Copper on its own is soft, conducts heat beautifully, and turns that familiar green given enough weather. Add a second metal and everything changes. A few percent of zinc gives you something you can machine at speed; tin gives you a casting that shrugs off salt water; nickel gives you a surface that stays bright on a ship's pipework for decades.
Walk into any UK metal merchant and you will see the three families lined up side by side, often looking almost identical in the rack. Here is what separates them, where each one earns its keep, and how to choose without over-specifying or overpaying.
What Actually Makes a Copper Alloy
Almost every copper alloy you will meet is copper plus one or two deliberate additions, plus small extras that control grain size, machinability or casting behaviour. The broad families are:
- Brass — copper and zinc, sometimes with lead, tin, aluminium or arsenic in small amounts.
- Bronze — properly, copper and tin. In industry the word is stretched to cover copper alloys built around aluminium, silicon or manganese instead.
- Cupronickel — copper and nickel, usually with controlled iron and manganese for seawater service.
The naming quirks matter more than they should. Manganese bronze, for example, is a high-strength brass. Gunmetal is a copper-tin-zinc alloy. If you are ordering stock, go by the standard designation (the EN or BS numbers, such as CW614N or CC491K) rather than the trade name.
Brass: The Machinist's Favourite
Brass covers everything from around 5% zinc up to roughly 45%. Below about 37% zinc you get alpha brass — ductile, easy to cold work, happy to be rolled, drawn into tube and deep drawn into cartridge cases. Push past that and a beta phase appears, which is harder, hot works well, but is more brittle when cold.
Corrosion and the dezincification problem
Brass resists fresh water and most atmospheres well, but zinc is the weak link. In certain waters, particularly warm or slightly acidic supplies, the zinc leaches out of the surface and leaves a spongy, copper-coloured residue. That is dezincification, and it is why plumbing fittings for aggressive water are specified as DZR — dezincification-resistant — brass, with small arsenic, antimony or phosphorus additions.
Brass is also prone to stress corrosion cracking. Parts that have been cold formed and left with internal stress can split along the grain in the presence of ammonia or some cleaning chemicals. A stress-relief anneal after forming prevents it.
Where you will find it
- Plumbing fittings, taps and valves
- Electrical terminals, pins and connectors
- Architectural ironmongery, hinges and decorative trim
- Instrument parts, gears and small turned components
Free-machining grades contain lead, which makes chips break cleanly and allows fast production on automatic lathes. It also means swarf and dust need proper handling. Check the safety data sheet, use extraction and suitable personal protection, and take professional advice if you are setting up a workshop process for the first time.
Bronze: Tough, Traditional, Castable
Tin bronze has been poured in Britain for centuries and remains the default choice for bearings, statues, bells and marine fittings. Tin additions of roughly 5–12% give good strength with excellent castability and a low coefficient of friction, which is why plain bearings still use it.
The modern bronze family
Phosphor bronze uses a small phosphorus addition as a deoxidiser. It is strong, springy and wears well — think leaf springs, contact strips and bearing bushes.
Aluminium bronze is a different animal, with aluminium as the main addition plus iron and nickel. It is one of the strongest copper alloys in common use, resists seawater well, and does not spark against steel, hence its use for pump impellers, valve seats and hardware on gas and petrochemical plant.
Silicon bronze casts and welds cleanly, which makes it popular for sculpture, architectural castings and corrosion-resistant fasteners.
Bronze is normally cast or hot worked rather than cold formed. It machines reasonably well but tends to produce long, stringy chips, so sharp tooling and a firm feed matter. Welding is possible on most grades, though aluminium bronze needs care with preheat and filler selection.
Cupronickel: Built for Seawater
Add 10–30% nickel to copper and you get a silver-coloured alloy with a stubborn resistance to seawater. The two standard grades are 90/10 and 70/30. The higher nickel version handles faster flow velocities and more demanding duty, which is why condensers, desalination plant and naval pipework have used it for generations.
Small additions of iron and manganese improve the protective oxide film that forms on the surface. Left alone in clean seawater, that film is what keeps the metal intact. In water polluted with sulphides the film is disrupted and corrosion can accelerate sharply — a reminder that cupronickel suits the environment it was designed for, not every wet job.
It also has a familiar domestic face. Cupronickel is the silvery alloy used for much of Britain's small change, chosen because it wears slowly, takes a crisp impression and does not tarnish into an unattractive mess in a pocket.
In the workshop, cupronickel work hardens quickly and needs regular annealing to stay formable. It machines with a gummy tendency, similar to pure copper, so keep speeds moderate and use plenty of cutting fluid. Brazing and welding are straightforward on the common grades.
Choosing Between Them
A short decision route that covers most jobs:
- High-volume machined part? Use free-machining brass.
- Seawater or fast flow? Aluminium bronze for hardware and strength, cupronickel for tube and pipework.
- Bearing, bush or casting? Tin bronze or phosphor bronze.
- Decorative or springy? Brass for bright work, phosphor bronze for springs and contacts.
- Welded frame or sculpture? Silicon bronze or a weldable cupronickel grade.
The nickel-bearing alloys are the most expensive of the three and the hardest to source in small quantities, so a standard stock size from a UK merchant is usually cheaper than a special order.
Before You Buy
Three habits will save you time and money. Write down the standard designation rather than the trade name, so you get the alloy you actually want. Ask for the condition — half-hard, quarter-hard, annealed, as-cast — because the same alloy behaves completely differently in each. And match filler and flux to the parent metal before you start welding, since mixing brass filler into a bronze joint rarely ends well.
Keep a small offcut of each alloy in the rack, marked with a paint dot and its designation. When a job arrives and you are not sure what is in your hand, the colour and the weight in the palm will usually settle it. Copper alloys reward that kind of care: choose the right one and it will outlast the machine it sits in.
Photo: alper_eral / Pixabay
