I often carry coins in a cheap plastic sandwich bag. A green solid soon appears. Is it copper oxide? How is the polyvinyl chloride (or polyethylene or polypropylene) greatly accelerating the reaction? Or are they? Are there other chemicals being formed?

Coin collector websites all say vinyl-containing compounds, and maybe other plastics, damage coins. But they don't say how....

P.S.: Coins are made of copper, nickel and zinc, right?

  • $\begingroup$ A cheap plastic bag shouldn't be made of PVC in the year 2018, i hope, and all other ordinary plastics surely don't react with copper. And I also doubt that PVC does! So this is most likely just a moisture problem. $\endgroup$ – Karl Jun 25 '18 at 18:36
  • $\begingroup$ Karl seems to be right, my bag is either polyethylene or (maybe) polypropylene... $\endgroup$ – Kurt Hikes Jun 25 '18 at 23:56

Far, far too broad a question. I don't know what is going on in your situation. I can, perhaps, clear up some misconceptions you may have. PVC is NOT commonly used in sandwich bags. PVDC is a close relative and used to be common as "saran wrap". But most sandwich bags are made out of polyethylene (PE). Chemically both PVC and PVDC degrade due to weathering and other things. Dehydrohalogenation is the reaction of organic chlorine compounds (both PVC and PVDC) which can be expressed in the partial formula C-HC-Cl → C=C + HCl (this is a crude simplification). Note that this reaction generates HCl, also known as hydrochloric acid (or muriatic acid). HCl is a powerful acid, attacking most metals. If you live by the ocean or in a desert chloride levels in the air (dust or mist) can be quite high and can accelerate corrosion dramatically. Coins are designed to be inert. Placed clean and dry into a sealed PE bag, they should not corrode. But. Whenever two different metals (or metal alloys) come into contact, a electrical potential is created. A random bag of coins will be sorta like a bag of little batteries. That is, you shouldn't be surprised if they react (discharge), especially in the presence of moisture. Neither copper nor nickel form green oxides, but both do form green chlorides and green carbonates. You need a source of chlorine for the chlorides, but the carbonates might form when simply exposed to air and moisture when a electrical potential exists. Sweat, sweaty hands, are potentially one source of chloride/chlorine, but as I stated up front I am not familiar with your situation, I carry (mixed) coins in my pocket for days at a time without any green forming.

  • $\begingroup$ You're right, Lio, my bag is polyethylene or polypropylene, not polyvinyl chloride. $\endgroup$ – Kurt Hikes Jun 25 '18 at 23:58
  • $\begingroup$ So the green stuff, in conclusion, does not involve the bag, just the coins plus carbon dioxide/ monoxide plus moisture. It is carbonate(s) of copper, nickel or zinc (I forgot about zinc). And having multiple coins and different metals touch creates a tiny electrochemical voltage, accelerating the process? Is that right? $\endgroup$ – Kurt Hikes Jun 26 '18 at 0:07
  • 2
    $\begingroup$ "A random bag of coins will be sorta like a bag of little batteries." Very nice analogy. +1 $\endgroup$ – Karl Jun 26 '18 at 18:35

Many polymers (including Polyvinyl Chloride) are water-resistant materials. The moisture that they trap causes metals such as copper to ionise on the surface and corrode (oxidise) into “rust” by readily reacting with oxygen.


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