Brass vs Stainless vs Copper-Nickel for Seawater — Marine Alloy Selection Guide

"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 yellow brass (CW617N, ~40% Zn) in seawater fails by three mechanisms simultaneously:

1. Dezincification — high Zn + chloride = aggressive selective leaching, typically failing within 3–18 months
2. Erosion-corrosion — chloride attacks the protective oxide film; flowing water removes it physically; the velocity threshold for CW617N in seawater is just 0.5 m/s
3. 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
• Cost relative to CW617N: ~1.3× per kg

Admiralty brass C44300 — naval condenser tube

Composition: ~71% Cu, 28% Zn, 1% Sn, plus arsenic. The tin addition gives slightly better impingement resistance than aluminium brass for 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) — propellers & shafts

→ 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 fittings: propeller shafts, rudder stocks, through-hull fittings. It is not immune to dezincification but the small tin addition modestly improves corrosion resistance versus standard CW617N. For seacocks and seawater valves, manganese bronze (C67500) is often preferred for higher strength.

Manganese bronze C67500 — seacocks, through-hulls, high-load

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) and slightly better marine corrosion behaviour. The dominant choice for seacock and through-hull fittings on yachts and commercial vessels.

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 the highest seawater velocity of any copper alloy and is essentially immune to chloride stress-corrosion cracking.

• Max recommended seawater velocity: 3.5 m/s (continuous service)
• Specifications: ASTM B466 (rod), ASTM B467 (tube), EEMUA 144
• Cost relative to CW617N: ~2.5–3× per kg

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 (~4× CW617N) and reserved for applications where 70/30 cost is justified by the service criticality.

Comparison — copper alloys vs 316L stainless for seawater

Which alloy when — by application

Seacocks & through-hull fittings (yacht / commercial)

Manganese bronze C67500 forged + CNC machined. Higher strength than naval brass; better seawater resistance than CW617N; acceptable cost.

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 kill the algal and bacterial 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, despite stainless being available at similar cost.

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

Sources & references

Marine-corrosion references:

Last reviewed: June 2026. Standards and regulatory references are checked at each review.