Super-austenitic stainless · UNS N08904 · EN 1.4539
SS 904L: the stainless steel built for sulphuric acid
In short: SS 904L is a super-austenitic stainless steel with about 25% nickel, 4–5% molybdenum and 1–2% copper, and carbon held to 0.020%. The copper and nickel give it resistance to sulphuric and phosphoric acids that ordinary stainless steels lack, so it is used in acid plants, phosphate fertiliser units and chemical process equipment.
Overview
SS 904L in brief
- High Ni-Mo-Cu for sulphuric and phosphoric acids
- PREN about 32–36
- Good resistance to chloride SCC
- UNS N08904: classed with nickel alloys
SS 904L was developed for sulphuric acid service. High nickel, molybdenum and copper give it resistance to reducing acids that attack SS 316L. Its PREN of about 32–36 gives good resistance to pitting, crevice corrosion and chloride stress corrosion cracking.
Because its UNS number is in the N (nickel alloy) series, SS 904L sits between stainless steels and nickel alloys, both technically and in price. It's used for acid plants, phosphoric acid evaporators, seawater-cooled heat exchangers, pulp bleaching and pollution control. It's non-magnetic and is welded with over-alloyed fillers.
Available as
SS 904L product forms
Select a form for its specification, sizes and weights.
Where it's used
Industries that specify SS 904L
Acid service
Each alloying element has a job
SS 904L's corrosion resistance comes from the combination, not any single element.[1]
- Nickel 23–28%
- Resists reducing acids and chloride stress corrosion cracking.
- Molybdenum 4–5%
- Pitting and crevice resistance in chlorides.
- Copper 1–2%
- Specifically improves resistance to sulphuric acid.
- Carbon ≤ 0.020%
- Keeps welds free from sensitisation.
Chemistry
Composition of SS 904L
- UNS
- N08904
- EN / W.Nr.
- 1.4539 · X1NiCrMoCu25-20-5
- Density
- 8 g/cm³
- PREN
- 32 – 36
| Element | Limit, % | Why it matters |
|---|---|---|
| C | 0.020 max | |
| Mn | 2.00 max | |
| P | 0.045 max | |
| S | 0.035 max | |
| Si | 1.00 max | |
| Cr | 19.0 – 23.0 | |
| Ni | 23.0 – 28.0 | High nickel: reducing acids and SCC |
| Mo | 4.0 – 5.0 | Pitting resistance |
| Cu | 1.0 – 2.0 | Sulphuric acid resistance |
| N | 0.10 max |
Limits per ASTM A240; other product forms and specifications differ slightly. Your order's specification and edition govern.
Where SS 904L earns its cost
Acid plants, fertiliser and chemicals
Markets with strong SS 904L demand: Indonesia (acid-leach nickel plants), France and Germany (chemicals).
Mechanical properties
Room-temperature minimums
ASTM A240: room-temperature specification values for the annealed condition.
Supply
SS 904L by product form
| Product form | Specification and designation |
|---|---|
| SS 904L pipes | ASTM B677 / ASTM A312 N08904 |
| SS 904L tubes | ASTM B677 / B674 |
| SS 904L plates | ASTM A240 / B625 N08904 |
| SS 904L sheets | ASTM A240 / B625 N08904 |
| SS 904L coils | ASTM A240 N08904 |
| SS 904L bars | ASTM B649 / A479 N08904 |
| SS 904L fittings | ASTM A403 WP904L |
| SS 904L flanges | ASTM A182 F904L |
| SS 904L bolts | SS 904L bolting (to order) |
| SS 904L nuts | SS 904L nuts |
| SS 904L washers | SS 904L washers |
- Specification: stainless (A312, A240) or nickel-alloy (B677, B625) route
- Acid concentration, temperature and impurities, for a suitability check
- Copper and molybdenum content confirmed on the certificate
- Matching SS 904L or higher-alloy filler
- Solution-annealed condition
Ordering
What to put on an order for SS 904L
Getting these on the enquiry avoids the usual back-and-forth and makes sure the certificate matches your specification.
SS 904L questions
Answers about SS 904L
- Why is SS 904L good in sulphuric acid?
Its high nickel and molybdenum resist reducing acids, and the copper addition specifically improves resistance to sulphuric acid. It was developed for that service.[1] Impurities such as chlorides and fluorides change the picture sharply, so check corrosion data for your actual acid.
- What is the PREN of SS 904L?
About 34–36 with typical composition (PREN = Cr + 3.3 Mo + 16 N); producers typically quote a minimum of about 34. That is well above SS 316L (about 24) but below super duplex and 6Mo grades.
- SS 904L or super duplex?
For seawater and high chlorides, super duplex 2507 (PREN 40+) and twice the yield strength usually win. For sulphuric and phosphoric acid, SS 904L's nickel and copper often make it the better choice.
- Is SS 904L a stainless steel or a nickel alloy?
A super-austenitic stainless steel. With about 25% nickel it sits between conventional stainless and nickel alloys, which is why it carries a UNS N-number (N08904) and pipe is supplied to nickel-alloy specifications such as ASTM B677 as well as stainless ones.[2]
Sources
References
Acid resistance depends strongly on concentration, temperature and impurities; use producer iso-corrosion data for design.
Handling sulphuric or phosphoric acid?
Send acid concentration, temperature and impurities with your list. We'll confirm whether SS 904L or a nickel alloy fits.