904L Stainless Steel: The Complete Technical Guide for Engineers & Buyers [2025]
When standard 316L isn’t enough — but a full nickel alloy like Hastelloy C-276 Hastelloy C-276 vs C-22 strains the budget — 904L stainless steel occupies the critical middle ground. It delivers near-nickel-alloy corrosion resistance at roughly 60–70% of the cost, making it the go-to super austenitic grade for severe chloride and acid environments where ordinary stainless steels fail within months.
Also known as UNS N08904, EN 1.4539, and “super austenitic” stainless steel, 904L was originally developed by Outokumpu to handle sulfuric acid in phosphoric acid plants. Today it’s specified across chemical processing, offshore oil and gas, pulp and paper, seawater cooling, pharmaceutical manufacturing, and flue gas desulfurization (FGD) systems — anywhere that chlorides and reducing acids attack standard grades.
904L (UNS N08904) is a super austenitic stainless steel with 23–28% nickel, 19–23% chromium, and 4–5% molybdenum — plus 1–2% copper for sulfuric acid resistance. Its PREN of ~36–43 outperforms 316L (PREN 23–28) and competes with duplex 2205 Duplex 2205 Stainless Steel Guide (PREN 34–38) while offering far better reducing acid resistance thanks to the copper addition. If your process involves H₂SO₄, H₃PO₄, hot chlorides, or mixed acid environments, 904L is almost certainly the most cost-effective solution.
1. Chemical Composition: What Makes 904L “Super Austenitic”
904L is classified as a “super austenitic” stainless steel — a category that sits between standard austenitics (304, 316) and full nickel-based alloys (Hastelloy, Inconel). The nickel content alone tells the story: at 23–28%, it’s more than double the 10–14% in 316L.
| Element | 904L (UNS N08904) | 316L (S31603) | 317L (S31703) | Duplex 2205 |
|---|---|---|---|---|
| Chromium (Cr) | 19.0–23.0% | 16.0–18.0% | 18.0–20.0% | 22.0–23.0% |
| Nickel (Ni) | 23.0–28.0% | 10.0–14.0% | 11.0–15.0% | 4.5–6.5% |
| Molybdenum (Mo) | 4.0–5.0% | 2.0–3.0% | 3.0–4.0% | 3.0–3.5% |
| Copper (Cu) | 1.0–2.0% | — | — | — |
| Carbon (C) max | 0.02% | 0.03% | 0.03% | 0.03% |
| Manganese (Mn) max | 2.0% | 2.0% | 2.0% | 2.0% |
| PREN | 36–43 | 23–28 | 27–34 | 34–38 |
2. Mechanical Properties: Strength, Toughness & Temperature Limits
| Property | 904L | 316L | Duplex 2205 |
|---|---|---|---|
| Tensile Strength (MPa) | 490–690 | 485 min | 620 min |
| Yield Strength 0.2% (MPa) | 220 min | 170 min | 450 min |
| Elongation (%) | 35 min | 40 min | 25 min |
| Hardness (HB) max | HB 230 | HB 217 | HB 293 |
| Density (g/cm³) | 8.0 | 8.0 | 7.8 |
| Max Service Temp (°C) | ~400 | ~870 | ~300 |
904L is not a high-strength alloy — if you need structural load-bearing capacity, duplex grades win hands down (2205 has double the yield strength). But where 904L excels is corrosion resistance at elevated temperatures in acidic environments. Its maximum continuous service temperature of approximately 400°C is limited by sigma phase precipitation, not mechanical weakening.
3. Corrosion Resistance: Where 904L Excels
3.1 Pitting & Crevice Corrosion (Chloride Resistance)
With a PREN (Pitting Resistance Equivalent Number) of 36–43, 904L dramatically outperforms 316L (PREN 23–28). The formula tells the story:
PREN = %Cr + 3.3 × %Mo + 16 × %N
For 904L: 21 + (3.3 × 4.5) + (16 × 0.06) = ~36–43 (depending on exact composition)
PREN directly correlates with resistance to localized pitting attack. In practical terms, 904L can handle seawater at ambient temperatures and chloride concentrations up to 5,000–10,000 ppm — environments where 316L would pit within weeks. However, for continuous seawater service (especially at elevated temperatures), super duplex 2507 (PREN >40) or 6% Mo super austenitics like 254 SMO are preferred.
