Tarnish on pennies: Is it rust, copper oxide, or copper sulfide

A. They are not coated with copper oxide when they are made, Jan. They are plated with copper metal, but the copper metal eventually tarnishes into copper oxide.

Why they are plated with copper is explained in other threads on this site: Copper got too expensive to continue to make pennies out of, so the mint started making them out of zinc. The copper plating offers some corrosion resistance, while leaving them looking like pennies are expected to look.

A. It is not copper oxide on tarnished pennies but copper sulfide.

I don't agree, Mr. Dupay. It is true that tarnishes and patinas can be more complicated than just a simple oxide--and often involve oxides, hydroxides, carbonates, sulfides, and more. But I believe that the common brown tarnish on pennies is primarily oxide not sulfide. The 'black' tarnish on silver is sulfide though.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

Sulfide forms a very tough tarnish on copper, but by far, the largest form of tarnish on copper will be an oxide.

James Watts
- Navarre, Florida
2004

A. Ask any chemist or metallurgist and they'll say it's a sulfide not an oxide. When copper forms an oxide coating (the green stuff) it's copper carbonate ⇦this on eBay or Amazon [affil links] And you have to heat copper to get the black oxide. So I have to say you're wrong Mr. Mooney.

I've been wrong a hundred times if not a thousand times on this forum, Simon, and always appreciate being corrected. You could be right, but you haven't given me any reason at all to re-examine my belief yet, and continuing an unsupported "is not!", "is so!", "is not!" debate would just be foolish, and a poor lesson for students who come here to learn about the scientific method.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

I looked up copper chloride, oxide and sulfide. Chloride is minorly soluble in water, so it is probably not that. Also it is greenish. Sulfide and Oxide of copper in the valence state of II are both a shade of black or very dark blue and are both insoluble in hot water. Considering that there is many more times more oxygen in the air than sulfide, my money goes with oxide. Note that you can smell hydrogen sulfide in the low ppm range. So, if you do not live in Newark, bet on the oxide.

James Watts
- Navarre, Florida
2004

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Ed. note: The open dumps/landfills of Secaucus have been gone for at least 4 decades, maybe more. Re-acquaint yourself with the Garden State :-)

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? Now, I know that I don't have the chemistry/metallurgy background that a lot of ya'll have. But it seems to me, that in order to have a sulfide, don't you have to have sulfur present ... and for an oxide, oxygen present? If it's copper sulfide, where does the sulfur come from?

Marc Green
anodizer - Boise, Idaho
2004

A. Well sulfur ⇦this on eBay or Amazon [affil links] is present in epson salt, when coal is burned with whatever amounts of sulfur, and also in many lawn fertilizers.

A. Ok, well, please do not call it rust on a penny because it's tarnish!

A. Jacob, you need to be more descriptive about the brown "stuff" on the outside of a penny (tartar or copper?)

Your friend, Hayden

A. Hi. But one thing it's not is rust. Rust is oxides of iron and there is no iron in pennies to get oxidized and form rust.

Regards,

A. Hi Julia. Yes, we're still here, but this is a public forum, not a homework hotline, and help is unlikely to be close enough to real-time to help students who wait until the last day. My own experience is that I sometimes find blue-green pennies between the boards of boardwalks and similar places, and I sometimes see blue-green corrosion on copper water pipes -- but I never see blue-green pennies in circulation and being handled.

This seems to indicate to me that the brown tarnish on pennies is quite abrasion resistant and stable, whereas the blue-green corrosion apparently wears off rather easily. I've noticed a similar effect on brass and copper awnings and roofs: where the rain drops land gently, I see a blue-green patina build up, but at drip lines from a roof above, I see only brown.

Although Wikipedia reports that Cu₂O is red to yellow, and that CuO is black, the coloration of copper corrosion products is very complex. The tarnish color can be affected by particle size, water of hydration, and other anions which may combine with the copper, like carbonates and sulfides. copper sulphate ⇦this on eBay or Amazon [affil links] and copper chloride are blue, so I'd say the corrosion that tends toward blue-green corrosion is Cu++. Copper cyanide is more brownish, so I'd say the browner corrosion is probably Cu+, and that Cu2O is the more prevalent tarnish which endures on pennies, which gradually converts to Cu++ oxides and salts -- but I don't actually know for certain what percentage of the typical brown tarnish is CuO, Cu2O, or other corrosion products.

Regards,

A. Hi Deane. Sorry, I have no idea. If your question is a school homework question, I hope someone can give you a theoretical answer.

But if it's a real world question, please try to explain your actual situation because that invariably produces more realistic & helpful answers than abstract questions do. Thanks.

Regards,