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Yes—Incoloy 825 (UNS N08825 / W.Nr. 2.4858 / NS142) does resist sulfuric acid corrosion, but "high temperature" here needs qualification. It performs very well in sulfuric acid across a broad concentration range up to approximately 120°C (250°F), and in some high-concentration (70–98%) sulfuric acid it can be used up to ~150°C. Beyond that, especially in dilute-to-medium sulfuric acid at elevated temperature, its corrosion rate rises sharply and a higher-molybdenum alloy such as Hastelloy C-276 is normally recommended.
Below is a practical breakdown based on ASTM B424/B425 material data and field isocorrosion charts.
Quick Answer: Incoloy 825 Sulfuric Acid Service Limits
|
H₂SO₄ Condition |
Typical Temperature Limit |
Expected Corrosion Rate* |
|---|---|---|
|
5–40% H₂SO₄ (dilute) |
≤ 60–80°C (140–175°F) |
< 0.13 mm/y (< 5 mpy) |
|
40–60% H₂SO₄ |
≤ 80–100°C (175–210°F) |
< 0.13 mm/y (< 5 mpy) |
|
70–98% H₂SO₄ (concentrated) |
≤ 120–150°C (250–300°F) |
< 0.13 mm/y (< 5 mpy) |
|
Boiling 40% H₂SO₄ |
Not recommended for continuous service |
> 0.5 mm/y |
*Values are typical for solution-annealed alloy in static, unaerated lab conditions per ASTM G31. Aeration, impurities (Cl⁻, F⁻), and velocity affect actual performance.
In short: Incoloy 825 is an excellent choice for warm sulfuric acid service, but it is not a universal "high-temperature boiling sulfuric acid" material.
Why Incoloy 825 Resists Sulfuric Acid
The alloy's resistance comes from its deliberately balanced chemistry (ASTM B424 / B425):
Ni 38–46% – stabilizes the passive film in both oxidizing and reducing environments, resists chloride stress corrosion cracking.
Cr 19.5–23.5% – forms a protective Cr₂O₃ passive layer, especially beneficial in moderately oxidizing sulfuric acid.
Mo 2.5–3.5% + Cu 1.5–3.0% – the key combination. Molybdenum promotes passivation in reducing acid; copper shifts the corrosion potential into the passive region and suppresses hydrogen evolution, greatly slowing attack in sulfuric acid.
Ti 0.6–1.2% – ties up carbon as TiC, preventing chromium carbide precipitation at grain boundaries (no sensitization → no intergranular corrosion after welding).
This Ni–Cr–Mo–Cu synergy makes Incoloy 825 far superior to 316L stainless steel in sulfuric acid and a cost-effective step up from 904L in many process applications.
Temperature & Concentration Boundaries You Should Know
Isocorrosion diagrams (Dechema / NACE references) show Incoloy 825 has a "passive window" in sulfuric acid:
Dilute acid (≤ 20%, up to ~80°C): Very good resistance; corrosion rate typically under 0.05 mm/y.
Medium concentration (20–60%, up to ~100°C): Good resistance in most cases; check aeration—oxygen accelerates corrosion in low concentrations.
High concentration (> 70%, up to ~120–150°C): Surprisingly good, because concentrated H₂SO₄ is less aggressive (low water activity). Many plants use 825 for 93–98% H₂SO₄ storage and transfer at ambient to moderately warm temperatures.
Above ~120°C in < 50% H₂SO₄: Corrosion rate climbs. For boiling dilute or中等 sulfuric acid above 100°C, consider upgrading to N06059, C-276 (Hastelloy C-276), or zirconium (in non-fluoride media).
When Incoloy 825 Is (and Isn't) the Right Choice
Choose Incoloy 825 for:
Sulfuric acid pickling tanks, heating coils, and transfer lines (≤ 80–100°C, various concentrations).
Acid storage tanks for 93–98% H₂SO₄ at ambient to warm conditions.
Mixed acid streams containing H₂SO₄ + H₃PO₄ or minor chlorides (where 316L fails by pitting/SCC).
Services requiring NACE MR0175 compliance (sour gas + trace H₂SO₄).
Do NOT specify Incoloy 825 for:
Continuously boiling dilute-to-medium sulfuric acid (> 100°C).
Strongly oxidizing media (e.g., hot concentrated HNO₃ + H₂SO₄)—higher Cr/Ni alloys are better.
Hydrofluoric acid (HF) service—825 has no special HF resistance.