3.2 Sulfuric Acid: The Deciding Advantage
This is the domain where 904L truly separates from the competition. The copper addition (1–2%) provides critical resistance to reducing acid environments — meaning non-oxidizing acids like sulfuric, phosphoric, and various organic acids. In H₂SO₄ concentrations up to 80% at temperatures below 40°C, 904L shows corrosion rates under 0.1 mm/year. By contrast, 316L and duplex 2205 (which lack copper) can experience rates of 1–5 mm/year in the same environment.
| Corrosive Environment | 904L | 316L | Duplex 2205 | Hastelloy C-276 |
|---|---|---|---|---|
| Seawater (ambient) | Good | Poor | Good | Excellent |
| H₂SO₄ (10%, 25°C) | Excellent | Poor | Poor | Excellent |
| H₃PO₄ (wet-process) | Excellent | Poor | Poor | Excellent |
| Hot Chlorides (>60°C) | Moderate | Very Poor | Poor–Moderate | Excellent |
| Organic Acids (acetic, formic) | Excellent | Moderate | Moderate | Excellent |
| Caustic (NaOH, up to 50%) | Good | Good | Limited | Excellent |
3.3 Stress Corrosion Cracking (SCC)
Like all austenitic stainless steels, 904L is susceptible to chloride stress corrosion cracking (Cl-SCC) above approximately 60°C. This is a critical limitation: in hot chloride service, duplex grades (2205 and 2507) are actually safer choices because their ferrite phase provides near-immunity to Cl-SCC. However, in reducing acid environments with chlorides, 904L’s acid resistance often outweighs its SCC vulnerability.
4. Welding 904L: Procedure & Filler Metal Selection
904L is readily weldable by all common methods (GTAW/TIG, SMAW, SAW, GMAW/MIG), but the choice of filler metal is critical. The goal is to maintain or exceed the base metal’s corrosion resistance in the weld zone.
| Welding Parameter | Recommendation |
|---|---|
| Filler Metal (GTAW) | ER 385 (ERNiCrMo-3 / ERNiCrMo-4 for higher corrosion resistance) |
| Filler Metal (SMAW) | E 385-16 / ENiCrMo-3 electrodes |
| Preheat | Not required (room temperature) |
| Interpass Temperature | Max 150°C (300°F) |
| Heat Input | Low-medium: 0.5–1.5 kJ/mm |
| Post-Weld Heat Treatment | Generally not required for service below 400°C |
| Shielding Gas (GTAW) | 100% Argon or Ar + 2–5% H₂ for improved penetration |
| Backing Gas | 100% Argon (critical for pipe root passes to prevent oxidation) |
5. Available Product Forms & ASTM Specifications
| Product Form | ASTM / ASME Specification | Typical Size Range |
|---|---|---|
| Plate & Sheet | ASTM A240 / ASME SA-240 | 0.5–80 mm thickness |
| Seamless Pipe | ASTM A312 / ASME SA-312 | 1/8″–24″ NB, SCH 5S–SCH 160 |
| Welded Pipe | ASTM A358 / ASTM A312 | 6″–48″ NB, various schedules |
| Tube | ASTM A213 / ASTM A269 | 6.35–76.2 mm OD |
| Bar & Rod | ASTM A276 / ASTM A479 | 6–300 mm diameter |
| Fittings (Butt Weld) | ASTM A403 WP904L | 1/2″–24″ NB |
| Forged Flanges | ASTM A182 F904L | 1/2″–48″ NB, Class 150–2500 |
6. Typical Industrial Applications
904L is specified where the corrosion resistance of 316L is insufficient but the cost of full nickel-based alloys (Hastelloy C-276, C-22, Inconel 625) cannot be justified. Here are the industries where it’s most commonly found:
| Industry | Typical Equipment | Why 904L? |
|---|---|---|
| Phosphoric Acid Production | Reactors, heat exchangers, piping, evaporators | Copper provides critical H₃PO₄ resistance; 316L fails within weeks |
| Sulfuric Acid Handling | Storage tanks, piping, acid coolers, distributors | Resists H₂SO₄ up to 80% at ambient; 316L useless above 10% |
| Oil & Gas (Offshore) | Seawater cooling lines, heat exchanger tubes, produced water piping | Handles chlorides + CO₂ + H₂S in produced water environments |
| Pulp & Paper | Digesters, bleach plant equipment, black liquor evaporators | Resistance to organic acids + chlorides in bleaching chemicals |
