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Key takeaways
- 90/10 copper-nickel is forgiving and resists fouling, but has a maximum velocity, typically around 3 to 3.5 m/s depending on size.
- Super duplex (25Cr) is strong and handles high velocity, but has a temperature limit in chlorinated seawater and needs careful welding.
- GRE (glass-reinforced epoxy) doesn’t corrode and is light, but needs attention to fire performance, supports and handling damage.
Why seawater is hard
Seawater is warm, salty, oxygenated and full of marine life. Carbon steel corrodes quickly, and ordinary stainless steels such as 316 suffer pitting and crevice corrosion. On top of that, operators add chlorine to stop marine growth, which makes the water more aggressive towards some alloys.
Seawater systems include cooling water, firewater, ballast, water injection and service water, so the material choice affects a lot of pipe.
90/10 copper-nickel
- Strengths: good general corrosion resistance, natural resistance to biofouling, easy to fabricate.
- Limits: erosion at high velocity. Design velocities are commonly kept to around 3 to 3.5 m/s for larger sizes and lower for small bore. It is also sensitive to polluted water with sulfides, and its mechanical strength is low, so walls and supports need care.
Super duplex stainless steel (25Cr)
- Strengths: high strength (thinner, lighter walls), excellent resistance to erosion at high velocity, good pitting resistance.
- Limits: crevice corrosion risk at higher temperatures in chlorinated seawater. Many projects limit it to around 20 to 30 °C in chlorinated seawater, following NORSOK M-001 or company rules. Welding must control heat input to keep the right ferrite-austenite balance and avoid harmful phases.
GRE (glass-reinforced epoxy)
- Strengths: no corrosion, light, low friction.
- Limits: temperature limits set by the resin system, low impact resistance, and performance in fire. For firewater, GRE must be qualified for fire exposure or kept wet, and dry deluge sections usually need a metal. Supports must be closer together and must not point-load the pipe. Design and qualification follow ISO 14692.
| Factor | 90/10 CuNi | 25Cr super duplex | GRE |
|---|---|---|---|
| Corrosion in seawater | Good | Very good (within temperature limits) | Excellent |
| Velocity tolerance | Limited | High | Moderate |
| Weight | Medium | Low (thin walls) | Very low |
| Fire performance | Good | Good | Needs qualification |
| Main risk | Erosion, sulfide pollution | Crevice corrosion when hot, weld quality | Impact, fire, joint quality |
Titanium is also used where the highest reliability is needed, for example in seawater heat exchangers and some firewater systems, at a higher material cost.
Code reference
NORSOK M-001 Materials selection, including limits for stainless steels in seawater.
ISO 14692 Glass-reinforced plastics piping for petroleum and natural gas industries.
ASTM B466 / EEMUA 234 Copper-nickel pipe and piping components.
What to take back to your desk
Write the seawater temperature, chlorination level and maximum velocity into the design basis before choosing. Most seawater failures come from a material used just outside its limit.
Educational content. Always design to your project specification.
