Every stainless steel has a temperature and a chloride level at which it stops being stainless and starts being a sacrificial anode. For 316L that limit arrives early; for 904L it arrives later; for 254SMO it arrives much later.
The three grades are not rivals - they are rungs on a ladder, and the engineer's job is to know which rung the service actually demands. This article compares the two upper rungs that buyers most often confuse: 904L (UNS N08904), the 4.5% molybdenum workhorse of chemical plant, and 254SMO (UNS S31254), the 6% molybdenum super-austenitic built for seawater and the worst chloride duties.
The difference is not a marketing story - it is written in the chemistry, the pitting resistance number, and the critical pitting temperature, and this article reads those lines for you.
The chemistry that separates them
| Element | 904L (N08904) | 254SMO (S31254) | What changes |
| Chromium | 19.0 - 23.0 | 19.5 - 20.5 | Similar chromium level |
| Nickel | 23.0 - 28.0 | 17.5 - 18.5 | 904L carries much more nickel |
| Molybdenum | 4.0 - 5.0 | 6.0 - 6.5 | The decisive step - 1.5-2% more Mo |
| Nitrogen | ~0.10 max | 0.18 - 0.25 | 254SMO uses nitrogen as a strength and film element |
| Copper | 1.0 - 2.0 | 0.5 - 1.0 | 904L carries copper for acid resistance |
| Carbon | 0.020 max | 0.020 max | Both low-carbon and weld-safe |
Two design choices separate the grades. 904L spends its alloy budget on nickel and copper - a formula tuned for reducing acids and general chemical service. 254SMO spends it on molybdenum and nitrogen - a formula tuned for the chloride attack that nickel does not stop. When a pipe must sit in warm seawater, molybdenum and nitrogen are the elements that count, which is why 254SMO was born as a seawater grade and 904L as a chemical-plant grade.
The pitting resistance number (PREN)
The industry shorthand for chloride resistance is the pitting resistance equivalent number: PREN = %Cr + 3.3 x %Mo + 16 x %N. Applied at mid-range chemistry:
| Grade | Molybdenum | PREN (typical range) | Corrosion class |
| 316L | 2.0 - 3.0 | 24 - 26 | General chemical, mild chlorides |
| 904L | 4.0 - 5.0 | 34 - 38 | Chemical plant, moderate chlorides |
| 254SMO | 6.0 - 6.5 | 42 - 46 | Seawater, severe chloride duty |
The step from 904L to 254SMO is roughly half the size of the step from 316L to 904L, but it is the step that matters most in practice: it crosses the threshold where warm seawater and chloride-contaminated process streams stop being a gamble. In standardized pitting tests (ASTM G48, ferric chloride), 254SMO's critical pitting temperature stands measurably above 904L's - the kind of margin that turns a ten-year inspection interval into a twenty-year one.
Mechanical comparison (pipe values)
| Property | 904L pipe | 254SMO pipe | Notes |
| Tensile strength | 490 MPa min | 650 MPa min | 254SMO is ~30% stronger |
| Yield strength 0.2% | 220 MPa min | 300 MPa min | Nitrogen does the work |
| Elongation | 35% min | 35% min | Both ductile |
The strength gap is another reason 254SMO wins seawater duty beyond corrosion: for the same pressure, the wall can be thinner, which partly offsets its higher price per tonne in the finished line.
Standards and designations
| Item | 904L | 254SMO |
| UNS | N08904 | S31254 |
| EN designation | 1.4539 | 1.4547 |
| Pipe standard | ASTM B673 (seamless) | ASTM A312 / A269 |
| Plate standard | ASTM A240 | ASTM A240 |
| Common trade name | 904L / Sandvik 2RK65 | 254SMO (trademark, Outokumpu) |
904L is a nickel alloy designation (the N in N08904 reflects its nickel-rich chemistry) while 254SMO is an austenitic stainless designation - a distinction that affects nothing in the mill order but explains why the two grades live in different standard families.
Choosing between them: the decision rule
- Chemical plant with sulphuric or phosphoric acid, moderate chlorides - 904L, and its copper content helps in reducing acids
- Warm seawater, offshore and desalination piping - 254SMO, where PREN and CPT decide the service life
- Chloride-contaminated heat exchangers with crevices and deposits - 254SMO, because crevice attack is where 904L's margin runs out
- Cost-sensitive duties below the pitting threshold - 904L remains the economical upper-rung choice
Welding notes for both grades
- 904L: weld with over-alloyed filler such as ERNiCrMo-3 (625-type) to keep the weld zone at or above parent strength
- 254SMO: use matching 254SMO filler or ERNiCrMo-3 for the most corrosive duties - never drop to 316-type filler
- Both are low-carbon, so no preheat and no PWHT; control heat input and interpass temperature
- For seawater tube bundles, purge the weld root and pickle after fabrication to restore the film
The bottom line
904L and 254SMO are not competing for the same job - they are two deliberate steps on the same chloride ladder. Buy 904L for chemical plant where acids, not chlorides, set the failure mode. Buy 254SMO where warm chlorides, seawater, or crevice geometries set it. The price gap between them is real, and so is the failure cost of choosing the lower rung in the wrong service. Read the PREN, run the G48 test, and let the operating temperature decide.
FAQ
Q: Is 254SMO always better than 904L?
A: No. 254SMO wins chloride and seawater duty outright, but 904L carries more nickel and copper, which help in reducing acids and general chemical service. The right choice depends on the failure mode of the actual duty.
Q: What does PREN actually tell the buyer?
A: It is a comparative index of pitting resistance, not a guarantee. 254SMO's PREN of 42-46 against 904L's 34-38 predicts better performance in standardized chloride tests, but service history and operating temperature still decide the order.
Q: Can 904L be welded to 254SMO?
A: Yes, with an over-alloyed filler such as ERNiCrMo-3. The weld metal becomes the highest-alloy member of the joint, which is the safe direction for dissimilar joints.
Q: Is 254SMO a duplex stainless steel?
A: No. It is a fully austenitic super-austenitic grade. Its high strength comes from nitrogen solid-solution strengthening, not from a ferritic phase.
Q: Which standard covers 904L pipe?
A: Seamless 904L pipe is supplied to ASTM B673, and plate to ASTM A240. 254SMO pipe is covered by ASTM A312/A269 and plate by ASTM A240.
