AISI 316L Stainless Steel (UNS S31603): Marine-Grade Properties vs UNS N08904 (904L) Super Austenitic Stainless Steel
Side-by-side engineering comparison of AISI 316L Stainless Steel (UNS S31603): Marine-Grade Properties (ASTM A240/A276) and UNS N08904 (904L) Super Austenitic Stainless Steel (ASTM A240/B625). Compare mechanical properties, chemical composition, density, yield strength, international equivalents, and typical applications to select the right material for your project.
Quick Verdict
Start with the corrosive environment. For marine, coastal, food, and moderate chloride service, 316L is the cost-effective, universally stocked workhorse (PREN 24-28)....
Lower cost and universal availability for general chloride service
Maximum pitting resistance for aggressive chemical and seawater environments
Quick Comparison
| Property | AISI 316L Stainless Steel | UNS N08904 (904L) Super A |
|---|---|---|
| Standard | ASTM A240/A276 | ASTM A240/B625 |
| Category | Stainless Steel | Stainless Steel |
| Density | 8.00 g/cm³ | 8.00 g/cm³ |
| Yield Strength | 170 MPa (25 ksi) | 220 MPa (32 ksi) |
| Tensile Strength | 485 MPa (70 ksi) | 490 MPa (71 ksi) |
| Key Applications | AISI 316L (UNS S31603) is a low-carbon (≤0.03%) austenitic stainless steel with 16-18% chromium, 10-14% nickel, and 2-3%... | UNS N08904 (904L) is a high-alloy austenitic stainless steel with elevated nickel (23-28%), chromium (19-23%), molybdenu... |
International Equivalents
| AISI 316L Stainless Equivalents | UNS N08904 (904L) Su Equivalents |
|---|---|
| EN 1.4404 | EN 1.4539 |
| X2CrNiMo17-12-2 | X1NiCrMoCu25-20-5 |
| SUS316L | SUS890L |
| 022Cr17Ni12Mo2 | 00Cr20Ni25Mo4.5Cu |
| AISI 316L | 904L |
How to Choose
| Choose AISI 316L Stainless Steel when... | 316L stainless steel delivers PREN 24-28 with 2-3% molybdenum, adequate for marine, coastal, and moderate chemical environments at moderate cost. |
| Choose UNS N08904 (904L) Super A when... | 904L (UNS N08904) is a super-austenitic stainless with 4.5% molybdenum, 1.5% copper, and high nickel (23-25%), achieving PREN 29-33 plus exceptional resistance to sulfuric acid and chloride stress corrosion cracking. |
Decision Checklist: Which Is Right for You?
| Criterion | AISI 316L Stainless | UNS N08904 (904L) Su |
|---|---|---|
| Lower Cost And Universal Availability For General Chloride Service | ✓ | |
| Maximum Pitting Resistance For Aggressive Chemical And Seawater Environments | ✓ | |
| Higher yield strength | ✓ | |
| Cost — 904L costs 2.5-4× more than 316L; reserve it for environments t... | ✓ | |
| Pitting resistance — 904L's PREN (29-33) exceeds 316L's (24-28) for ag... | ✓ | |
| Uniform corrosion — 904L's copper addition excels in sulfuric acid; 31... | ✓ |
Selection Guide
Start with the corrosive environment. For marine, coastal, food, and moderate chloride service, 316L is the cost-effective, universally stocked workhorse (PREN 24-28). Upgrade to 904L (UNS N08904, super-austenitic) only when the environment defeats 316L — concentrated or hot sulfuric acid, severe chloride pitting, seawater immersion where 316L pits, or where a single corrosion failure outweighs the 2.5-4× cost premium. Rule of thumb: if 316L has failed in your process, 904L is the first logical upgrade; if it has not, save the money.
Key Decision Factors
- Cost — 904L costs 2.5-4× more than 316L; reserve it for environments that actually defeat 316L
- Pitting resistance — 904L's PREN (29-33) exceeds 316L's (24-28) for aggressive chlorides, seawater, and acid service
- Uniform corrosion — 904L's copper addition excels in sulfuric acid; 316L does not
- Availability — 316L is stocked universally in all forms; 904L has longer lead times and limited stock
When to Use Each
Use AISI 316L Stainless for:
316L's 2-3% molybdenum resists chloride pitting for coastal balustrades, boat fittings, and offshore topsides within its PREN (24-28) envelope.
316L is the ASME BPE food-grade standard for mixers, storage tanks, and piping carrying food acids and CIP chemicals.
316L is standard for clean room and purified water (WFI) piping, where its pitting resistance and smooth finishes meet GMP requirements.
Use UNS N08904 (904L) Su for:
904L's 1.5% copper and high nickel (23-25%) provide exceptional resistance to hot sulfuric acid — the standard choice for acid plants and sulfur handling where 316L corrodes rapidly.
With PREN 29-33, 904L resists chloride pitting and crevice corrosion in seawater with better margins than 316L, used where duplex or super-duplex is not warranted.
904L's high nickel and molybdenum provide resilience to chloride-induced SCC — specified for process internals and plates in aggressive chloride streams.
Frequently Asked Questions
What is the main difference between AISI 316L Stainless Steel (UNS and UNS N08904 (904L) Super Austen?
AISI 316L Stainless Steel (UNS S31603): Marine-Grade Properties (ASTM A240/A276) provides 170 MPa (25 ksi) yield strength at 8.00 g/cm³ density, while UNS N08904 (904L) Super Austenitic Stainless Steel (ASTM A240/B625) delivers 220 MPa (32 ksi) at 8.00 g/cm³. The choice depends on whether your application prioritizes lower cost and universal availability for general chloride service or maximum pitting resistance for aggressive chemical and seawater environments.
Can AISI 316L Stainless Steel (UNS be substituted for UNS N08904 (904L) Super Austen?
In many applications, these materials can be cross-referenced, but direct substitution should always be verified against specific project specifications, especially for lower cost and universal availability for general chloride service, maximum pitting resistance for aggressive chemical and seawater environments, and operating environment. Consult your engineer of record.
Is 904L stronger than 316L?
No. 316L has higher yield strength (170-210 MPa) than 904L (220 MPa is actually 904L's; both are austenitic with similar design allowables for most applications). The reason to choose 904L is NOT strength — it is corrosion resistance. 904L wins on resistance to sulfuric acid, chloride pitting, and crevice corrosion, not on mechanical properties. For strength-critical corrosive service, consider duplex 2205 (450 MPa yield) instead.
When is 904L the right upgrade from 316L?
Upgrade to 904L when 316L pits or suffers chloride SCC in service, when the process handles hot concentrated sulfuric acid, or when a single corrosion-induced failure (downtime, contamination, leak) costs more than 2.5-4× the material premium. Common trigger industries: chemical processing, pulp & paper bleach plants, seawater systems, and acid recovery. If 316L is performing fine, you do not need 904L.
Is 904L suitable for seawater piping?
Yes, but judge margins carefully. 904L (PREN 29-33) resists seawater pitting better than 316L but is not immune. For continuous seawater immersion with high velocities or crevices, super-duplex 2507 (PREN 42+) or 6Mo super-austenitics offer higher PREN. 904L is a reasonable, cost-moderate choice for moderately aggressive seawater where 316L fails but super-duplex is over-specified.