Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex 2507 in Seawater Applications
Super Duplex 2507 (UNS S32750 / EN 1.4410) is the standard alloy for high-chloride seawater service where standard 2205 duplex and 6 percent molybdenum austenitic stainless steels are insufficient. The combination of 25 percent chromium, 3.5 to 4 percent molybdenum, and 0.24 to 0.32 percent nitrogen gives a PREN of 41 or higher and a critical pitting temperature above 50 degrees Celsius per ASTM G48 Method E. Critical crevice temperature above 35 degrees Celsius per ASTM G48 Method F.
In moving seawater up to about 6 metres per second velocity, super duplex 2507 resists pitting, crevice corrosion, and chloride stress corrosion cracking that typically defeats lower-grade stainless steels. TorqBolt supplies 2507 fasteners, bar, plate, pipe, fittings, flanges, and forgings to seawater system specifications worldwide, with mill test certificates per EN 10204 Type 3.1 (Type 3.2 third-party-witnessed available on request).
Why Super Duplex 2507 in Seawater Service
Seawater is an aggressive electrolyte: ~19000 ppm chloride, dissolved oxygen, biological activity (sulfate-reducing bacteria producing H2S in stagnant pockets), and velocity-induced erosion. Material selection for seawater piping and components is governed by:
- Pitting resistance (PREN, CPT per ASTM G48 Method E), 2507 sits at PREN 41+, CPT above 50 degrees Celsius
- Crevice corrosion resistance (CCT per ASTM G48 Method F), 2507 above 35 degrees Celsius
- Chloride stress corrosion cracking resistance, duplex microstructure resists CSCC much better than austenitic 316L
- Sulfide stress cracking when stagnant biological activity produces H2S, 2507 qualified per NACE MR0175 with hardness ≤ 28 HRC
- Erosion resistance at design velocities up to about 6 m/s
Super Duplex 2507 Seawater Design Data
| Parameter | Value | Reference |
|---|---|---|
| PREN | 41 to 43 typical | Cr + 3.3 times Mo + 16 times N |
| Critical pitting temperature (CPT) | ≥ 50 degrees Celsius | ASTM G48 Method E |
| Critical crevice temperature (CCT) | ≥ 35 degrees Celsius | ASTM G48 Method F |
| Maximum design velocity | About 6 m/s (continuous), higher for short transients | NORSOK M-001 guidance |
| Maximum continuous service temperature | 250 degrees Celsius (sigma-phase risk above 300) | NORSOK M-630 |
| Hardness limit (sour seawater) | 28 HRC max | NACE MR0175 / ISO 15156-3 |
| Ferrite content | 35 to 55 percent (base), 35 to 65 percent (weld) | NORSOK M-630 |
Where Super Duplex 2507 Is Specified in Seawater Systems
- Offshore platform sea-water lift systems, caisson piping, lift pumps, sea-chest, fire-water ring main
- Ship and FPSO sea-chest and ballast piping, replacing 90/10 copper-nickel where biofouling and erosion are issues
- Subsea cooling-water piping, manifolds, jumpers, distribution headers
- Seawater scrubbers in FGD, wet absorber inlet ducts and quenchers handling chloride-rich brine (cross-link to FGD applications)
- Reverse osmosis desalination high-pressure brine piping, covered in detail at desalination applications
- Heat exchanger tubesheets and channels, for fire water pre-heating and process cooling
- Subsea wellhead christmas trees and manifolds, covered at marine and offshore
Super Duplex 2507 vs Competing Alloys in Seawater
| Alloy | PREN | CPT (G48 E) | Cost Index | Seawater Verdict |
|---|---|---|---|---|
| 316L (austenitic) | ~25 | About 15 degrees Celsius | 1.0 | Pitting + CSCC failure typical; not recommended |
| 2205 (standard duplex) | 34 to 38 | About 30 degrees Celsius | 1.5 to 1.8 | Borderline; OK for cool seawater, marginal at tropical temperatures |
| 6Mo (254 SMO, AL6XN) | 43 to 45 | About 60 degrees Celsius | 2.5 to 3.0 | Excellent but more expensive than 2507 |
| Super Duplex 2507 | 41 to 43 | ≥ 50 degrees Celsius | 2.0 to 2.5 | Best value at typical seawater temperatures and velocities |
| Zeron 100 (S32760) | 41 to 44 | ≥ 50 degrees Celsius | 2.2 to 2.7 | Equivalent performance, slight Cu / W chemistry difference (see 2507 vs Zeron 100) |
| Inconel 625 (Ni-base) | 50+ | ~ 80 degrees Celsius | 4.0 to 5.0 | Used only when seawater is hot AND aggressive (high temperature, chlorination); 2507 cheaper at typical conditions |
Super Duplex 2507 Installation Notes for Seawater Service
- Welding per NORSOK M-601 with filler ER2594 (overmatching) or ER2553 (matching); heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius, see welding procedure
- Pickling and passivation after fabrication: 20 percent nitric + 5 percent HF, then citric or nitric passivation. Pre-existing iron contamination from carbon-steel tooling must be removed before commissioning.
