Duplex vs Super Duplex Steel: Which Should You Specify?
By Ragnar Metals & Tubes Engineering Team • May 20, 2026 • 6 min read
If you are specifying piping or fittings for a demanding environment — offshore, chemical processing, or seawater desalination — you have likely asked: should I use Grade 2205 duplex or Grade 2507 super duplex? This guide answers that question with numbers, not guesswork.
What Is Duplex Stainless Steel?
Duplex stainless steels have a mixed (duplex) microstructure of approximately 50% austenite and 50% ferrite. This two-phase structure gives them two major advantages over standard austenitic grades like SS 316L:
- Roughly twice the yield strength — so thinner walls are possible, reducing material cost and weight.
- Much better resistance to chloride stress-corrosion cracking (SCC) — the failure mode that destroys SS 316L in seawater and chloride-rich environments.
Grade 2205 (Duplex) — UNS S31803/S32205
Grade 2205 is by far the most widely used duplex steel globally. Key data:
| Property | Value |
|---|---|
| UNS designation | S31803 / S32205 |
| PREN (Pitting Resistance Equivalent Number) | > 34 |
| Yield strength (min) | 450 MPa |
| Tensile strength (min) | 655 MPa |
| Relevant standards | ASTM A790 (pipe), A815 (fittings), A182 F51 (flanges) |
| Max chloride service (approx.) | Up to ~25,000 ppm at ambient temperature |
Best for: Process plant piping, heat exchanger shells, desalination pre-treatment, pulp & paper, bridges and structural applications where weight matters.
Grade 2507 (Super Duplex) — UNS S32750
Grade 2507 pushes both the corrosion resistance and strength envelope further:
| Property | Value |
|---|---|
| UNS designation | S32750 |
| PREN | > 41 |
| Yield strength (min) | 550 MPa |
| Tensile strength (min) | 795 MPa |
| Relevant standards | ASTM A790 (pipe), A815 WP2507 (fittings), A182 F55 (flanges) |
| Max chloride service (approx.) | Seawater at elevated temperatures; >35,000 ppm |
Best for: Subsea flowlines, offshore topsides, seawater lift pumps, FPSOs, chemical injection systems, aggressive acid-gas wells (NACE MR0175 compliant).
Grade 2205 Duplex Pipe
Grade 2507 Super Duplex Pipe
The PREN Number — What It Actually Means
PREN = %Cr + 3.3 × %Mo + 16 × %N. The higher the number, the better the pitting corrosion resistance in chloride environments. As a rule of thumb:
- PREN > 25: suitable for moderately aggressive environments
- PREN > 34: suitable for seawater service (Grade 2205)
- PREN > 40: suitable for hot seawater and aggressive sour-gas wells (Grade 2507)
Cost Comparison
Super duplex 2507 carries a price premium of approximately 30–50% over duplex 2205, primarily due to its higher molybdenum (4%) and nitrogen content. For most applications where 2205 is technically adequate, the cost savings of 2205 are significant over the project lifecycle.
Quick Decision Guide
| Situation | Recommended Grade |
|---|---|
| Seawater at ambient temperature | 2205 or 2507 — both are suitable |
| Hot seawater (>35°C) or aggressive brine | 2507 |
| Chemical plant (HCl, H2SO4 at moderate concentration) | 2205 |
| Subsea, deep-sea flowlines | 2507 |
| Structural / bridges | 2205 (best strength-to-cost) |
| NACE MR0175 sour service (<4.5% H2S) | 2205 (hardness-controlled) |
| Aggressive H2S + high chlorides simultaneously | 2507 |
Need Material Certification or a Quote?
Ragnar Metals & Tubes supplies both Grade 2205 and 2507 pipe, tube, fittings, flanges, and round bars with MTC 3.1, NACE MR0175 compliance, and third-party inspection (SGS/BV/DNV). Call or message for lead times and pricing.
WhatsApp Send EnquiryRelated pages: Duplex Steel Pipe • Super Duplex Pipe • Duplex Flanges
