How do you rank alloys for seawater service?
Seawater alloy selection is usually decided by one number — the pitting resistance equivalent number, or PREN — combined with the maximum metal temperature the part will see. PREN is calculated from the composition as Cr + 3.3 × Mo + 16 × N, and experience in marine service has produced a simple working rule: grades below PREN 40 will suffer pitting and crevice corrosion in aerated seawater, and grades above 40 perform reliably. Temperature moves the threshold, because chloride pitting accelerates strongly above about 30 °C.
| Alloy | UNS | PREN (approx.) | Flowing seawater | Stagnant or chlorinated |
|---|---|---|---|---|
| 316L | S31603 | 24 | Not recommended | Fails — crevice attack |
| Duplex 2205 | S32205 | 35 | Moderate duty, below 30 °C | Risky above 30 °C |
| Super duplex 2507 | S32750 | 42 | Suitable | Suitable with care |
| 904L | N08904 | 35 | Moderate | Marginal |
| 254SMO (6Mo) | S31254 | 43 | Suitable | Suitable |
| Monel 400 | N04400 | — | Excellent, different mechanism | Excellent |
| Inconel 625 | N06625 | 45 | Excellent | Excellent |
| Titanium Grade 2 | R50400 | — | Effectively immune | Effectively immune |
| Titanium Grade 5 | R56400 | — | Effectively immune | Effectively immune |
Why does 316L fail in seawater?
316L (UNS S31603) is an austenitic stainless steel protected by a passive chromium oxide film, and chloride ions break that film down locally. In seawater the breakdown starts as crevice corrosion under gaskets, bolt heads and deposits, and it begins at metal temperatures as low as 25 to 30 °C. This is why the failures are so often found under a flange gasket rather than on an open, flowing surface.
- Crevice corrosion: the dominant failure mode, typically under deposits and joints
- Pitting: initiates above roughly 30 °C, faster in chlorinated seawater
- Stress corrosion cracking: a risk where the part is both stressed and warm
- Biofouling: creates oxygen-depleted zones that accelerate crevice attack
When is duplex stainless steel the right answer?
Duplex 2205 (UNS S32205) is the standard upgrade and is widely used for splash zones, firewater systems, and moderate-temperature seawater piping. Its PREN of roughly 35 sits just under the working threshold, so it is specified where temperatures stay below about 30 °C and the surfaces are kept clean. Super duplex 2507 (UNS S32750) crosses the threshold with a PREN above 40 and also brings roughly double the yield strength of 316L, which lets designers reduce wall thickness on high-pressure seawater lines.
- Duplex 2205: splash zone, firewater, seawater piping below 30 °C
- Super duplex 2507: flowing seawater, subsea manifolds, desalination high-pressure lines
- Both grades require correct welding and heat balance — the ferrite-austenite ratio must be controlled
- Both are susceptible to hydrogen embrittlement if cathodically protected incorrectly
Why do nickel alloys and titanium beat the stainless grades?
Because they change the mechanism rather than just slowing it down. Monel 400 (UNS N04400), a nickel-copper alloy, resists seawater by forming a protective film that is stable in chlorides and is one of the few materials that performs well in high-velocity seawater. Inconel 625 (UNS N06625) combines a very high PREN with enough molybdenum and niobium to resist both pitting and crevice attack. Titanium Grade 2 (UNS R50400) is effectively immune to seawater corrosion at ambient temperature, and its oxide film reforms instantly if damaged.
- Monel 400: pump shafts, valves, marine fasteners, high-velocity seawater
- Inconel 625: seawater systems where high strength and crevice resistance are both needed
- Titanium Grade 2: heat exchanger tubing, desalination plant, where life-cycle cost dominates
- Titanium Grade 5 (Ti-6Al-4V): same corrosion resistance with structural strength
What about galvanic corrosion and velocity?
Seawater systems rarely fail from a single material choice. Galvanic effects between dissimilar metals, and erosion-corrosion at high flow velocities, decide the outcome as often as PREN does.
| Situation | Risk | Mitigation |
|---|---|---|
| Stainless coupled to titanium or graphite | Accelerated attack of the less noble metal | Insulate the joint, or use a compatible pair |
| Flow above about 3 m/s in copper alloys | Erosion-corrosion | Move to Monel 400 or titanium |
| Stagnant seawater in 316L pipework | Crevice and under-deposit attack | Drain during shutdowns, or upgrade to 6Mo |
| Chlorinated seawater | Pitting and crevice attack accelerate | Super duplex, 254SMO, 625 or titanium |
| Cathodic protection with super duplex | Hydrogen embrittlement risk | Control potential, or specify 6Mo instead |
Which standards apply to seawater service materials?
The following standards cover the grades discussed here, and the corrosion test methods matter as much as the material specifications:
- ASTM G48 — pitting and crevice corrosion resistance test methods (used to verify PREN claims)
- ASTM A240 / A240M — chromium and nickel stainless steel plate, sheet and strip (316L, 2205, 2507, 904L)
- ASTM A276 — stainless steel bars and shapes for marine fasteners and shafts
- ASTM B446 — nickel-chromium-molybdenum-niobium alloy bar (Inconel 625)
- ASTM B164 — nickel-copper alloy bar and rod (Monel 400)
- ASTM B338 — seamless and welded titanium and titanium alloy tubes
- ASTM B265 — titanium and titanium alloy strip, sheet and plate
Frequently asked questions
Is 316L ever acceptable in seawater?
In practice it is used for intermittent, drained, low-temperature systems where experience in that specific service supports it — but it is not a safe default. Crevice corrosion under gaskets begins well below 30 °C, and the first inspection often finds a leaking joint rather than a visible pit. Most projects that start on 316L for seawater end up on 2205 or 2507.
Is titanium worth the cost for seawater piping?
For heat exchanger tubing and long-life systems, often yes. Titanium Grade 2 has a lower purchase price volatility, effectively unlimited seawater corrosion life and allows thin walls, which offsets the higher price per kilogram. Where the part also has to carry structural load, Grade 5 raises strength without losing corrosion performance.
What is the difference between Grade 2 and Grade 5 titanium?
Grade 2 (UNS R50400) is commercially pure titanium — excellent corrosion resistance, moderate strength, and the most weldable form. Grade 5 (UNS R56400, Ti-6Al-4V) is an alpha-beta alloy with roughly double the strength and the same seawater resistance, used where the part is structural. Gr2 for tubing and liners, Gr5 for stressed components.
Does Hangbo Alloy supply seawater service materials?
Hangbo Alloy supplies 316L, duplex 2205, super duplex 2507, 904L, 254SMO, Monel 400, Inconel 625 and titanium Grade 2 and Grade 5 as bar, plate, seamless tube, forgings and welding wire, with EN 10204 3.1 mill test certificates and third-party inspection by SGS, BV or TUV on request.