Incoloy 825 (UNS N08825): Corrosion-Resistant Nickel Alloy for Chemical Processing
ASTM B423 / ASTM B425 · Published: 2026-06-01 · Updated: 2026-06-02
Incoloy 825 (UNS N08825) is a nickel-iron-chromium alloy with molybdenum and copper additions, providing exceptional resistance to sulfuric acid, phosphoric acid, and seawater corrosion. The molybdenum (2.5-3.5%) provides pitting resistance in...
Incoloy 825 (UNS N08825) is a nickel-iron-chromium alloy with molybdenum and copper additions, providing exceptional resistance to sulfuric acid, phosphoric acid, and seawater corrosion. The molybdenum (2.5-3.5%) provides pitting resistance in chloride environments; the copper (1.5-3.0%) provides resistance to reducing acids (sulfuric, phosphoric) — a unique combination not available in any stainless steel. Incoloy 825 is the standard material for chemical process equipment handling mixed acid environments, phosphoric acid evaporators, and offshore oil & gas production equipment exposed to sour service (H2S-containing) conditions.
The alloy is approved for pressure vessel construction under ASME Boiler and Pressure Vessel Code Section VIII Division 1 up to 538°C. It is included in NACE MR0175/ISO 15156 for sour service (H2S-containing environments) at hardness levels up to 35 HRC in the solution-annealed condition. Unlike Inconel 625 which derives its strength from niobium precipitation hardening, Incoloy 825 is solid-solution strengthened only — it cannot be age-hardened and is used in the annealed condition. Welding with matching filler metal (ERNiCrMo-3 / Inconel 625 filler) is standard practice.
Quick Facts
| Category | Nickel Alloy |
| Standard | ASTM B423 / ASTM B425 |
| Density | 8140 kg/m³ |
| Yield Strength | 240 MPa (annealed) |
| Tensile Strength | 590 MPa |
Global Equivalents & Cross-Reference
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Frequently Asked Questions
Why choose Incoloy 825 over 316L for sulfuric acid service?
316L has almost zero resistance to sulfuric acid at concentrations above 10% and temperatures above 25°C. The copper content in 825 (1.5-3.0%) provides a catalytic surface for the formation of a protective cupric sulfate film in sulfuric acid, enabling corrosion rates below 0.1 mm/year in 20-50% H2SO4 at up to 80°C. For sulfuric acid above 80°C or above 50% concentration, Incoloy 825's corrosion rate increases, and Alloy 20 or Hastelloy C276 may be required.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu