAlY overlay or aluminide — underlying Nimonic 263 then sees lower metal T.

7. Summary Oxidation Takeaways

• Nimonic 263 forms a protective Cr₂O₃ scale​ in air from ~ 700°C upward; 20% Cr is sufficient for continuous protection to ≈ 980°C (1800°F)​ in static or normally cycled combustion air.

• Above ~ 975–1000°C, CrO₃ volatilization​ gradually thins the scale → not recommended for continuous > 980°C oxidising service.

• Cyclic scale adherence is good (no rare-earth benefit as in Haynes 188, but adequate for aero-engine start/stop cycles).

• Oxidation performance is similar to Inconel 625 / Hastelloy X, slightly below Inconel 617 / Haynes 188 in ultimate T limit or cyclic retention.

• Primary selection driver for Nimonic 263 is 650–850°C creep strength + weldability (SAC-resistant) + acceptable oxidation to 980°C​ — ideal for combustor / transition duct liners.