Stainless Steel and Nickel Alloy Pipes for Marine Engineering and Desalination
Recommended Pipe & Tube Materials
Seawater contains about 19,000 mg/L of chloride, making it one of the most aggressive natural environments for metals. Standard 304 and 316 stainless steels often suffer pitting and crevice corrosion in seawater service. This article explains which seawater-resistant stainless steel, super duplex and nickel alloy pipes are used for offshore platforms, ships and desalination plants.
The Corrosion Challenges of Seawater
- Pitting corrosion: small, deep pits that can perforate a pipe wall
- Crevice corrosion: attack under gaskets, flanges and deposits
- Chlorination and elevated temperature: biocide dosing and warm seawater increase corrosion risk
- Stagnant conditions: low-flow or standby lines are much more prone to attack than flowing seawater
Using PREN to Compare Materials
PREN = %Cr + 3.3 × %Mo + 16 × %N. As a working rule, seawater service generally calls for materials with PREN above about 40.
| Material | Typical UNS | Approx. PREN | Seawater suitability |
|---|---|---|---|
| 316L | S31603 | 23-28 | Limited; not for stagnant or warm seawater |
| Duplex 2205 | S32205 | 34-36 | Borderline; ambient, well-flushed systems only |
| 904L | N08904 | 32-36 | Limited in warm seawater |
| Super duplex 2507 | S32750 | 40+ | Good; widely used |
| 6Mo (254 SMO) | S31254 | 42-45 | Good; widely used |
| Alloy 625 | N06625 | 50+ | Excellent, including hot seawater |
Typical Applications
Offshore Platforms and FPSOs
Fire-water systems, seawater lift and injection lines, cooling water, produced-water handling and hydraulic control lines. Super duplex 2507 is the workhorse thanks to its high strength (thinner walls, lower topside weight) and chloride resistance.
Shipbuilding
Ballast water treatment systems, scrubber (exhaust gas cleaning) piping, cooling and fire-fighting lines. Grades such as 2205, 2507 and 6Mo are used depending on temperature and chlorination.
Seawater Desalination (RO, MSF, MED)
- Reverse osmosis (RO): high-pressure feed and brine lines are commonly duplex 2205 or super duplex 2507; low-pressure permeate lines may use 316L.
- MSF / MED thermal plants: heat exchanger tubes and brine piping use super duplex, 6Mo, and nickel alloys where temperatures and chlorine levels are high.
Coastal and Subsea Structures
Subsea manifolds, umbilicals and instrumentation tubing typically use super duplex or Alloy 625 tubing.
Recommended Standards
- ASTM A790 / A928 (duplex pipe), ASTM A789 (duplex tube)
- ASTM A312 (6Mo austenitic pipe), ASTM A269 (tube)
- ASTM B444 (Alloy 625 pipe and tube)
- NORSOK M-650 and project-specific specifications for qualified manufacturers, where required
Selection and Installation Tips
- Define the maximum temperature, chlorination level and whether the line is ever stagnant
- Use welding procedures suited to duplex and 6Mo to maintain the ferrite/austenite balance and avoid intermetallic phases
- Pickle and passivate welds to restore the corrosion-resistant surface
- Avoid crevices in flange and gasket design, and flush or drain standby lines
- Email: info@duplex-stainless-steel.com
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Frequently Asked Questions
Q1: Can 316L stainless steel pipe be used for seawater?
Only in limited cases, for example short-term, low-temperature or well-flushed systems. For continuous seawater service, super duplex 2507, 6Mo or nickel alloys are more reliable.
Q2: What is the difference between super duplex 2507 and 254 SMO?
2507 is a duplex steel with roughly twice the yield strength of austenitic 6Mo, allowing thinner walls. 254 SMO is a fully austenitic steel with excellent crevice corrosion resistance and good formability. The choice depends on strength needs, temperature and fabrication method.
Q3: What pipe materials are used in RO desalination plants?
High-pressure feed and brine lines typically use duplex 2205 or super duplex 2507; low-pressure or permeate lines can use 316L or non-metallic materials.
Q4: Do you supply pipe with NORSOK or third-party certification?
We can supply pipes with EN 10204 3.1/3.2 certificates and arrange third-party inspection. Please tell us your project specification when you inquire.
Request a Quote for Seawater-Resistant Pipes
Send us your grade, standard, OD × wall thickness × length, quantity and destination port, and we will reply with pricing and lead time within 24 hours.