For seawater service, standard CW617N brass is the wrong material — it dezincifies. The correct copper-alloy choices are aluminium brass C68700 (condenser tubes, low-velocity service), admiralty brass C44300 (condenser tubes, naval), naval brass C46400 (shafts and hardware above the waterline), and copper-nickel 90/10 C70600 or 70/30 C71500 (high-velocity seawater piping). For wetted assemblies that include brass, 316L stainless is often specified instead — at a higher part cost, because it is slower to machine. This guide compares each alloy against seawater service requirements.
"Marine grade" is one of the most overused specifications in industrial procurement. The reality is that there is no single marine alloy — there's a set of options, each tuned to a specific combination of velocity, oxygen content, temperature, and exposure pattern. This guide is the engineering decision tool for getting the right copper alloy (or right stainless) into seawater service.
What seawater actually does to brass
Seawater is the most aggressive natural environment most metals encounter. The chemistry that drives brass failure:
- Chloride concentration ~19,000 mg/L — drives chloride pitting, stress-corrosion cracking initiation
- Dissolved oxygen 5–8 mg/L — supports both general corrosion and microbiologically influenced corrosion (MIC)
- pH ~8.0 — alkaline but with local acidification possible in crevices
- Temperature 0–30°C in normal service; up to 80°C in waste-heat coolers
- Velocity — 0.5–4 m/s in piping systems
- Biological activity — biofilms, barnacles, marine growth
Standard forging brass (CW617N, CuZn40Pb2, about 40% Zn) in seawater fails by three mechanisms simultaneously:
- Dezincification — high Zn + chloride = aggressive selective leaching; depending on temperature and flow, parts can fail within months to a few years
- Erosion-corrosion — chloride attacks the protective oxide film; flowing water removes it physically; brass has the lowest velocity tolerance of the copper alloys
- SCC potential — biological ammonia + residual stress = stress-corrosion cracking risk
The right copper alloys for seawater
Aluminium brass C68700 — the condenser-tube workhorse
Composition: ~77% Cu, 21% Zn, 2% Al, plus a trace of arsenic (typically 0.02–0.06%) as a dezincification inhibitor. The aluminium forms a thin, adherent aluminium-oxide protective film that resists chloride breakdown. Standard alloy for power-station condenser tubes, ship-board heat exchangers and low-velocity seawater service.
- Max recommended seawater velocity: 2.0 m/s
- Specifications: ASTM B111, ASTM B543
- Service temperature: up to 80°C
Admiralty brass C44300 — naval condenser tube
Composition: ~71% Cu, 28% Zn, 1% Sn, plus arsenic. Its velocity tolerance is below that of aluminium brass (1.5 m/s against 2.0 m/s), so it is used in low- to moderate-velocity service. Historically the dominant naval condenser-tube alloy; still widely specified.
- Max recommended seawater velocity: 1.5 m/s
- Specifications: ASTM B111
- Service temperature: up to 80°C
Naval brass C46400 (CuZn39Sn1) — shafts & marine hardware
→ Full C46400 naval brass datasheet (composition, properties, EN CW719R cross-reference)
Composition: ~60% Cu, 39% Zn, ~1% Sn. Naval brass is forged and machined for marine hardware: propeller shafts, rudder stocks, tube sheets and fittings above the waterline. It is not immune to dezincification but the small tin addition modestly improves corrosion resistance versus standard CW617N. It is not a material for seacocks and through-hull fittings below the waterline — those are made from gunmetal (leaded red bronze such as CC491K) or DZR brass, as ISO 9093 requires corrosion-resistant materials.
Manganese bronze C67500 — propellers, shafts, high-load hardware
Composition: ~58% Cu, 40% Zn, with Mn, Fe, Al, Sn additions. Significantly higher strength than naval brass (Rm 480–620 MPa vs 380–480 MPa) but with about 40% zinc it can dezincify in seawater, so it is used for propellers, shafts and highly loaded hardware that can be inspected — not for seacocks and through-hull fittings.
