Article
904L (UNS N08904, EN 1.4539) is a highly alloyed austenitic stainless steel. It was developed for sulphuric acid service and is now used wherever 316L or 317L cannot handle the corrosion, but a nickel alloy would be overkill.
Composition
| Element | 904L % | 316L % (for comparison) |
|---|---|---|
| Chromium | 19–23 | 16–18 |
| Nickel | 23–28 | 10–14 |
| Molybdenum | 4–5 | 2–3 |
| Copper | 1–2 | – |
| Carbon | 0.020 max | 0.035 max |
Why it performs
- Copper improves resistance to sulphuric and phosphoric acid.
- High nickel gives strong resistance to chloride stress corrosion cracking.
- Molybdenum raises pitting and crevice corrosion resistance; PREN is around 34–36.
- Very low carbon avoids sensitisation during welding.
Applications
- Sulphuric and phosphoric acid plants
- Pulp and paper bleaching
- Seawater cooling and offshore utility systems
- Pharmaceutical and fine chemical processing
- Flue gas cleaning
Alternatives to compare
| Grade | Strength | When to consider |
|---|---|---|
| 2205 duplex | About twice the yield of 904L | Chlorides with higher pressure; lower cost |
| 6Mo (254 SMO, 1.4547) | Higher | More severe seawater and chloride service |
| Alloy 20 | Similar | Hot sulphuric acid |
Ordering
Pipe 3" SCH 40S, ASTM B677 / ASTM A312 TP904L, seamless, solution annealed, EN 10204 3.1
904L pipe may be supplied to ASTM B677 or A312 depending on the mill; state which specification your project requires.
Frequently asked questions
Why is 904L used for sulphuric acid?
Its copper and high nickel content give it much better resistance to sulphuric acid than standard stainless steels.
Is 904L stronger than 316L?
Its strength is similar to 316L. Duplex 2205 is much stronger if strength is a priority.
What is the UNS number of 904L?
UNS N08904, EN 1.4539.
Key takeaways
- 904L is a super-austenitic grade with high Ni, Mo and Cu.
- Excellent in sulphuric acid and chloride environments.
- Compare with 2205 duplex and 6Mo before choosing.
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