Duplex vs Super Duplex Stainless Steel: 2205 (S31803) vs 2507 (S32750) Selection Guide
Duplex stainless steels — austenitic-ferritic two-phase grades — sit between regular 316L and the high-Mo nickel alloys in the corrosion-resistance ladder. The choice almost always comes down to 2205 (UNS S31803/S32205) or its super-duplex sibling 2507 (UNS S32750/S32760). The PREN gap between them is roughly 7 points, but the price gap is 25-30%, so picking right is worth real money over the project life.
This guide pulls together the data engineers actually use during selection meetings: chemistry, mechanical and corrosion benchmarks, NORSOK and NACE envelopes, weldability and a 4-step decision tree. References from ASTM, IMOA and the major mills are at the bottom. Skip to the verdict for the bottom line.
Quick verdict — when 2507 beats 2205
Both grades resist chloride stress-corrosion cracking far better than 304/316. Super duplex extends the envelope on three axes: chloride concentration, temperature and H2S partial pressure.
| Service condition | Default choice | Why |
|---|---|---|
| Chemical-process aqueous, < 60 °C, < 1,000 ppm Cl⁻ | 2205 | PREN 35 is comfortably above the operating envelope |
| Chemical / process at 80-150 °C, 1,000-3,000 ppm Cl⁻ | 2507 | 2205 CPT (~50 °C) is exceeded; 2507 CPT (~75 °C) holds margin |
| Sub-sea oil & gas pipework | 2507 | NORSOK MDS D45; sour-service envelope is wider |
| Storage tank for brine, ambient | 2205 | Cost-effective; NORSOK MDS D44 covers it |
| FGD scrubber wet plenum | 2507 | Acid + chloride combination at 50 °C exceeds 2205 margin |
| Onshore separator vessel, mild sour | 2205 | Within MR0175 envelope at typical pressures |
| Seawater heat-exchanger tubing | 2507 | Higher PREN protects gasket-line crevices |
Push to 2507 when chlorides exceed 1,000 ppm and temperature exceeds 60 °C, or whenever NORSOK MDS D45 is specified. Otherwise 2205 saves the budget without sacrificing safety.
Chemistry & PREN
| Element (wt %) | 2205 (UNS S32205) | 2507 (UNS S32750) |
|---|---|---|
| Chromium (Cr) | 22.0 – 23.0 | 24.0 – 26.0 |
| Nickel (Ni) | 4.5 – 6.5 | 6.0 – 8.0 |
| Molybdenum (Mo) | 3.0 – 3.5 | 3.0 – 5.0 |
| Nitrogen (N) | 0.14 – 0.20 | 0.24 – 0.32 |
| Carbon (C) | ≤ 0.030 | ≤ 0.030 |
| Manganese (Mn) | ≤ 2.00 | ≤ 1.20 |
| Silicon (Si) | ≤ 1.00 | ≤ 0.80 |
The PREN formula PREN = Cr + 3.3·Mo + 16·N gives:
- 2205: PREN ≈ 35-38 (varies with N content)
- 2507: PREN ≈ 41-45 (the 0.30% N is critical)
Note that nitrogen contributes 16× per percent — that is why super duplex grades push N to 0.30%. It also stabilises the austenite phase and bumps strength.
Mechanical properties
| Property (annealed plate, room temp) | 2205 | 2507 | For reference: 316L |
|---|---|---|---|
| Yield strength 0.2% (min) | 450 MPa / 65 ksi | 550 MPa / 80 ksi | 170 MPa / 25 ksi |
| Tensile strength (min) | 620 MPa / 90 ksi | 800 MPa / 116 ksi | 485 MPa / 70 ksi |
| Elongation in 50 mm | 25% | 15% | 40% |
| Hardness (Brinell, max) | 293 HB | 310 HB | 217 HB |
| Charpy impact at –46 °C | ≥ 60 J | ≥ 45 J | ≥ 60 J |
| Density | 7.80 g/cm³ | 7.79 g/cm³ | 8.00 g/cm³ |
Duplex grades have ~2× the yield strength of 316L, which often allows 20-30% wall-thickness reduction in pressure vessels. That alone can offset the material premium on large fabrications.
Corrosion: CPT, CCT, SCC
| Test | 2205 | 2507 |
|---|---|---|
| CPT in 6% FeCl₃ (ASTM G48 A) | ≈ 35-50 °C | ≈ 75-85 °C |
| CCT in 6% FeCl₃ (ASTM G48 B) | ≈ 20 °C | ≈ 45 °C |
| SCC in 33% LiCl boiling | Pass (no cracks at 280 hr) | Pass |
| SCC in chloride at 200 °C, drop test | Marginal | Pass |
| Resistance to CO₂ (sweet service) | Excellent | Excellent |
Sour-service envelope
Per NACE MR0175 / ISO 15156:
- 2205 (S32205): typical limits 200 °F (93 °C), pH ≥ 4.5, H₂S partial pressure ≤ 100 mbar (1.5 psi). Slightly different for cold-worked material.
