Low-Carbon Super-Austenitic Stainless for Sulfuric & Chloride Service
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UNS NUMBER
N08904
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ALLOY FAMILY
Super Austenitic
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STRUCTURE
100% Austenite
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UTS (MIN)
≥490 MPa
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MAX TEMP
~400°C
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DENSITY
8.00 g/cm³
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Stainless Steel 904L (UNS N08904) is a low-carbon super-austenitic stainless with high nickel (23–28%), molybdenum (4–5%), and copper (1–2%), giving PREN 36–38 and strong resistance to sulfuric acid and chloride pitting. It bridges the gap between 316L and nickel alloys for aggressive chemical and seawater service.
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Cr
19–23%
Oxidation + acid resistance
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Ni
23–28%
Austenite; acid resistance
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Mo
4–5%
Pitting / crevice
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Cu
1–2%
Sulfuric acid resistance
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UTS
≥490 MPa
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YS 0.2%
≥220 MPa
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Elong.
≥35%
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Service
-196~400°C
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What is Stainless Steel 904L?
904L was developed for sulfuric-acid handling and other aggressively corrosive media that defeat 316L. The high nickel and molybdenum raise PREN to 36–38, while the 1–2% copper specifically improves resistance to reducing acids such as sulfuric and phosphoric. Carbon is kept very low (≤ 0.020%) so the grade is immune to sensitization — no need for stabilization or post-weld annealing.
It is a cost-effective step above 316L/317L where chlorides and acids coincide, and it is far cheaper than a full nickel alloy. The penalty versus duplex/super-duplex is lower strength and a higher nickel price. For the most aggressive seawater (warm, high chloride), the 6Mo super-austenitics AL6XN and 254SMO (PREN 42–48) are the next upgrade.
Chemical Composition (Weight %)
| Element |
Min (%) |
Max (%) |
Role in Alloy |
| Chromium (Cr) |
19.0 |
23.0 |
Oxidation + acid resistance |
| Nickel (Ni) |
23.0 |
28.0 |
Austenite; acid resistance |
| Molybdenum (Mo) |
4.0 |
5.0 |
Pitting / crevice resistance |
| Copper (Cu) |
1.0 |
2.0 |
Sulfuric / phosphoric acid resistance |
| Carbon (C) |
— |
0.020 |
Very low; no sensitization |
| Manganese (Mn) |
— |
2.00 |
Deoxidizer |
| Silicon (Si) |
— |
1.00 |
Deoxidizer |
| Phosphorus (P) |
— |
0.045 |
Impurity |
| Sulfur (S) |
— |
0.035 |
Impurity |
| Iron (Fe) |
Balance |
— |
Matrix former |
★ High nickel (23–28%) + molybdenum (4–5%) + copper (1–2%) and ultra-low carbon (≤0.020%) give PREN 36–38 with no sensitization risk. 904L is the classic sulfuric-acid grade.
Mechanical Properties
Values are for the solution-annealed condition per ASTM B625 (plate), B677 (pipe), B649 (bar), and B366 (fittings).
| Condition / Form |
UTS (MPa) |
YS 0.2% (MPa) |
Elong. % (min) |
Hardness (max) |
| Minimum (ASTM B625 / B677) |
≥490 |
≥220 |
35 |
90 HRB (200 HBW) |
| Typical mill-annealed |
530–650 |
240–320 |
40–50 |
75–90 HRB |
Modulus of Elasticity 195 GPa. Strength comes from high alloy + solid solution; not hardenable. Lower strength than duplex, so wall thicknesses are larger for the same pressure.
Physical Properties
| Property |
Value |
Note |
| Density |
8.00 g/cm³ |
Above 316L |
| Melting Range |
1300–1390°C |
Solidus ~1300°C |
| Thermal Conductivity (100°C) |
~13 W/m·K |
Lower than 316 |
| Thermal Expansion (20–100°C) |
15.0 ×10⁻⁶ /K |
Similar to austenitics |
| Specific Heat |
500 J/kg·K |
At 21°C |
| Electrical Resistivity |
0.95 µΩ·m |
At 21°C |
| Magnetic Property |
Non-magnetic |
Fully austenitic |
| PREN (Pitting Resistance) |
36–38 |
PREN = %Cr + 3.3×%Mo + 16×%N |
Corrosion Resistance
Sulfuric & Phosphoric Acid
The copper addition makes 904L notably resistant to sulfuric and phosphoric acids at moderate concentrations and temperatures — its defining application versus 316L.
