90/10 Copper-Nickel Pipe (C70600): The Seawater Piping Default and How to Buy It

Quick answer: 90/10 Copper-Nickel (UNS C70600) is the standard marine piping alloy: 10% nickel gives it the best combination of seawater corrosion resistance, biofouling resistance and fabricability of any common tube material. For shipboard, desalination and offshore seawater systems, it is usually the default over stainless — but it needs the right weld filler, proper dezincification-resistant fittings, and Fe content in the 1.0–1.8% band to achieve its rated performance.

Why copper-nickel for seawater at all?

Seawater is the harshest common piping environment: high chloride, dissolved oxygen, biofouling organisms and velocity effects that pit most alloys. 90/10 Cu-Ni handles all of them because the nickel shifts the corrosion product to a dense, adherent film and the small iron addition accelerates film formation. The result is decades of service in raw seawater without chlorination on many systems.

Forms and standards

• Pipe and tube: ASTM B466/B467 (seamless/welded), B608 for welded, and B819 for small tube. The 90/10 grade is C70600; the stronger 70/30 grade is C71500 for higher velocity.
• Fittings and flanges: C70600-compatible (MIL-spec or ASTM B61/B62 bronze-free versions) — never mix with ordinary brass, which dezincifies in seawater.
• Plate and rod: B122 for fabrication of custom manifolds and wear plates.

Three things that decide whether Cu-Ni actually lasts

1. Iron content in range. 90/10 relies on 1.0–1.8% Fe for its protective film. Off-spec low-iron material corrodes faster — verify it on the MTR.
2. Velocity envelope. C70600 likes 1–3 m/s flow; below ~0.5 m/s fouling creeps in, above ~4 m/s erosion-corrosion starts. Design the line, don't just size the pipe.
3. Weld procedure. Use matching Cu-Ni filler (ERCuNi or the equivalent) and keep the joint clean; contamination from steel wire or carbon pickup causes the classic selective-phase corrosion at the weld.

Fabrication and installation notes

• Cu-Ni work-hardens; bend with proper radius and anneal if heavily formed.
• Avoid galavanic coupling to dissimilar metals without isolation — coupling Cu-Ni directly to steel still drives steel corrosion.
• For desalination and offshore, specify cleaned and plug-capped ends the same way stainless tube is protected, because debris in the bore causes local attack.

What should the C70600 RFQ include?

• Standard and form: pipe per B466/B467, or tube per B819, seamless vs welded.
• Size, wall schedule and length; straight vs coiled for long runs.
• Iron-content confirmation in the MTR, plus chemistry and hydrostatic or eddy-current test.
• Fittings compatibility note (C70600-compatible, no dezincifying brass).
• End use: "RFQ: shipboard seawater cooling, 2 m/s" gives the supplier the velocity context to recommend 90/10 vs 70/30.

FAQ

Q: 90/10 or 70/30 for my seawater line? A: 90/10 (C70600) is the default for general seawater at moderate velocity. Step up to 70/30 (C71500) only where velocity is high or you need more strength; it costs more and is harder to fabricate.

Q: Does copper-nickel need cathodic protection like steel? A: No — Cu-Ni protects itself with its surface film. Where it is coupled to steel in the same system, the steel still needs its own protection; the Cu-Ni does not.

Q: Why did my Cu-Ni weld corrode near the seam? A: Almost always contamination — steel wire, carbon pickup, or wrong filler. The weld metal must be Cu-Ni; a stainless or steel residue sets up selective-phase corrosion right at the joint.

Q: Is copper-nickel better than Monel for seawater? A: They overlap. Monel 400 resists very high-velocity seawater and has higher strength, but costs more and is harder to obtain; 90/10 Cu-Ni is the cost-effective workhorse for the bulk of seawater piping.