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Incoloy 800H Oxidation Resistance
Incoloy 800H (UNS N08810) holds useful oxidation resistance to 982 deg C (1800 deg F) in still air and to approximately 1093 deg C (2000 deg F) for short-term excursions. The protective scale is chromium oxide (Cr2O3) supported by a thin sublayer of aluminium oxide (Al2O3) and silicon oxide (SiO2) from the residual Al and Si content. The 19 to 23 percent chromium window is what sets the Cr2O3 film stability against isothermal mass loss, and the 30 to 35 percent nickel content is what suppresses internal oxidation by raising the Cr activity in the matrix near the scale-metal interface. Continuous isothermal exposure mass loss at 982 deg C is approximately 0.5 mg per cm2 per 1000 hours after the parabolic-rate stable regime is established. Cyclic exposure with full cool-down to ambient lifts the mass loss by approximately 3 to 5 times due to differential thermal contraction stresses that crack the Cr2O3 film and expose fresh metal to oxidation. This page documents the isothermal and cyclic mass loss curves, the oxide spalling threshold and the comparison to 310H, 800 and 800HT.
Isothermal Mass Loss in Still Air (Incoloy 800H)
| Temperature | Mass loss after 1,000 hr (mg/cm2) | Mass loss after 10,000 hr (mg/cm2) | Penetration depth (mm) | Service classification |
|---|---|---|---|---|
| 650 deg C (1200 deg F) | < 0.05 | < 0.5 | < 0.005 | Excellent, negligible |
| 815 deg C (1500 deg F) | ~ 0.2 | ~ 2.5 | ~ 0.03 | Excellent, design factor allowance 0.5 mm/40 years |
| 871 deg C (1600 deg F) | ~ 0.3 | ~ 4.0 | ~ 0.04 | Good, ASME Section VIII cap exceeded for 800H |
| 927 deg C (1700 deg F) | ~ 0.4 | ~ 6.0 | ~ 0.07 | Good, design path is 800HT |
| 982 deg C (1800 deg F) | ~ 0.5 | ~ 8.0 | ~ 0.10 | Acceptable, published service ceiling for 800HT |
| 1093 deg C (2000 deg F) | ~ 1.5 | ~ 25 | ~ 0.30 | Short-term excursion only |
Indicative isothermal mass loss in still air per the Special Metals technical bulletin. Service environment (water vapour, sulphur, alkali contaminants) shifts the curves.
Cyclic Oxidation Mass Loss (15-minute heat + 5-minute cool cycles)
| Temperature | Isothermal 1,000 hr (mg/cm2) | Cyclic 1,000 hr (mg/cm2) | Cyclic-to-isothermal ratio |
|---|---|---|---|
| 815 deg C | ~ 0.2 | ~ 0.6 | 3.0x |
| 927 deg C | ~ 0.4 | ~ 1.6 | 4.0x |
| 982 deg C | ~ 0.5 | ~ 2.5 | 5.0x |
Oxide Spalling Threshold
The Cr2O3 film is a brittle ceramic with a thermal expansion coefficient approximately 30 percent below the Incoloy 800H matrix. On thermal cool-down from the service temperature, the differential contraction stresses the Cr2O3 film in compression. When the film thickness exceeds approximately 5 micrometres and the cool-down rate exceeds approximately 50 deg C per minute, the film spalls from the matrix and exposes fresh metal to re-oxidation. The practical spalling threshold for Incoloy 800H is approximately 100 thermal cycles from 982 deg C to ambient. Beyond this, the residual chromium in the matrix near the surface is depleted by the repeated oxidation cycles and the mass loss accelerates onto a near-linear curve. The Al2O3 sublayer from the residual Al content is what extends the spalling threshold beyond 100 cycles in 800HT, where the higher Al content (0.25 percent minimum vs 0.15 percent in 800H) supports a continuous inner Al2O3 layer.
Environmental Modifiers (Water Vapour, Sulphur, Alkali)
- Water vapour: 10 percent steam in the atmosphere lifts the 982 deg C mass loss by approximately 2x via volatile chromium hydroxide CrO2(OH)2 evaporation from the scale surface. Critical for steam superheater service.
- Sulphur (SO2 + H2S): sulphidation of the chromium scale lifts the 815 deg C mass loss by approximately 3 to 5x. Sulphur partial pressure above 10 Pa shifts the failure mode to internal sulphidation.
