Incoloy 825
Incoloy 825 (UNS N08825) is a nickel-iron-chromium alloy with molybdenum, copper, and titanium additions. It was engineered for aqueous corrosion resistance in reducing acid environments. Its chemistry sits between austenitic stainless steels and high-nickel alloys, giving it good resistance to sulfuric and phosphoric acids at moderate service temperatures.
1. Chemical Composition
The table below lists the elemental composition of Incoloy 825. Molybdenum and copper work together to resist reducing acid attack, while titanium stabilizes the alloy against sensitization.
| GRADE | UNS Designation |
Standard (Pipe) |
Chemical Requirement (Max) | ||||||||||
| Ni | Cr | Fe | Mn | C | Cu | Si | S | Al | Ti | Mo | |||
| Incoloy 825 | N08825 | B423 | 38.0-46.0 | 19.5-23.5 | =22.0 | =1.0 | =0.05 | 1.5-3.0 | =0.5 | =0.03 | =0.2 | 0.6-1.2 | 2.5-3.5 |
2. Mechanical Properties
The values below apply to Incoloy 825 seamless pipe and tube in the annealed condition. This is the standard delivery condition for corrosion-service process piping.
| Grade | Condition & Size | Standard | Tensile Strength , MPa (ksi) |
Yield Strength, MPa (ksi) |
Elongation , % |
| N08825 | Hot-finished annealed | B423 | =517(75) | =172(25) | =30 |
| N08825 | Cold-worked annealed | B423 | =586(85) | 241 (35) | =30 |
3. Equivalent Grade
The table below maps Incoloy 825 (UNS N08825) to its equivalent designations under EN/DIN, JIS, ISO, GB/T, and GOST standards for international procurement reference.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| Incoloy 825 | N08825 | - | NS142 | NCF825 | - | 2.4858 | - |
4. Key Technical Advantages
- Resistance to Reducing Acids: The 3% Mo and 2.25% Cu in Incoloy 825 give it effective resistance to sulfuric acid across a wide concentration range (up to 40% H2SO4 at temperatures below boiling) and to wet-process phosphoric acid. Molybdenum-free stainless steels fail by uniform dissolution in these conditions.
- Titanium Stabilization Against Sensitization: Titanium at 0.6–1.2% preferentially bonds with carbon to form TiC, preventing chromium carbide formation at grain boundaries. This stops sensitization during welding and slow cooling, so intergranular attack does not occur in the as-welded condition, even without post-weld heat treatment.
- Chloride Pitting and Crevice Corrosion Resistance: The high nickel base (38–46%) combined with molybdenum gives substantially better pitting and crevice corrosion resistance than duplex or 316 stainless steels in chloride environments. This qualifies the alloy for combined acid-chloride service in chemical process piping.
Technical Note: Incoloy 825 is not intended for high-temperature structural service. Its operating window for corrosion applications is primarily aqueous, covering temperatures up to about 200°C. Above this range, other Alloy 800 family variants or high-nickel alloys should be evaluated based on specific process chemistry.
5. Common Manufacturing Standards
ASTM B423: Standard Specification for Seamless Pipe and Tube — Nickel-Iron-Chromium-Molybdenum-Copper Alloy (UNS N08825). This is the primary governing standard for Incoloy 825 seamless pipe and tube in all corrosion-service applications.
ASTM B407: Standard Specification for Seamless Pipe and Tube — Nickel-Iron-Chromium Alloy (covers related Incoloy family products).
DIN/EN Standards:
- EN 10216-5: Seamless steel tubes for pressure purposes — nickel alloy grades (Material No. 2.4858)
- DIN 17459: Seamless circular tubes made of nickel alloys
JIS Standards:
- JIS H 4552: Nickel and nickel alloy seamless pipes and tubes
GB/T Standards:
- GB/T 15007: Corrosion-resistant alloy grades (NS1402 designation)
GOST Standards:
- GOST 14965: Tubes from nickel and nickel alloys
Summary Table — Standard Equivalents:
| ASTM | EN/DIN | JIS | GB/T | GOST |
| ASTM B423 | EN 10216-5 / DIN 17459 | JIS H 4552 | GB/T 15007 | GOST 14965 |
6. Primary Applications
- Sulfuric Acid Production and Handling: Absorption towers, heat exchangers, and transfer piping in contact-process H2SO4 plants and oleum storage systems. Sulfuric acid concentrations and temperatures vary widely across these operations.
- Phosphoric Acid Processing: Evaporator tubes and slurry piping in wet-process phosphoric acid plants, where fluoride-contaminated, high-temperature phosphoric acid creates a particularly aggressive corrosion environment.
- Oil and Gas Sour Service: Downhole tubing, flowlines, and surface processing equipment in H2S and CO2-bearing production fluids. The alloy's resistance to sulfide stress cracking and general acid corrosion suits NACE MR0175 applications.
- Marine and Seawater Desalination: Heat exchanger tubing in multi-effect distillation (MED) and multi-stage flash (MSF) desalination plants, where combined seawater corrosion and scale formation must be managed over decades of continuous operation.
- Pollution Control Equipment: Flue gas desulfurization scrubbers and wet electrostatic precipitator internals exposed to dilute sulfuric acid condensate, chloride, and fluoride under both oxidizing and reducing conditions.