Pitting & Crevice Corrosion
PREN 36–38 gives strong resistance to chloride pitting and crevice corrosion, far better than 316L. Still below the 6Mo grades (AL6XN/254SMO, PREN 42–48) in warm seawater.
General & Stress Corrosion
High nickel gives excellent resistance to chloride stress-corrosion cracking. Fully austenitic and non-magnetic.
Weld Sensitization
Ultra-low carbon (≤0.020%) means 904L does not sensitize after welding; post-weld annealing is not required.
Stainless Steel 904L vs Similar Grades — Selection Guide
| If you need... |
Choose... |
Why |
| Sulfuric / phosphoric acid |
904L |
Copper-boosted acid resistance |
| Chloride above 316L, ambient |
904L |
PREN 36–38 |
| Warm seawater / extreme Cl⁻ |
AL6XN / 254SMO |
PREN 42–48 |
| General chloride, cheaper |
316L / 317L |
Lower cost |
| Cost-no-object extreme |
Nickel alloy (C276) |
Max resistance |
Applicable Standards
| Standard |
Product Form |
| ASTM B625 |
Plate, sheet, strip |
| ASTM B677 |
Seamless pipe & tube |
| ASTM B674 |
Welded pipe & tube |
| ASTM B366 |
Butt-weld fittings (WPN08904) |
| ASTM B564 |
Forgings / flanges |
| ASTM B649 / B625 |
Bar, rod |
HT PIPE Supply — Stainless Steel 904L Product Forms
| Product Form |
Specifications |
HT PIPE Supply |
| Seamless Pipe |
ASTM B677 |
✅ Available |
| Welded Pipe |
ASTM B674 |
✅ Available |
| Forged Flanges |
ASTM B564, ASME B16.5 |
✅ In stock |
| Butt-Weld Fittings |
ASTM B366 WPN08904 |
✅ Available |
| Plate / Sheet |
ASTM B625 |
✅ Available |
| Bar / Rod |
ASTM B649 |
✅ Available |
HT PIPE supplies 904L in pipe, flanges, fittings, plate, and bar for sulfuric-acid and aggressive chloride service, with full MTRs and PMI on request.
Project Experience — Stainless Steel 904L
HT PIPE supplies 904L for sulfuric/phosphoric acid plants and aggressive chloride service worldwide.
| Project |
Product |
Specification |
| Sulfuric Acid Plant (Asia) |
904L Pipe + Fittings |
B677, B366 |
| Phosphoric Acid Process (Africa) |
904L Plate + Forgings |
B625, B564 |
| Aggressive Chloride System (Europe) |
904L Flanges + Pipe |
B564, B677 |
We advise 904L vs 254SMO selection and supply matching ERNiCrMo-3 (625) filler.
| FREQUENTLY ASKED QUESTIONS |
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FAQ — Stainless Steel 904L
| What is 904L best at? |
| Resisting sulfuric and phosphoric acids thanks to its 1–2% copper, plus chloride pitting/crevice corrosion (PREN 36–38) well above 316L. It bridges 316L and nickel alloys. |
| Is 904L better than 316L? |
| Yes for aggressive acid and chloride service: PREN 36–38 vs ~26, and copper improves sulfuric-acid resistance. The trade-off is higher cost (more nickel) and lower strength. |
| Does 904L need post-weld heat treatment? |
| No. Ultra-low carbon (≤0.020%) prevents sensitization, so welded fabrications stay corrosion-resistant as-welded. |
| What filler is used for 904L? |
| ERNiCrMo-3 (Inconel 625 type) is standard to match corrosion performance. Using 316L filler would undermatch and fail first. |
| When should I step up to 254SMO? |
| When chlorides are extreme — warm seawater, high chloride with temperature — because 254SMO (PREN 42–45) and AL6XN (PREN 45–48) outlast 904L in those conditions. |
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Need Stainless Steel 904L for Aggressive Service?
HT PIPE supplies 904L (UNS N08904) pipe, flanges, fittings, plate, and bar for sulfuric-acid and aggressive chloride service with full MTR documentation. Contact our technical team for material selection and welding advice.
Request a Quote |
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