Transient glory: Copper corrosion by sweat

A marathon runner's glory is transient (if only medals is what you are looking at)

Over the christmas holidays, I was sorting my race medal collection, neatly stored in a wooden box. To my mild horror, I found that some of my bronze-coloured medals were decaying, being covered with a greenish-white crumbly salt, for which the term patina would be more than flattering. Moreover, something was chewing away tiny pieces from the edges of my tin medals! It’s not only the body that gets older.
So I sat down to think and research what might have happened. Many of my medals were made from bronze, a mixture or alloy of mostly copper (around 90%) with a few percent tin and often small amounts of other metals, mainly zinc. Other medals are made from brass, which is a mixture of copper with zinc only. Anyway, both alloys are mostly made up from copper. Copper is a noble metal, which means that it should normally not rust in air or water like for example iron does. However, when an alloy containing copper comes into contact with chloride (present in common salt) in a wet environment, corrosion will inevitably occur. This is also called bronze disease, and this is what happens:
1) Oxygen from the air oxidises the copper metal (Cu), which means, it takes an electron (e⁻) away from the copper atom and transforms it into a positively charged ion:

Cu → Cu⁺+ e⁻

2) The cuprous ion (Cu⁺) reacts with the chloride ion to form an insoluble white salt, which is called cuprous chloride:

Cu⁺ + Cl⁻ → CuCl

3) The cuprous chloride reacts with atmospheric moisture and another oxygen molecule to form a green salt (cupric chloride). The fuzzy white/green salt is a mixture of the white cuprous chloride and the green cupric chloride. And some hydrochloric acid is also produced (HCl):

 4 CuCl + 4 H₂O + O₂ → CuCl₂·3 Cu(OH)₂ (green salt) + 2 HCl

4) Another copper atom is oxidised by air to the cuprous ion, which reacts with the chloride ion from hydrochloric acid to form even more of the white cuprous chloride.

Cu⁺ + Cl⁻ → CuCl + e⁻

… and the circle of copper oxidation continues repeats from here! We have a chain reaction, helped about or catalysed by the chloride ions, that speeds up the corrosion of copper.
But where does the nasty chloride come from? Now, a medal is what someone hangs around your neck after you have produced quite a bit of sweat. When you exercise and sweat, your body loses salt (mostly sodium chloride) in your sweat, in which there are ample of chloride Ions around.
Now what happened next and why is the tin medal corroded as well? As said before, copper is a "noble" metal that prefers to keep its electrons to itself. And if necessary, it takes them at somebody else's expense, in this case snatching it from a less "noble metal" in direct contact. Like the tin (Sn) medal it was lying on top of (Sorry Tegla!):

Cu²⁺ +2 e⁻ → Cu 
Sn  → Sn²⁺ +2 e⁻ 

This process is called bimetallic corrosion and can happen wherever there are two different metals in direct contact which each other. So basically what I had built involuntarily was a copper-tin battery (though a very expensive one)! If you put together a pint of sweat, a copper and a tin medal, you can easily build your own and generate electricity.
But what could be done next to rescue my medals from decay? Firstly, remove the chlorides (the green-white-fuzzy salt), by giving medals a thorough clean. The same goes for after wearing them. Secondly, take care to keep the metals in a dry environment. My storage in wooden boxes was probably not optimal in the moist North-Western European climate, so now they are in a metal box. Thirdly, take care not to put copper and tin in direct contact. Or in general, don't store treasures where moths eat them and rust destroys them.
 

Bronze disease of the bronze medal on the right, and corrosion on the tin medal on the left.
Sorry Tegla!