- 2507 (S32750): typical limits 450 °F (232 °C) — but practically 290 °F (143 °C) is the design limit because of impact-toughness considerations, pH ≥ 3.5, H₂S partial pressure ≤ 350 mbar (5 psi).
The exact limits depend on the latest revision of the standard and the qualification package the mill provides. Always cross-check with the project metallurgist before specifying.
Weldability & fabrication
Duplex grades are sensitive to heat input. Out-of-spec welds form sigma phase (FeCr intermetallic) which is brittle and corrodes preferentially.
- Heat input: 2205 = 0.5-2.5 kJ/mm, 2507 = 0.5-1.5 kJ/mm. Lower for super duplex because the upper bound for sigma precipitation is tighter.
- Inter-pass temp: 150 °C (2205), 100 °C (2507).
- Filler: ER2209 for 2205, ER2594 for 2507. Slightly higher Ni than parent metal to push the as-welded ferrite into the 30-50% target.
- Backing gas: pure argon or 95Ar/5N2 nitrogen-bearing for super duplex roots.
- Solution anneal after weld: usually skipped for routine joints, but mandatory for thick-wall sour-service tie-ins.
- Required testing: ASTM A923 method A or B on the weld metal and HAZ.
Cost and lead time
For 10 mm × 1500 mm × 6000 mm hot-rolled, solution-annealed plate FOB China, Q2 2026 ranges:
| Form | 2205 (USD/kg) | 2507 (USD/kg) | Premium |
|---|---|---|---|
| Plate, 10 mm | 4.8 – 5.5 | 6.4 – 7.5 | +25-35% |
| Sheet, 2 mm | 5.5 – 6.5 | 7.5 – 9.0 | +30-40% |
| Seamless pipe, sch 40S | 6.5 – 8.0 | 9.5 – 12.0 | +40-50% |
| Welded pipe (EFW), sch 10S | 5.0 – 6.5 | 7.5 – 9.5 | +45-55% |
Stock availability on 2205 plate up to 50 mm and pipe up to DN300 is good in major distribution hubs. 2507 is thinner-stocked above 30 mm thickness or DN200; expect 12-16 week mill rolling for non-stock sizes.
4-step selection flowchart
Application matrix
| Application | Default grade | Reason |
|---|---|---|
| Onshore process tanks (mild Cl⁻) | 2205 | PREN 35 is enough; cost wins |
| Offshore topsides piping | 2507 | NORSOK MDS D45; sour + Cl⁻ |
| Desalination MED evaporator tubes | 2507 | Hot brine + thermal cycling |
| Pulp digester housing | 2205 | Caustic + low Cl⁻; 2507 is overkill |
| Subsea umbilicals (Cu-bearing zones) | 2507 | Sour service + high Cl⁻ |
| Heat exchanger U-tubes, brackish water | 2205 | PREN 35 covers ambient brackish |
| FPSO produced-water injection lines | 2507 | H₂S + Cl⁻ + temperature |
| Bridge structural, marine atmospheric | 2205 | Cost vs PREN trade-off |
For more on related grades, see our duplex stainless steel range (2205, 2507, 2304, lean-duplex 2101). For a deeper materials primer, the IMOA Practical Guidelines and the Outokumpu duplex handbook are the two best free resources.
Frequently Asked Questions
Is 2507 super duplex always better than 2205?
What chloride concentration should push me from 2205 to 2507?
Can I weld 2205 to 2507 in the same joint?
Why is duplex limited to 300 °C?
Do duplex grades work in seawater above 50 °C?
Can I use duplex stainless in hydrochloric acid?
What standards cover 2205 and 2507 plate and pipe?
How do I verify there is no sigma phase?
References
- [1]ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip — ASTM International
- [2]ASTM A790 — Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe — ASTM International
- [3]ASTM A923 — Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels — ASTM International
- [4]NORSOK M-630 — Material Data Sheets and Element Data Sheets for Piping (MDS D44, D45, D55) — Standards Norway
- [5]IMOA Practical Guidelines for the Fabrication of Duplex Stainless Steel — International Molybdenum Association
- [6]NACE MR0175 / ISO 15156 — Materials for use in H2S-containing environments — AMPP / ISO
- [7]ASTM G48 — Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels — ASTM International
- [8]Pitting Resistance Equivalent Number (PREN) — Wikipedia — Wikipedia
- [9]Duplex stainless steel — Wikipedia — Wikipedia
- [10]Outokumpu Duplex Stainless Steel Handbook — Outokumpu
- [11]Sandvik SAF 2507 — technical datasheet — Sandvik Materials Technology
- [12]DNV-OS-F101 — Submarine pipeline systems (offshore code) — DNV
