904L (UNS N08904) and 254SMO (UNS S31254, also called 6Mo) are high-alloy austenitic stainless steels that outclass 316L in corrosive service. 254SMO adds about 6% molybdenum and 0.2% nitrogen, raising its critical pitting temperature far above 904L in seawater and chloride environments.
1. Chemical Composition Comparison (%)
| Element | 904L (UNS N08904) | 254SMO / 6Mo (UNS S31254) |
|---|---|---|
| Chromium (Cr) | 19.0 – 23.0 | 19.5 – 20.5 |
| Nickel (Ni) | 23.0 – 28.0 | 17.5 – 18.5 |
| Molybdenum (Mo) | 4.0 – 5.0 | 6.0 – 6.5 |
| Copper (Cu) | 1.0 – 2.0 | ≤0.75 |
| Nitrogen (N) | — | 0.18 – 0.22 |
| Carbon (C) | ≤0.02 | ≤0.02 |
| PREN (approx) | ~35 | ~43 |
2. Mechanical Properties (Typical)
| Property | 904L | 254SMO (6Mo) |
|---|---|---|
| Tensile Strength (MPa) | ≥490 | ≥650 |
| Yield Strength 0.2% (MPa) | ≥220 | ≥310 |
| Elongation (%) | ≥35 | ≥35 |
| Hardness | ≤90 HRB | ≤95 HRB |
3. Key Technical Differences
- Corrosion: 254SMO's higher Mo and N give a PREN of ~43 vs 904L's ~35, significantly better in seawater and chlorides.
- Strength: 254SMO has markedly higher yield strength, allowing thinner walls.
- Cost: 254SMO is generally more expensive due to higher alloying.
- Niche: 904L is excellent for sulfuric/phosphoric acid; 254SMO dominates seawater and brackish water systems.
5. Key Application Areas
904L application areas
- Sulfuric and phosphoric acid processing plants
- Chemical storage and transfer systems
- Pulp and paper bleach plants
- Pharmaceutical and food processing equipment
254SMO (6Mo) application areas
- Seawater cooling and desalination systems
- Offshore oil & gas topside piping
- Ballast water and brackish water treatment
- High-chloride FGD and scrubber systems
Frequently asked questions
Is 254SMO always better than 904L?
In chloride-rich and seawater service, yes. For sulfuric acid without chlorides, 904L is often sufficient.
What does PREN mean?
Pitting Resistance Equivalent Number — a higher value indicates better pitting resistance.
Can 254SMO replace 904L?
Mechanically and corrosion-wise yes, but the cost premium may not be justified for mild service.
Is 254SMO weldable?
Yes, but use matching high-Mo filler and control heat input to avoid hot cracking.
Which is more available?
904L is more common; 254SMO is a specialty grade with longer lead times.
Does Hangbo supply both?
Yes, in plate, pipe, tube, and fittings with full MTRs and PMI.