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 Replies: 7 / Views: 1,468 |
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Pillar of the Community
United States
1699 Posts |
I have heard a little bit, hear and there, about coins where there metals have reacted with other coin metals. Which metals react with which other metals, and does this happen if they are not directly touching? Any other imformation about this would be helpful. Thanks.
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Pillar of the Community
Canada
598 Posts |
I am familiar with galvanic action in dissimilar metals causing corrosion... esp. in the presence of moisture(and the old potato/lemon(<acids) battery with iron/copper electrodes or similar combinations). The coin metals would have to be touching to re-act within a reasonable amount time(lifetime(?)!). There are more chemistry types here than I'll pretend to be, but this is an interesting point with many bi-metallic coins in circulation(eg. twoonie. Perhaps not in the U.S. yet.) But I'm sure the compositions are researched to minimize this action. As for coin storage. I don't have much stuff in tubes other than junk and it's all the same metal. I therefore don't have any dissimilar metals contacting one another. Paper flips are cheap and I see no real reason to store copper touching silver, etc. But I 'spose old piggy banks would have some green gunk goin' on. You would be rite as a collector to keep any dissimilar metals from touching. I would say copper wouild be the biggest culprit and newer compositions the least. But I forget most of the valence electron stuff from chemistry to tell you why.  |
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New Member
United States
15 Posts |
Not much help, but do know about 10 + years ago I stopped mixing coins when I soaked/cleaned them in acetone.
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Bedrock of the Community
United States
20753 Posts |
What you mentioned is basically true of all metals. When any two dissimalar metals touch each other there is a slight electrical current created as the ambient temperature is modified to any extent. Some metals react more electrically than others when touching and some mixtures of those metals create a much larger current.
For example even in magnetics the mixtures of some metals create powerfull magnet strengths. AlNiCo is one of those which actually is greater than Iron in magnetics.
In industry the touching of dissimalar metals is utilized as a thermocouple system of temperature measurements. Due to some metals being more reactive than others, a carefull study is made for different situations. In a pipe or air or gas duct a threaded hole, called a thermowell, is implanted. Another item called a thermocouple head is installed there. this item is openable so connections can be made inside. A conduit with two, well insulated, shielded, dissimilar metals are installed and connected together inside the thermohead. At the other end of those wires is a gauge. Any changes in temperature at the thermocouple end creates a current that is recorded at that gauge. This is an extreamely accurate means of measuring changes in temperature. This system of temperature measurements is utilized in Nuclear and Fossil Power stations and the gauges are in the main control rooms. Many other industry organizations also use this system.
So in answer to your question, yes, the touching of two dissimalar metals can and does create a reaction.
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Pillar of the Community
Canada
598 Posts |
Then there's a thermo-pile... a series/parallel arrangement of thermo-couples used to generate sufficient current to actually operate a mechanical device. Electricity is generated by heat, using dissimilar metals. But I'm still not gonna get my lab books out!  If you have about a 4 foot length of copper pipe, same as household plumbing, and drop a small AlNiCo(aluminum,nickel,cobalt) magnet down it... the eddy currents generated will cause the magnet to levitate below the bottom of the pipe... it's way cool. Those magnets have an extremely hi gauss to weight ratio. You can tear one outta your hard drive. Bonne chance!  Edited by IBGolden
06/12/2009 12:09 am
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Moderator
 United States
188130 Posts |
Quote:
You can tear one outta your hard drive. I have several of them stuck to a file cabinet near my desk. I love watching people try to pull them off!  |
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Bedrock of the Community
United States
20753 Posts |
Of course to drag this out even further some metals touching other metals are advantageous to one of both of them. A good example is the Aluminized coatings used on some buildings steel outer structures. Aluminum is one of those metals that as Oxydation starts, the Oxide of Aluminum stays on creating a protective coating. If applied to steel or mixed with steel in the proper proportions, a coating starts and then stays. Examples are the Federal Buildings in Chicago where the buildings are all Steel on the outside and have stayed free of Oxydation. One of the worst metals to touch each other is Aluminum and Copper. Since most industries utilize Copper for Grounding systems, in many areas a Cathodic Protection system is warranted due to such ingrediants in the soils. Of course this is all rather boring but when you work with it all day you feel like telling someone. So to sum it up, keep your coins away from each other. And keep them away from me too.  |
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Pillar of the Community
Canada
598 Posts |
To further add to the tedium at hand...  Quote:
One of the worst metals to touch each other is Aluminum and Copper.
just carl ... and that is why in some jurisdictions, *some genius,  some veritable management wizard, allowed the use of aluminum wiring.  This wiring must now(in specified applications; eg. --- nursing homes) be paired with copper approved devices (outlets,switches,lamp bases, etc.)... with the old aluminum wiring bonded to the new copper devices; with special connectors utilizing a di-electric gell from the original aluminum wiring to copper pig tails to the new copper rated devices. The aluminum oxidization would cause a resistive load at junction points, heating up... and burning down. *Stay in school kids, stay in school  >>>edit... And I have zinc strip stories too! Edited by IBGolden
06/12/2009 12:08 am
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Replies: 7 / Views: 1,468 |
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Posted 06/10/2009 8:42 pm
