Views: 2 Author: Monica Publish Time: 2026-06-29 Origin: Site
Table of Contents
Super Duplex S32750 is the most cost-effective high-performance alloy for chloride-rich seawater desalination. Its PREN value of ~42, yield strength of ~550 MPa, and proven service record in reverse-osmosis and multi-effect distillation plants make it a direct upgrade over 316L and a leaner alternative to 6Mo super-austenitic grades.
● PREN ≈ 42 → resists pitting/crevice corrosion in aerated seawater up to ~35–40 °C.
● Yield strength ~550 MPa → allows thinner wall sections and lower system weight than austenitic grades.
● Used in SWRO high-pressure piping, seawater intake risers, brine heaters, and LT-MED evaporators.
● Must be welded with controlled heat input to preserve corrosion resistance.
Property | S32750 (Super Duplex 2507) |
Typical PREN | ~42 |
Min. Yield Strength | ~550 MPa |
Tensile Strength | 750–800 MPa |
Critical Pitting Temp. (CPT) | ~75–80 °C (lab) |
Relative Material Cost | ~3–4× 316L |
Super Duplex S32750—also known as Alloy 2507 or 1.4410—is a 25Cr-7Ni-4Mo-0.27N stainless steel whose microstructure is roughly 50 % austenite and 50 % ferrite. That dual-phase structure delivers roughly twice the strength of standard austenitic grades and a PREN above 40, the threshold engineers use to separate "seawater-safe" alloys from marginal ones.
It's composition as follow:
Element | Weight % | Function |
Chromium (Cr) | 24.0–26.0 | Forms the passive oxide film that blocks rust. |
Nickel (Ni) | 6.0–8.0 | Stabilizes austenite phase and improves toughness. |
Molybdenum (Mo) | 3.0–5.0 | Raises resistance to pitting and crevice attack. |
Nitrogen (N) | 0.24–0.32 | Strengthens both phases and boosts PREN. |
Iron (Fe) | Balance | Base metal. |
PREN (Pitting Resistance Equivalent Number) is calculated as %Cr + 3.3×%Mo + 16×%N. For S32750, this lands near 42, well above the ~24–26 of 316L and comparable to 6Mo super-austenitic grades such as S31254.
The ferrite-austenite mix also gives S32750 a yield strength around 550 MPa—about three times that of 316L. In practical terms, a desalination pipe can be made with a thinner wall, reducing weight and weld length without sacrificing pressure rating.
S32750 resists the three failure modes that kill ordinary stainless steel in desalination plants: chloride pitting, crevice corrosion, and stress-corrosion cracking—while costing less over a 20-year lifecycle than nickel-base or 6Mo alternatives.
Seawater is not just "salty water." It contains ~19,000 ppm chloride, dissolved oxygen, microorganisms, and abrasive solids. In reverse-osmosis (SWRO) plants, pressures reach 55–70 bar. In low-temperature multi-effect distillation (LT-MED) plants, brine concentrates further as it evaporates. Both environments demand more than 316L can reliably deliver.
A 2020 Springer corrosion study compared S32750 with standard duplex S32205 in simulated LT-MED evaporator conditions. The result: S32750 showed fewer pitting traces than S32205, and its pitting potential dropped more slowly as temperature and salinity increased. In other words, S32750 keeps its protective film where lesser alloys begin to break down.
Major desalination contractors specify S32750 for:
●High-pressure RO feed and permeate piping
●Seawater intake and brine-discharge risers
●Energy-recovery-device manifolds
●LT-MED evaporator tube bundles and brine heaters
●Firefighting water systems on offshore platforms and plant sites
For cold-to-moderate seawater under mechanical load, S32750 is usually the sweet spot: stronger than 316L and 904L, cheaper than S31254 or Inconel 625, and corrosion-resistant enough for continuous seawater service.
Alloy / UNS | PREN | Yield (MPa) | CPT (°C) | Best Use Case | Relative Cost |
316L / S31603 | 24–26 | ~170 | ~15 | Freshwater, splash, short-term exposure | 1× |
904L / N08904 | 35–38 | ~220 | 45–55 | Warm, moderate-salinity process streams | ~3× |
S32750 / 2507 | ~42 | ~550 | 75–80 | High-pressure seawater, structural piping | ~3–4× |
S31254 / 6Mo | 42–44 | ~300 | 70+ | Warm seawater, chemical compatibility needed | ~4–5× |
Inconel 625 / N06625 | >50 | ~414 | >85 | Hot seawater, sour service, splash zones | ~8–10× |
If the service is continuous seawater and the temperature stays below ~35–40 °C, S32750 gives you most of the corrosion resistance of 6Mo grades at roughly two-thirds the material cost—plus a large strength advantage.
