If you are selecting a stainless for chloride-bearing process fluid and you have heard the words 904L, 254SMO and AL-6XN in the same meeting, the most common mistake is to over-spec. All three are good austenitic grades; they differ mainly in PREN, in form availability, and in how much of the price premium is actually doing useful work in your service.
PREN (Pitting Resistance Equivalent Number) is a calculated value: PREN = Cr + 3.3 × Mo + 16 × N. It is a useful first filter for ranking chloride pitting resistance, but it is not a complete design tool — temperature, chloride concentration, pH, oxidative species (Fe³⁺, Cu²⁺) and crevice geometry all change the answer.
| Parameter | 904L (N08904) | AL-6XN (N08367) | 254SMO (S31254) |
|---|---|---|---|
| UNS / EN | N08904 / 1.4539 | N08367 / — | S31254 / 1.4547 |
| Typical Cr / Mo / N | 20 / 4.3 / 0.05 | 20.5 / 6.2 / 0.22 | 20 / 6.1 / 0.20 |
| PREN (typical) | ~ 34 | ~ 43 | ~ 43 |
| Cu addition | 1.0 – 2.0 % | — | — |
| Yield strength (min) | ~ 220 MPa | ~ 300 MPa | ~ 300 MPa |
904L is the cheapest of the three and is the correct answer wherever 316L has failed by pitting or crevice corrosion in moderate chloride service — wet-process phosphoric acid, dilute sulphuric acid coolers, and many FGD duties. The copper addition gives it a useful edge in reducing acid environments.
The extra molybdenum and nitrogen in 254SMO and AL-6XN push PREN into the low 40s, which is where seawater, concentrated chloride brines, and oxidative process streams become safely handled. Specify either when:
Mechanically and in PREN they are near-identical. The differences are commercial: 254SMO (Outokumpu / EN 1.4547) has the broader European stockist network and is more commonly available in plate and tube; AL-6XN (Allegheny / N08367) is the answer US-spec jobs tend to gravitate to. Choose the one that is in stock on your standard form at the mill you already work with.
Start with 316L. Move to 904L when 316L pitting is the failure mode. Move to 254SMO or AL-6XN only when you are actually in the chloride-temperature envelope that justifies the PREN. The price steps are real — make sure the premium is buying margin you actually need.
Send us your drawing or specification for a quotation — we reply within one business day.