| Pharmaceutical / Food | Reactor vessels, clean-in-place systems, sterile processing lines | Excellent resistance to cleaning chemicals (acids, caustics) at elevated temperatures |
| Petrochemical / Refining | Crude overhead condensers, amine units, sour water strippers | Handles naphthenic acids + chlorides + H₂S simultaneously |
| FGD (Flue Gas Desulfurization) | Absorber vessels, outlet ducting, mist eliminators | Handles acidic condensate (pH 1–3) with chlorides and fluorides |
7. 904L vs Alternatives: Cost-Performance Analysis
| Grade | Relative Cost (316L = 1.0) | PREN | Acid Resistance | Best Application |
|---|---|---|---|---|
| 316L | 1.0× | 23–28 | Poor | Mild environments, architectural, general purpose |
| 317L | 1.3–1.5× | 27–34 | Moderate | Mild chloride environments, FGD scrubbers |
| Duplex 2205 | 1.5–2.0× | 34–38 | Poor | High strength + seawater; avoid reducing acids |
| 904L ⭐ | 2.5–3.5× | 36–43 | Excellent | Sulfuric/phosphoric acid + chlorides; the cost-sweet-spot |
| Super Duplex 2507 | 2.5–3.5× | 40–45 | Poor | Seawater + high strength; avoid acids |
| 6% Mo (254 SMO / AL-6XN) | 3.5–5.0× | 43–48 | Good | Seawater + high temperature; moderate acids |
| Hastelloy C-276 | 8.0–12.0× | ~65+ | Excellent | Severe mixed-acid + chloride; budget is secondary |
8. Current Market Price Reference (2025)
| Product Form | Typical Size | Price Range (USD/kg) |
|---|---|---|
| Sheet (304L reference) | 3 mm × 1500 × 3000 mm | $3.50–$5.00 |
| Sheet (316L) | 3 mm × 1500 × 3000 mm | $5.50–$7.50 |
| Sheet (904L) | 3 mm × 1500 × 3000 mm | $12.00–$18.00 |
| Seamless Pipe (904L) | 2″ SCH 40S | $35.00–$55.00/kg |
| Butt Weld Fittings (904L) | 2″ Sch 40S, 90° LR Elbow | $45.00–$65.00/kg |
Prices vary significantly by quantity, delivery location, and certification requirements. Request a project-specific quotation for accurate pricing.
Need 904L for Your Chemical Processing Project?
We stock 904L in pipe, sheet, plate, and fittings — fully traceable to ASTM A240/A312/A403 with EN 10204 3.1 MTR certification. We also stock nickel alloys (Hastelloy C-276, Inconel 625) for the most severe environments.
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Frequently Asked Questions
Q: Is 904L better than 316L for seawater?
Yes — significantly. 316L will pit in seawater within months (PREN 23–28), while 904L (PREN 36–43) can handle ambient-temperature seawater for years. However, for continuous submerged seawater service or elevated temperatures, super duplex 2507 or 6% Mo grades (254 SMO / AL-6XN) are preferred.
Q: Can 904L replace Hastelloy C-276?
In some applications, yes — particularly for sulfuric and phosphoric acid handling at moderate temperatures. But C-276 has fundamentally broader corrosion resistance (especially to hot chlorides and mixed oxidizing-reducing acids) and a much higher temperature ceiling. Never substitute without a qualified corrosion engineer’s review of your complete process chemistry, temperature range, and upset conditions.
Q: Is 904L magnetic?
No. 904L is fully austenitic in the annealed condition and is non-magnetic. After cold working, it may develop slight magnetic permeability in heavily deformed areas (a few percent — far less than duplex grades, which are always ferromagnetic).
Q: What is the difference between 904L and 6% Mo super austenitics?
6% Mo grades (254 SMO / UNS S31254, AL-6XN / N08367) have higher molybdenum (6.0–7.0% vs 4–5%) and higher nitrogen (0.18–0.25%), giving them PREN values of 43–48 vs 904L’s 36–43. They’re better for hot seawater and high-chloride brines. However, 904L’s copper gives it an edge in reducing acid resistance (H₂SO₄, H₃PO₄). The choice depends on whether your main threat is chlorides (→ 6% Mo) or acids (→ 904L).
