316L vs 904L Acero Inoxidable Súper Austenítico
When 316L corrodes in months and nickel alloys seem too expensive, 904L fills the gap — delivering PREN 43 with superior sulfuric acid resistance at a fraction of nickel alloy cost.
| Propiedad | 316L | 904L |
|---|---|---|
| UNS | S31603 | N08904 |
| Nickel (Ni) | 10.0 – 14.0% | 23.0 – 28.0% |
| Chromium (Cr) | 16.0 – 18.0% | 19.0 – 23.0% |
| Molybdenum (Mo) | 2.0 – 3.0% | 4.0 – 5.0% |
| Copper (Cu) | — | 1.0 – 2.0% |
| Tensile Strength | ≥ 485 MPa | ≥ 490 MPa |
| Yield Strength | ≥ 170 MPa | ≥ 220 MPa |
| PREN | 24 – 28 | 35 – 39 |
| Critical Pitting Temp | 15 – 25°C | 40 – 55°C |
| H₂SO₄ Resistance | Poor above 10% conc. | Excellent to 60°C at all conc. |
| Cl-SCC Resistance | Susceptible above 80°C | Highly resistant up to 120°C |
| Density | 8.00 g/cm³ | 8.00 g/cm³ |
| Relative Cost | 1.0× (base) | 3.0 – 4.0× |
The Copper Difference: Why 904L Handles Sulfuric Acid
904L's 1-2% copper addition is the defining differentiator — it provides unique resistance to reducing acids, especially sulfuric acid (H₂SO₄). Copper promotes the formation of a protective Cu-rich film on the metal surface that resists attack by H₂SO₄ at concentrations where 316L would suffer rapid general corrosion. This is the same mechanism that makes Incoloy 825 effective — but at roughly half the cost.
Elegir 316L Cuando:
- Standard chloride service (PREN 26 sufficient)
- No sulfuric or phosphoric acid exposure
- Budget-sensitive projects
- General offshore and desalination (pre-treatment)
Elegir 904L Cuando:
- Sulfuric acid present at any concentration
- PREN > 35 required but full nickel alloy too expensive
- Pollution control (FGD, acid scrubbers)
- Phosphoric acid production and handling
- Seawater cooling above 35°C continuously
Conclusion
904L bridges the gap between commodity stainless (316L) and premium nickel alloys (825/C-276). At 3-4× the cost of 316L but only 50% of the cost of Incoloy 825, it is the cost-optimal choice for sulfuric acid service and high-chloride environments where 316L fails within months. For mixed acid service with sulfuric acid present, 904L is often the first material to try before specifying nickel alloys.