Incoloy 800 Corrosion Resistance Performance in Oxidation, Carburization & Chloride Environments

Incoloy 800 (UNS N08800 / W.Nr. 1.4876 / NS111) is fundamentally an iron-nickel-chromium austenitic alloy stabilized with Al and Ti. Its corrosion resistance differs markedly from stainless steels and from higher-nickel alloys like Incoloy 825 or Hastelloy C-276. It is nota universal acid-resistant alloy—its strength lies in high-temperature gaseous environments (oxidation, carburization, sulfur-bearing atmospheres)​ and in resisting chloride-induced stress corrosion cracking (SCC) in aqueous service, rather than in resisting strong reducing acids. Below is a detailed, application-oriented breakdown.

High-Temperature Oxidation Resistance (Air / Combustion Gases)

The 19–23% chromium content forms a continuous, adherent Cr₂O₃ (chromia) scale on the metal surface when exposed to air or oxygen-containing combustion gases.

• Continuous service temperature:​ Up to 870°C (1600°F)​ in stagnant or mildly flowing air without excessive scale spallation.

• Intermittent / cyclic exposure:​ Short-term excursions to 980–1100°C​ are tolerable, though prolonged exposure above ~900°C causes grain coarsening and slow scale thickening.

• Performance vs. stainless:​ Superior to Type 309/310 SS in long-term cyclic heating because Al + Ti additions stabilize the grain structure and slow chromia-scale spalling.

• Limitation:​ In highly reducingcombustion gases (very low O₂, high CO, presence of sulfur/vanadium), the protective Cr₂O₃ may break down—consider 800H/800HT with controlled grain size or higher-Cr alloys for severe sulfur/ash corrosion.

Resistance to Carburization & Metal Dusting (Endothermic / Cracking Furnaces)

Incoloy 800 resists carbon penetration in carburizing atmospheres (endothermic gas, CO, CH₄ mixtures) significantly better than 300-series stainless steels.

• Mechanism:​ The Cr₂O₃ scale plus Al/Ti stabilization inhibits inward diffusion of carbon. Unlike high-carbon low-alloy steels, 800 does not readily form internal chromium carbides that would deplete the matrix and embrittle the part.

• Typical service:​ Radiant tubes, muffle furnaces, and ethylene cracker convection sections operating at 650–950°C in carburizing or metal-dusting-prone atmospheres.

• Best practice:​ For severecarburizing above 900°C or in metal-dusting conditions, specify 800HT (N08811)​ with coarse grain (ASTM 5 or coarser) and Al+Ti ≥0.85%, which offers improved carburization resistance over standard 800.

• Quantitative note:​ Isocarburization tests show 800 has ~1/3 to 1/5 the carburization depth of 310S stainless after 1000 h at 1000°C in typical endothermic gas—exact values depend on gas composition and dew point.

Aqueous Corrosion & Chloride Stress Corrosion Cracking (SCC) Resistance

Incoloy 800 is moderatelyresistant to general corrosion in many neutral-to-alkaline waters and some oxidizing acids (e.g., dilute nitric acid), but it is not​ recommended for reducing acids like sulfuric or hydrochloric acid—use Incoloy 825 or C-276 for those.

Its standout aqueous advantage over 18-8 and 316 stainless steels is:

• Chloride Stress Corrosion Cracking (Cl⁻ SCC):​

  800 is highly resistant to transgranular Cl⁻-induced SCC that commonly cracks Type 304/316 SS in hot (>60°C) chloride solutions. This is due to its high Ni content (30–35%), which suppresses the susceptibility of the austenite to chloride-SCC initiation.

• Pitting / Crevice Corrosion Threshold:​

  PREN (Pitting Resistance Equivalent Number) ≈ %Cr + 3.3×%Mo ≈ 19–23 (no Mo in standard 800) → moderate. In seawater or brackish water with high chlorides and stagnation, pits caninitiate—less resistant than 904L or 6Mo super-austenitics, and far less than duplex or super duplex in splash-zone service.

• Typical aqueous uses:​ Nuclear secondary-side feedwater heaters, high-purity water systems, alkaline (NaOH/KOH) evaporators up to ~300°C, dilute nitric acid coolers.

Sulfide / Sulfur-Bearing Atmosphere Resistance (Brief Note)

In reducing sulfurous gases (H₂S, SO₂ in low-O₂ environments), 800 shows acceptable resistance below ~650–700°C. Above this, or in high-velocity flue gas with ash deposition (vanadium pentoxide, etc.), sulfidation attack accelerates—800H/800HT helps somewhat, but high-Cr Ni-base alloys (e.g., 600H, 601, or Al-treated grades) may be required depending on pS₂ and temperature.

Summary Table – Incoloy 800 Corrosion Suitability