A copper part left on a bench for a month changes colour, and a copper roof left for thirty years turns green. Neither is rust. The vocabulary matters, because the fix for tarnish, the fix for a patina and the fix for a corroded busbar are three different conversations.
Does copper rust?
No. Rust is specifically iron oxide, so only iron and steel rust. Copper corrodes by a different route: in air it forms a thin copper oxide that darkens to brown, and over years, with moisture and sulphur compounds, it builds the green patina of copper carbonate and sulphate. In aggressive conditions - chlorides, ammonia, or contact with a dissimilar metal in a wet environment - copper can pit instead. For a part this matters twice: the patina on exposed copper is stable and protective, so a green copper roof is fine, but tarnish on a contact surface is not, because the oxide layer is not conductive and raises contact resistance at busbars, terminals and grounding points.
Rust, corrosion, tarnish, patina
- Rust - hydrated iron oxide, and only that. It flakes off, exposes fresh metal and keeps going, which is why steel needs protection and copper does not.
- Tarnish - the first stage on copper: a thin brown or purple-brown oxide, forming in days to weeks indoors. Cosmetically obvious, electrically resistive.
- Patina - the long-term green sulphate and carbonate layer. Adherent, protective, and the reason architectural copper is left unpainted.
- Pitting - localised attack that goes into the metal. Chlorides and ammonia cause it, and a fitting that shows a green drip marks rather than an even film is a case of pitting, not of patina.
What speeds it up
- Moisture and salt - coastal and road-salt environments turn the surface green far faster than dry indoor air.
- Sulphur compounds - rubber and some packaging release sulphur that stains copper in storage; this is why copper parts should not be packed against ordinary rubber.
- Ammonia and ammonium compounds - cleaning agents and some cutting fluids; they attack copper aggressively and cause stress-cracking.
- Dissimilar metal contact - copper against aluminium or steel in a wet joint creates a galvanic cell, and the less noble metal loses.
- Heat - hot copper oxidises faster, which is worth remembering for a housing whose whole purpose is to carry heat.
When it actually matters on a part
- Electrical contact surfaces - tarnish is not conductive. Busbars and terminals are usually tin- or silver-plated for exactly this reason, and a bare copper contact face should be cleaned and jointing compound used.
- Thermal paths - a copper sensor housing is chosen for conductivity, and a heavy oxide layer on the seating face adds a small thermal resistance. On a controlled interface, that is worth specifying as plated or as bare with a defined flatness.
- Appearance - lacquer, clear coating or a plated finish keeps the bright colour. Uncoated copper is expected to change.
- Function inside the part - the patina on the outside of a drawn housing does not affect the thread, the seal or the electronics inside, and most industrial copper parts are accepted with it.
Keeping copper bright
- Store parts dry and do not pack them against rubber or sulphur-bearing paper.
- Clean oxide before joining: a light abrasive or a commercial cleaner, then assemble promptly.
- Specify a coating when appearance matters - lacquer for indoor decorative parts, tin or nickel plating where conductivity or solderability matters.
- Keep copper and aluminium apart in wet joints, or use a bimetallic transition and a jointing compound.
- Write the requirement on the drawing: "bright dipped and lacquered" and "mill finish, tarnish accepted" are two different parts at two different prices.
Key point
Copper does not rust; it tarnishes, then patinates, and in chlorides or ammonia it can pit. On an exposed architectural or decorative part that is normal. On a contact face or a controlled thermal interface it is not, so the drawing should say which of the two the part is.
Frequently asked questions
Does copper rust in water?
No, it cannot rust, because rust is iron oxide. Copper in water forms an oxide and eventually a green patina; in water carrying chlorides or ammonia it can pit instead.
What is the green layer on copper?
It is a patina of copper carbonate and sulphate, formed over years of exposure to air and moisture. It is stable and protective, unlike rust, which keeps eating into the metal.
Is tarnished copper still conductive?
The metal underneath is, but the oxide film on the surface is not, and it raises contact resistance at joints and terminals. That is why contact surfaces are plated or cleaned before assembly.
Does copper rust in salt water?
It does not rust, but salt water accelerates the patina and can pit the metal, particularly where it is in contact with a less noble metal or where flow is turbulent.