Seawater is salty, oxygenated and electrically conductive: it attacks each metal in its own way. For a valve, a fitting or a pipe, three questions arise. How does the alloy behave on its own? What happens when it touches another metal? What flow velocity can it withstand?
Brass: avoid it below the waterline, unless it is DZR
Brass is an alloy of copper and zinc. In seawater it can dezincify: the zinc leaches out and leaves a porous, weak network of copper (Practical Boat Owner). The part keeps its shape but loses its strength, which makes it dangerous on a seawater intake. The Copper Development Association shows a 60/40 brass valve stem destroyed this way in less than a year. The dezincification test method, ISO 6509-1, is intended for alloys with more than 15 % zinc.
DZR brass (grade CW602N) contains a small addition of arsenic that almost entirely inhibits dezincification. Parts are marked CR or DZR (Practical Boat Owner). In the catalogue, the DZR grade is shown on product pages and is available as a filter in threaded ball valves.
Chrome-plated or nickel-plated brass is still brass. The seawater intake configurator does not offer chrome-plated brass and recommends bronze or stainless steel below the waterline.
Bronze: the material for hull fittings
Bronze is an alloy of copper and tin. Some foundry bronzes used for valves (gunmetals) also contain zinc and lead. For example, grade CC499K (EN 1982), used for the body of Guidi 2300 ball valves, contains 4 to 6 % tin and 4 to 6 % zinc: well below the 15 % threshold. It is the usual material for through-hull fittings below the waterline (Practical Boat Owner). In the catalogue: through-hulls, seawater strainers, strainers and the Guidi brand.
Nickel aluminium bronzes are complex alloys: their seawater performance depends on composition and heat treatment, and too low an aluminium content is enough to cause selective phase attack (Copper Development Association). On the catalogue's butterfly valves, it is mostly the disc material.
316 stainless steel: at ease in flowing water, vulnerable in crevices
Stainless steel owes its resistance to a passive layer. In the galvanic series in seawater, passive 316 stainless steel is among the most noble metals. But in slow-moving or poorly aerated water, and in shielded areas, it can become active (ASTM G82). Crevices promote crevice corrosion: flanges, threads, deposits. Above 1.5 m/s, the risk decreases because deposits do not form (Alleima). The Copper Development Association notes that 316 stainless steel is liable to pit, particularly in crevices, when the system is not in use.
Molybdenum improves crevice corrosion resistance (Alleima): this is what sets 316 apart from 304. For severe conditions, more highly alloyed stainless steels, duplex or super-austenitic, perform better. In the catalogue: stainless steel pipe in 316L and 304L, and flanged ball valves in stainless steel.
90/10 copper-nickel: the pipe for seawater systems
90/10 copper-nickel (CuNi10Fe1Mn) was developed for seawater service and has been used in it for more than half a century. It resists corrosion well and limits marine biofouling (Powell and Michels). Its limit is velocity: for pipes of 100 mm and larger, British Standard BS MA 18 allows 3.5 m/s, less for smaller sizes. Commissioning matters: flush with clean water and avoid prolonged exposure to polluted, sulphide-bearing water (Copper Development Association). In the catalogue: copper-nickel pipe in 90/10, for welding or press-fitting, whose product pages show Lloyd's or RINA, and fittings with a 3.2 certificate.
Combining two metals: the galvanic series
Two different metals in electrical contact in seawater form a cell: the less noble one corrodes faster. The further apart they are in the galvanic series, the greater the risk. Surface areas matter too: a large noble surface connected to a small, less noble part is the worst case (Copper Development Association).
Order in flowing seawater, from most active to most noble (ASTM G82, figure 1): zinc, aluminium alloys, steel and cast iron, brasses, bronzes and copper-nickels, passive 316 stainless steel.
In practice:
- stay within one family of alloys: copper-alloy valves on copper-alloy pipework (Copper Development Association);
- a stainless steel valve screwed straight onto a bronze through-hull forms a couple; the configurator only offers parts in compatible materials, and the cart in the customer area flags the couple and suggests the same part in the reference material, or an insulating fitting;
- an anode, or contact with steel, cancels the biofouling resistance of copper-nickel (Powell and Michels).
At a glance
| Material | Use in seawater | Watch out for |
|---|---|---|
| Brass | Out of immersion | Dezincification below the waterline |
| DZR brass | Immersed parts, if no bronze | Check the CR or DZR marking |
| Bronze | Through-hulls, strainers, valves | Coupling with stainless steel |
| 316 stainless steel | Flowing water | Crevices and stagnant water |
| 90/10 copper-nickel | Seawater piping | Velocity limit, commissioning |
Frequently asked questions
Does brass corrode in seawater?
Yes, when immersed: it dezincifies and becomes porous. Below the waterline, prefer bronze or stainless steel; failing that, DZR brass marked CR or DZR.
Does 316 stainless steel rust in seawater?
It performs well in flowing water. In stagnant water, under a deposit or in a thread, it can suffer crevice corrosion or pitting, especially during long shutdowns with the system full.
Can a stainless steel valve be screwed onto a bronze through-hull?
It forms a galvanic couple: the two metals are far apart in the seawater series. Choose a bronze valve, or separate the metals with an insulating fitting.
Sources
- Vyv Cox, "Dezincification resistant (DZR) skin fittings explained", Practical Boat Owner, 16/07/2025: pbo.co.uk.
- Nick Vass, "Skin fittings and seacocks explained", Practical Boat Owner, 10/07/2025: pbo.co.uk.
- ISO 6509-1:2014, Corrosion of metals and alloys — Determination of dezincification resistance of copper alloys with zinc — Part 1: Test method: boutique.afnor.org.
- Guidi, data sheet 2300 (CC499K bronze body): rpmdiffusion.com; CC499K composition to EN 1982: kmmbronze.com.
- Copper Development Association, "Effect of Composition and Microstructure on the Seawater Corrosion Resistance of Nickel Aluminum Bronze" (archived copy, 22/01/2025): web.archive.org.
- ASTM G82, Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance, figure 1, reproduced in the Alaska Harbor Electrical Guidelines (1997), appendix C: dot.alaska.gov.
- Alleima, "Crevice corrosion": alleima.com.
- Copper Development Association, Seawater System Design: Guidelines (archived copy, 14/12/2024): web.archive.org.
- C. A. Powell and H. T. Michels, "Copper-Nickel for Seawater Corrosion Resistance and Antifouling: A State of the Art Review", CORROSION/2000, paper 00627, NACE (archived copy, 22/05/2025): web.archive.org.