Overview
Inconel 625 (UNS N06625) and Incoloy 825 (UNS N08825) are both nickel-based alloys widely used in chemical processing. The key distinction: Inconel 625 contains significantly more nickel (≥58%), molybdenum (8–10%), and niobium, making it a high-performance corrosion and oxidation-resistant alloy. Incoloy 825 adds copper and titanium at a lower nickel level (38–46%), optimized for sulfuric acid resistance at a more economical cost.
Chemical Composition Comparison
| Element | Inconel 625 (N06625) | Incoloy 825 (N08825) |
|---|---|---|
| Nickel (Ni) | ≥ 58% | 38–46% |
| Chromium (Cr) | 20–23% | 19.5–23.5% |
| Molybdenum (Mo) | 8–10% | 2.5–3.5% |
| Iron (Fe) | ≤ 5% | ~30% (balance) |
| Niobium (Nb) | 3.15–4.15% | — |
| Copper (Cu) | — | 1.5–3.0% |
| Titanium (Ti) | — | 0.6–1.2% |
PREN and Pitting Resistance
- Inconel 625: PREN ≈ 51–54 (Cr~21, Mo~9)
- Incoloy 825: PREN ≈ 29–33 (Cr~21.5, Mo~3)
With nearly double the PREN value, Inconel 625 offers dramatically superior resistance to pitting and crevice corrosion in chloride-rich environments such as seawater, FGD scrubbers, and offshore applications.
Mechanical Properties (Room Temperature, Annealed)
| Property | Inconel 625 | Incoloy 825 |
|---|---|---|
| Tensile Strength | ≥ 827 MPa | ≥ 586 MPa |
| Yield Strength | ≥ 414 MPa | ≥ 241 MPa |
| Elongation | ≥ 30% | ≥ 30% |
| Density | 8.44 g/cm³ | 8.14 g/cm³ |
Inconel 625 is significantly stronger due to solid-solution strengthening from its high molybdenum and niobium content.
ASTM Standards
| Product Form | Inconel 625 | Incoloy 825 |
|---|---|---|
| Sheet/Plate | ASTM B443 | ASTM B424 |
| Seamless Tube | ASTM B444 | ASTM B163 |
| Bar | ASTM B446 | ASTM B425 |
| Forgings | ASTM B564 | ASTM B564 |
Weldability
Both alloys offer excellent weldability. Inconel 625 has outstanding resistance to hot cracking due to niobium stabilization, making it the preferred weld overlay alloy. Neither requires post-weld heat treatment for corrosion resistance.
Cost Comparison
Inconel 625 typically costs 1.5–2 times more than Incoloy 825 per kilogram, driven by higher nickel (≥58% vs 38–46%) and molybdenum (8–10% vs 2.5–3.5%) content. Approximate pricing:
- Inconel 625: $25–40/kg for standard tubes and sheets
- Incoloy 825: $12–20/kg for standard tubes and sheets
Selection Guide
- Seawater, offshore, FGD scrubbers, high chloride? → Inconel 625 (PREN > 50)
- Sulfuric acid, moderate concentration, moderate temperature? → Incoloy 825 (Cu provides H₂SO₄ resistance)
- High strength required at elevated temperature? → Inconel 625
- Budget-limited, mild chloride environment? → Incoloy 825
- Sour gas (NACE MR0175)? → Inconel 625 preferred