- Alkali (Na, K from fuel impurities or process side): alkali sulphates flux the Cr2O3 scale at 700 to 870 deg C in a hot-corrosion attack mode. Boiler superheater and gas turbine combustor exposure is the typical service envelope.
- Chloride (HCl, Cl2, NaCl): chloride attack on the Cr2O3 film initiates at 540 deg C and accelerates above 700 deg C. Waste incineration and chlorinated hydrocarbon process service requires Inconel 625 or alloy 600 rather than 800H.
Oxidation Comparison vs 310H, 800 and 800HT
| Alloy | UNS | Cr percent | Mass loss at 982 deg C, 1000 hr (mg/cm2) | Cyclic spalling threshold (cycles to 982 deg C) |
|---|---|---|---|---|
| 304H stainless | S30409 | 18-20 | not qualified | not qualified |
| 310H stainless | S31009 | 24-26 | ~ 0.3 | ~ 80 |
| Incoloy 800 | N08800 | 19-23 | ~ 1.0 | ~ 60 (no carbon-grain control) |
| Incoloy 800H | N08810 | 19-23 | ~ 0.5 | ~ 100 |
| Incoloy 800HT | N08811 | 19-23 | ~ 0.4 | ~ 200 (Al2O3 sublayer) |
Test Methods + Documentation
Isothermal oxidation testing is run to ASTM G54 in still air or controlled atmosphere with mass measurement at scheduled intervals. Cyclic oxidation testing is run to ASTM G54 with the cycle profile specified in the procurement document. Heat-specific oxidation testing is not standard practice for production lots; the published Special Metals data is the design reference for Incoloy 800H. Service-side oxidation monitoring on installed assets is by ultrasonic wall-thickness measurement at scheduled inspection intervals, see ASME VIII pressure vessel inspection for the API 510 + API 570 inspection envelope.
Engineering Implications
- The 0.5 mg/cm2 isothermal mass loss at 982 deg C, 1,000 hours translates to approximately 0.5 mm wall thickness loss over 40 years of continuous service, designed-in as the corrosion allowance for high-temperature heat exchanger tubes.
- Cyclic service (process-side thermal cycling, decoking cycles, daily start-stop) cuts the effective oxidation life by 3 to 5x, design with a larger corrosion allowance for cyclic ethylene cracker tube service.
- Above 100 thermal cycles to 982 deg C, the Cr2O3 spalling rate accelerates, specify 800HT (UNS N08811) instead of 800H for highly cyclic service above 900 deg C.
- Steam-containing atmospheres reduce the published mass-loss life by approximately 2x, confirm the design corrosion allowance against the documented service-side steam partial pressure.
- Sulphur and alkali atmospheres demand alternative materials (Inconel 600 or 625) rather than 800H above 815 deg C.
- The Al2O3 sublayer in 800HT extends the cyclic life by 2x but does not lift the isothermal mass loss, choose between 800H and 800HT on the service cycle profile, not on the peak temperature alone.
Frequently Asked Questions
What is the maximum continuous service temperature in oxidation?
982 deg C (1800 deg F) for Incoloy 800H in still air per the Special Metals technical bulletin. 1093 deg C (2000 deg F) for short-term excursion. Service in water vapour, sulphur or alkali atmospheres reduces the ceiling.
Why does cyclic oxidation cause more mass loss than isothermal?
The Cr2O3 scale has a thermal expansion coefficient approximately 30 percent below the metal matrix. On cool-down the scale is stressed in compression and spalls when the film thickness exceeds approximately 5 micrometres and the cool-down rate exceeds approximately 50 deg C per minute. The spalled scale exposes fresh metal to re-oxidation.
Is 800HT more oxidation-resistant than 800H?
In isothermal exposure the difference is small (approximately 20 percent lower mass loss for 800HT). In cyclic exposure the difference is approximately 2x in favour of 800HT due to the Al2O3 sublayer supported by the higher residual Al content.
Does 800H oxidise in vacuum or inert atmosphere?
No, in pure vacuum or argon there is no oxidation. In a partial vacuum with residual oxygen (above approximately 10^-4 Pa) the chromium evaporates from the surface and depletes the Cr content in the near-surface zone. For vacuum-furnace heat-treating fixtures above 900 deg C, the chromium loss is the controlling life-limiter.
What corrosion allowance should I design with for 800H in 815 deg C continuous service?
Typical practice is 1.5 to 3.0 mm corrosion allowance for 30 to 40 year design life in continuous-service ASME Section VIII pressure vessels. For cyclic service add 2 to 3x. Confirm against the heat-specific service environment.
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