Incoloy 825 vs Inconel 625
These two nickel alloys look similar on paper but are optimized for completely different missions. 825 dominates in acid processing at moderate temperatures; 625 owns high-temperature strength and oxidation resistance.
| Property | Incoloy 825 | Inconel 625 |
|---|---|---|
| UNS | N08825 | N06625 |
| Nickel | 38.0 – 46.0% | ≥ 58.0% |
| Chromium | 19.5 – 23.5% | 20.0 – 23.0% |
| Molybdenum | 2.5 – 3.5% | 8.0 – 10.0% |
| Copper | 1.5 – 3.0% | — |
| Titanium | 0.6 – 1.2% | — |
| Niobium | — | 3.15 – 4.15% |
| Tensile Strength | ≥ 586 MPa | ≥ 827 MPa |
| Yield Strength | ≥ 241 MPa | ≥ 414 MPa |
| Max Service Temp | 540°C | 980°C |
| H₂SO₄ Resistance | Excellent (Cu addition) | Good |
| H₃PO₄ Resistance | Excellent | Good |
| Seawater Resistance | Excellent (Cu) | Excellent (Mo) |
| Relative Cost | 0.75 – 0.85× | 1.0× (base) |
Choose Incoloy 825 When:
- Sulfuric or phosphoric acid processing
- Pickling operations and acid regeneration
- Pollution control (FGD, scrubbers)
- Seawater-cooled heat exchangers (moderate temp)
- Budget-sensitive acid service (15-25% cheaper than 625)
Choose Inconel 625 When:
- High-temperature service above 540°C
- Aerospace engine and exhaust components
- Nuclear reactor applications
- High-pressure subsea (strength + corrosion)
- Where fatigue and thermal cycling resistance matter
The Key Difference: Copper vs Niobium
825's 1.5-3.0% copper addition is the defining differentiator — it provides unique resistance to sulfuric acid that even 625 cannot match. This is why 825 is the standard material for sulfuric acid coolers, pickling tanks, and pollution control scrubbers. 625's 3.15-4.15% niobium enables precipitation hardening for extraordinary high-temperature strength — up to 980°C. For acid processing below 540°C, 825 is the better and more economical choice. For anything involving high temperature, 625 is irreplaceable at its price point.