GRADE PROFILE
904L stainless steel — 1.4539 / X1NiCrMoCu25-20-5
904L is a super austenitic with very high nickel, 24 to 26%, 4 to 5% molybdenum and — crucially — 1.2 to 2% copper. The copper is its speciality: resistance to reducing acids, sulphuric acid in particular, which ordinary stainless steels cannot take.
- Super austenitic
- PREN 34–36
- Cost index 340
- Welded · Seamless · Heat exchanger
- Ø 10 to 273 mm
When to choose it
In sulphuric or phosphoric acid, in bleaching circuits and in surface treatment. That is where the copper works, on a mechanism — corrosion in reducing conditions — that PREN does not describe at all.
Its very high nickel also gives it practical immunity to chloride stress corrosion cracking, which makes it a valid answer where that mechanism is the dominant risk and duplex is ruled out for fabrication or temperature reasons.
When not to choose it
Outside acidic media. At cost index 340 against 175 for a 2205 of equivalent PREN, 904L is a poor purchase on any application where pitting governs. Judging it by its PREN leads systematically to overpaying.
In hot seawater without reservation: a PREN of 35 is no more sufficient than on a 2205.
Where 317L covers the requirement. On dilute acids the intermediate step exists and costs far less.
Composition
| Element | % |
|---|---|
| C | ≤ 0.02 |
| Cr | 19–21 |
| Ni | 24–26 |
| Mo | 4–5 |
| Cu | 1.2–2 |
| N | ≤ 0.15 |
| Mn | ≤ 2 |
Mechanical properties
We only publish mechanical properties tied to a specific standard and product form — welded tube, seamless tube and bar do not share the same values, and the gap routinely exceeds 30%. The figures that apply to your specification appear on the material certificate; ask us at enquiry stage.
Designations and standards
| Reference | Designation |
|---|---|
| EN (numeric) | 1.4539 |
| EN (symbolic) | X1NiCrMoCu25-20-5 |
| ASTM / AISI | 904L |
| UNS | N08904 |
| JIS | — |
| AFNOR | — |
Tube product standards :
- EN 10216-5
- ASTM A312
- ASTM B677
Availability
Welded, seamless and heat exchanger tube, Ø 10 to 273 mm, plus fittings, flanges and bar. Partner stock or mill depending on the size and quantity. EN 10204 3.1 material certificates supplied with the goods.
Related grades
- 317L (S31703) — the step below for dilute acids.
- 254 SMO (1.4547) — the seawater super austenitic, PREN 44, where the issue is saline rather than acidic.
- Alloy 825 (N08825) — the nickel route where acidic conditions exceed what a stainless can take.
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Frequently asked questions
- 904L or 2205, at identical PREN?
- 2205, at almost half the price, unless the medium is a reducing acid. The PREN of 35 is the same, but it says nothing about copper or sulphuric acid resistance, which are the reason 904L exists.
- Why does copper improve acid resistance?
- It alters the kinetics of the cathodic reaction in reducing conditions and sharply lowers the general corrosion rate. It is a different mechanism from chromium passivation, and no pitting index captures it.
- Does it weld like an ordinary austenitic?
- The principles are the same, but an over-alloyed filler metal is needed to avoid depleting the weld in molybdenum and copper. A 316L-type filler would cancel the grade's advantage.