Choose 6Mo or Inconel 625 only when the process temperature, chloride concentration, or sour-gas risk justifies the premium.
S32750 used wherever seawater is handled under pressure or where downtime is expensive: intake systems, high-pressure RO trains, brine handling, and evaporator bundles.
Plant Section | Typical Components | Why S32750? |
Seawater intake | Risers, screening pipes, pump columns | Resists biofouling-assisted crevice corrosion and wave fatigue. |
SWRO high-pressure loop | HP feed piping, manifolds, valves, flanges | Strength handles 55–70 bar; PREN resists concentrated brine. |
Energy recovery | PX / isobaric device manifolds | Fatigue strength and corrosion resistance in pulsating flow. |
LT-MED / MSF | Evaporator tubes, brine heaters, flash chambers | Lower pitting tendency than S32205 in warm brine. |
Utility systems | Seawater cooling, firewater lines | Thin-wall, high-strength piping reduces support steel and installation cost. |
S32750 is covered by globally recognized ASTM, ASME, EN, and NORSOK specifications. Specifying the right product form and condition is as important as selecting the grade itself.
Standard | Product Form | Key Requirement |
ASTM A240 / ASME SA-240 | Plate, sheet, strip | Chemical limits, annealing, mechanical properties. |
ASTM A790 / ASME SA-790 | Seamless and welded pipe | Duplex microstructure, corrosion testing optional. |
ASTM A182 / ASME SA-182 | Forged fittings, flanges | Forging ratio, heat treatment, hardness limits. |
ASTM A479 / ASME SA-479 | Bars and shapes | Mechanical properties for bolting and structural use. |
EN 10088-2 / 10028-7 | European plate and sheet | 1.4410 designation, CE marking support. |
NORSOK M-630 / MDS D55 | Offshore project use | Tight chemistry, impact, and corrosion test requirements. |
NACE MR0175 / ISO 15156 | Sour service | Limits on hardness and temperature for H₂S exposure. |
When buying material for a desalination project, request certificates showing actual PREN, ferrite content (typically 35–55 %), and—where required—ASTM G48 corrosion test results (Method A or C).
Yes. Although the alloy costs more per kilogram than 316L, its higher strength and longer corrosion-free life usually produce the lowest total installed and operating cost for critical seawater circuits.
Lifecycle cost has three parts: material, fabrication/installation, and maintenance. S32750 affects all three:
●Material: ~3–4× the per-kilogram price of 316L.
●Fabrication: Thinner walls mean less welding wire, fewer supports, and faster erection.
●Maintenance: Fewer leaks, inspections, and emergency shutdowns over 20+ years.
For a high-pressure SWRO train handling millions of cubic metres of water per year, a single unplanned shutdown can cost more than the entire alloy upgrade. That is why owners increasingly view S32750 not as a premium material, but as insurance against corrosion-related failure.
What does PREN > 40 mean in practice?
PREN > 40 signals that the alloy is generally resistant to pitting and crevice corrosion in ambient seawater. It is the unofficial cutoff engineers use when specifying materials for continuous marine service.
Can S32750 replace 316L in an existing desalination plant?
Often yes, but not always directly. Because S32750 is stronger, wall thickness can usually be reduced. Any replacement must be re-rated for pressure, supports, and weld procedures. It is best treated as an upgrade project, not a like-for-like swap.
Is S32750 suitable for hot seawater (>50 °C)?
Its laboratory CPT is ~75–80 °C, but practical limits depend on chloride concentration, oxygen level, and crevice geometry. For sustained service above ~40 °C with crevices, many engineers step up to 6Mo super-austenitic or nickel-base alloys.
What welding filler is used for S32750?
ER2594 is the most common matching filler for S32750. It restores the austenite-ferrite balance and maintains corrosion resistance in the weld metal.
How does S32750 compare with 2205 duplex?
S32750 has higher chromium, molybdenum, and nitrogen than 2205 (S32205), giving it higher PREN, strength, and pitting resistance. In seawater desalination, S32750 is the safer choice when chloride exposure is continuous or severe.
Need S32750 plate, pipe, fittings, or bar for your desalination project? Contact our team for mill test reports, NORSOK/ASTM-certified stock, and technical support on welding and fabrication.
