(Wikipedia/Jonathan Zander) Native copper.
Does Copper Corrode in Salt Water?
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JANUARY 03, 2008
April Holladay, HappyNews Citizen Journalist

Q: Does copper corrode in salt water? I plan to build a salt-water hot tub and wonder if the heater copper pipes will corrode in the "marine" environment.
Bert, Albuquerque, New Mexico
A: Seawater leaves copper-nickel alloys practically intact. General corrosion rates for 90-10 and 70-30 copper-nickel alloys range between 2.5 and 25 microns per year. That's not much: 25 microns — only twice the thickness of a human hair — in a whole year.
Copper pipes will easily last the lifetime of a hot-tub heater, especially if the copper is a copper-nickel alloy.
What protects the alloys? An oxide film forms on copper's surface, which stops further corrosion quickly, much like the chromium oxide film that keeps stainless steel shiny. The film can make the copper look brown, greenish-brown, or brownish-black.
The film gets better with time. In flowing water, the corrosion rate decreases continually over at least a 14-year period.
That's the answer, but there's more about copper:
• Copper beads, found in Iraq, date back to 9000 BC.
• Egyptian metal workers first cast copper to shape in molds ( c . 4000 BC), reduced ores to metal with fire and charcoal, and intentionally alloyed copper with tin to form bronze ( c . 3500 BC).
• Copper got its name from the Romans, who called it the "ore of Cyprus" since Cyprus supplied almost all Rome's copper. In Latin, that's aes Cyprium. The Romans soon shortened that to cyprium and later corrupted it to cuprum and finally we anglicized it to "copper".
• The greatest known deposit of copper is in rock formed by volcanic activity in the Andean Mountains of Chile.
• Copper, present in humans as a trace element, helps catalyze hemoglobin formation in our blood.
• Almost all coins contain copper.
• In World War II, the Germans laid mines in shallow waters, even in the Thames Estuary. Magnetic sensors detected when a ship's steel hull passed above and triggered the mines. The exploding ships were devastating allied and neutral shipping. Copper to the rescue. Workers simply attached a copper strip around the hull and connected the strip to ship's power. A current passed through the strip, neutralized the ship's magnetic field, and defeated the mines.
• Policemen are called "cops" or "coppers" because their uniforms used to have copper buttons.
Further Reading:
Copper through the ages, Copper Development Association
(Answered Oct. 11, 2002; updated Dec. 4, 2007)