- Cathodic protection compatibility, 2507 can be coupled to less-noble metals (carbon steel, 316L) in seawater without dissolving them, but isolating CP zones avoids hydrogen embrittlement of high-strength bolting
- Chlorination tolerance, 2507 tolerates continuous free chlorine up to about 1 to 2 ppm at ambient seawater temperature; higher dosing risks pitting at long-term exposure
- Crevice management, gasketed flanges should use spiral-wound CG-style PTFE-filled or graphite-filled gaskets to minimise crevice geometry
Recommended TorqBolt 2507 Products for Seawater Applications
- 2507 pipe, ASTM A790 seamless and welded for sea-water headers and risers
- 2507 tube, ASTM A789 for heat exchangers
- 2507 fittings, ASTM A815 WP-S32750 buttweld elbows, tees, reducers
- 2507 flanges, ASTM A182 F53 weld neck and blind
- 2507 stud bolts, ASTM A1082 / API 20F for pipe-flange connections in seawater
- 2507 heavy hex nuts, ASTM A1082 in matching S32750 chemistry, 8MA dimensional pattern
- 2507 flat washers, UNS S32750 chemistry (F436 dimensional pattern in S32750 by special order)
- 2507 plate, for tank construction, sea-chest plate
Related Application Pages
- Oil and gas, sour-service compliance per NACE MR0175
- Desalination, high-pressure brine in MSF, MED, RO plants
- Marine and offshore, subsea hardware, christmas trees
- FGD, wet flue gas desulfurization
- Chemical processing, dilute HCl, H2SO4, HF service
Super Duplex 2507 Seawater FAQ
Why is Super Duplex 2507 specified for seawater service?
Because PREN above 41 and CPT above 50 degrees Celsius (per ASTM G48 Method E) give the alloy enough margin against pitting and crevice corrosion in full-strength seawater. Standard 2205 duplex (PREN 34 to 38) and austenitic 316L (PREN ~25) suffer pitting and chloride stress corrosion cracking at typical seawater temperatures.
What is the maximum seawater velocity for Super Duplex 2507 piping?
About 6 metres per second continuous, with brief transients at higher velocity. NORSOK M-001 provides design guidance. Excessive velocity causes erosion-corrosion and removes the protective passive film faster than it regenerates.
Can Super Duplex 2507 handle chlorinated seawater?
Yes, up to about 1 to 2 ppm continuous free chlorine at ambient seawater temperature. Higher chlorination doses or hot chlorinated seawater (above 30 degrees Celsius) approach the pitting threshold and require more conservative material selection (6Mo or Inconel 625).
Is Super Duplex 2507 better than 6Mo (254 SMO, AL6XN) for seawater?
Both perform well in seawater. 6Mo has slightly higher PREN (43 to 45 vs 41 to 43) and higher CPT (~60 vs ~50 degrees Celsius), but costs 25 to 50 percent more. For typical ambient-to-warm seawater service, 2507 delivers equivalent service life at lower installed cost. 6Mo is preferred where seawater temperature exceeds 50 degrees Celsius or chloride concentration is elevated.
Can Super Duplex 2507 fasteners be used in seawater?
Yes. Use stud bolts and heavy hex nuts to ASTM A1082/A1082M in UNS S32750 (the dedicated standard for super duplex bolting) and API 20F for petroleum and natural gas service. Hardness must be 28 HRC maximum for NACE MR0175 sour-service compliance. PTFE or molybdenum disulfide thread lubricant is recommended to prevent galling on assembly.
What welding filler should I use for Super Duplex 2507 in seawater?
ER2594 (overmatching, slightly higher PREN than the base) is the most common choice for seawater service per AWS A5.9. ER2553 (matching) is acceptable for less aggressive applications. Heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius. Argon plus 2 to 5 percent nitrogen shielding for GTAW root pass.
What surface finish is recommended for Super Duplex 2507 in seawater?
Pickled and passivated finish per ASTM A380. After fabrication, full pickling (20 percent nitric + 5 percent HF) removes any iron contamination from carbon-steel tools and restores the chromium-rich passive film. Then citric or nitric passivation. Avoid mechanical polishing finer than Ra 0.8 micron, which can trap chloride films.
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