Copper-nickel 90/10 (C70600) — the high-end seawater piping alloy
Composition: ~89% Cu, 10% Ni, 1.4% Fe, 0.5% Mn. Cu-Ni 90/10 is the standard alloy for seawater piping in ship-building, offshore oil & gas platforms, and seawater desalination plants. It tolerates a far higher seawater velocity than the brasses — 3.5 m/s in continuous service — and is essentially immune to chloride stress-corrosion cracking.
- Max recommended seawater velocity: 3.5 m/s (continuous service)
- Specifications: ASTM B466 (seamless pipe and tube), ASTM B467 (welded pipe), ASTM B151 (rod and bar), EEMUA 144
Copper-nickel 70/30 (C71500) — extreme service
Composition: ~69% Cu, 30% Ni, 0.7% Fe, 1% Mn. Higher Ni content than C70600 gives even better high-velocity erosion resistance. Used in nuclear-station seawater cooling, navy ship critical systems, and offshore platforms. Significantly more expensive and reserved for applications where 70/30 cost is justified by the service criticality.
Comparison — copper alloys vs 316L stainless for seawater
| Alloy | Max velocity (m/s) | Pitting | SCC risk | Antifouling |
|---|---|---|---|---|
| CW617N | lowest | High (dezincification) | Medium | Yes (natural copper antifouling) |
| Naval brass C46400 | 1.0 | Medium | Medium | Yes |
| Manganese bronze C67500 | 1.5 | Low | Medium | Yes |
| Aluminium brass C68700 | 2.0 | Low | Low | Yes |
| Cu-Ni 90/10 (C70600) | 3.5 | Very low | Very low | Yes |
| Cu-Ni 70/30 (C71500) | 4.5 | Very low | Very low | Yes |
| 316L stainless | 5.0+ | Medium (crevice issue) | Medium | No — biofilm + barnacle growth |
| Super-austenitic 6Mo (UNS S31254) | 6.0+ | Low | Low | No |
Which alloy when — by application
Seacocks & through-hull fittings (yacht / commercial)
Gunmetal / leaded red bronze (for example CC491K, 85-5-5-5) or DZR brass, to ISO 9093. High-zinc alloys such as CW617N, naval brass or manganese bronze are not suitable for seacocks and through-hull fittings — dezincified through-hulls are a known cause of flooding.
Condenser tubes (power station, marine heat exchangers)
Aluminium brass C68700 is the default. For high-velocity tube banks or cooling-water from intake systems with grit / sand entrained, switch to Cu-Ni 90/10.
Seawater piping (ship engine room, offshore)
Cu-Ni 90/10 (C70600) — the industry standard. Pre-fabricated brazed and welded systems. For nuclear-grade or military, step up to 70/30 (C71500).
Propellers, shafts, rudder stocks
Manganese bronze C67500 for the bulk of recreational and small-commercial work. Nickel-aluminium bronze C95800 for large commercial and naval applications.
Decorative trim & deck fittings (low corrosion service)
Naval brass C46400 — adequate for decorative parts that are washed regularly with fresh water.
Antifouling — the unsung copper advantage
One frequently-overlooked seawater advantage of copper alloys is their natural antifouling action. Copper ions released slowly from the surface inhibit the bacteria, algae and larvae that would otherwise initiate marine fouling (barnacles, mussels, seaweed). Stainless steel has no equivalent protection — stainless seawater piping fouls dramatically within months and requires regular chemical cleaning. Copper-nickel piping in the same service can run for decades with minimal fouling.
This is why ships' seawater service systems are almost universally copper alloy rather than stainless steel, even where stainless would be cheaper to buy.
Sources & references
- ASTM B111 — Copper and copper alloy seamless condenser tubes
- ASTM B466 / B467 — Copper-nickel seamless pipe and tube
- EEMUA 144 — Recommendations for the use of copper-nickel pipework in seawater service
- CDA Marine Applications Library
- European Copper Institute — copper-nickel publications
- Brassland custom brass parts
- Brassland Corrosion Guide
Frequently asked questions
What is the best fitting material for seawater?
Why does ordinary brass fail in seawater?
When is stainless steel used in marine fittings?
Sources & references
Marine-corrosion references:
Last reviewed: September 2026. Standards and regulatory references are checked at each review.