Article
Seawater is one of the most corrosive environments piping faces. Chlorides, dissolved oxygen, marine organisms and temperature changes combine to attack many metals. Choosing the right material is the difference between decades of service and early failure.
Why seawater is so aggressive
- High chloride content (around 19,000 ppm) causes pitting and crevice corrosion
- Biofilms on surfaces raise corrosion potential
- Higher temperatures, as in desalination, accelerate attack
- Crevices under gaskets and deposits are especially vulnerable
Material options
| Material | Seawater suitability |
|---|---|
| 316L (PREN ~24) | Fine for atmospheric marine exposure; not reliable in warm, stagnant seawater |
| Duplex 2205 (PREN ~35) | Good for brackish water and some seawater duties at moderate temperature |
| Super duplex 2507 (PREN > 40) | Widely used for seawater, firewater and offshore systems |
| 6Mo super austenitic | Seawater and high-chloride process service |
| Copper-nickel 90/10, 70/30 | Traditional for seawater cooling, good biofouling resistance |
Desalination plants
Reverse osmosis plants use high-pressure seawater and brine lines where super duplex and 6Mo grades are common. Lower-pressure product water and chemical dosing lines often use 316L or duplex. Thermal desalination uses a mix of materials depending on temperature and stage.
Shipbuilding and offshore
Ships and platforms use stainless for firewater, ballast, hydraulic and process systems, and require class society approval (DNV, ABS, LR, BV) and certificates for many items.
Design tips
- Avoid stagnant seawater in stainless lines during shutdowns
- Minimise crevices and use suitable gaskets
- Specify PREN minimums for duplex and super duplex
- Control heat input in welding duplex to keep the phase balance
Frequently asked questions
Is 316L suitable for seawater piping?
It is fine for marine atmospheres but generally not reliable for immersion in warm or stagnant seawater, where duplex or super duplex is preferred.
Why is super duplex popular offshore?
It combines high strength with PREN above 40, giving excellent resistance to seawater corrosion.
What materials are used in RO desalination high-pressure lines?
Mainly super duplex and 6Mo super austenitic stainless steels.
Key takeaways
- Seawater attacks most stainless grades through chlorides and crevices.
- Super duplex and 6Mo are standard for seawater service.
- Avoid stagnant water and crevices; specify PREN minimums.
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