Weird degenerating coin

[ScubaJorgen]

I recently found remnants of a burglary in a small lake. A.o. there were a couple of foreign coins. Based on the remnants (membership cards, etc) I could deduce that the stuff had been there for about 12 years.

One of the coins happened to be a German 5 Pfennig coin. Cleaning it, it revealed 1950 (5 years after WWII).

The weird part :icon10: was that the coin had been split: it consists of only two thin slabs, the head side and the tail side. The middle part seems to be vanished!

I've been told that after the war copper was scarce. Maybe the coins had some inferior :death2: middle layer (zinc? during the war there were pure zinc coins). The coin has been on a nickle(?) 50 F Belgium coin. Though the Belgium coin was dissolved partly, it was not in the place where the German coin touched it. The photo probably is more illustrative.

Does anybody know about German coins? Is this a redox reaction?

(Note: the photo may be unavailable in july/august 2006)

 

[Diver0001]

German 5p coins of that era were iron coated with brass. Maybe what you're seeing is the brass layer that didn't corrode with the iron on the inside corroded away.

I'm not a coin expert. Just call this a best guess.

R..

 

[ScubaJorgen]

Ah, that explains it! 12 years under water may dissolve the iron.

There however are no signs of rust, so it is probably not oxidation. It may be a reductor-oxidator reaction, like

Fe + Cu2+ -> Fe2+ + Cu

The inferior metal dissolves, the less inferior metal solidifies (or whatever it is called).

 

[Diver0001]

Well....I know less about chemistry than you do. I suppose anthing's possible. It could have just been eaten away by that fresh Dutch lake water too....

R..

 

[WizFromOz]

http://it.geocities.com/coinpc/Germany_5_pfennig.html

Worth between $70-$1000 in mint condition, sorry about the state of your find.
Seems like they brass plated steel blanks between '49-'50

I was in Germany in '55 and they had rubbishy coinage even back then

WizFromOz

 

[ScubaJorgen]

Worth between $70-$1000 in mint condition, sorry about the state of your find.

No wonder why they are so valuable nowadays: they don't survive time...

Guys, thanks for the information!

 

[DrSteve]

Ah, that explains it! 12 years under water may dissolve the iron.

There however are no signs of rust, so it is probably not oxidation. It may be a reductor-oxidator reaction, like

Fe + Cu2+ -> Fe2+ + Cu

The inferior metal dissolves, the less inferior metal solidifies (or whatever it is called).

But what you show above is an example of oxidation of the Fe (oxidation is loss of electrons) and reduction of Cu. The other thing is Fe2+ is green and Fe3+ is the standard red/brown rust. I'm guessing it would simply be a case of rusting which etches away in between the two layers:
2H2O + O2 + 4e- -> 4OH- STD electrode potential, E= +0.4 V
Fe -> Fe3+ + 3e- STD electrode potential, E= +0.04 V
Net reaction: 12H2O + 3O2 + 12e- + 4Fe -> 4Fe3+ + 12e- + 12OH- E=+0.44 V
Free energy of reaction = -nFE
A reaction is spontaneous if Delta G is -ve

So I've just proved that rusting should take place. Don't shoot me...I'm a chemist but not an electrochemist.

 

[ScubaJorgen]

But what you show above is an example of oxidation of the Fe (oxidation is loss of electrons) and reduction of Cu. The other thing is Fe2+ is green and Fe3+ is the standard red/brown rust. I'm guessing it would simply be a case of rusting which etches away in between the two layers:
2H2O + O2 + 4e- -> 4OH- STD electrode potential, E= +0.4 V
Fe -> Fe3+ + 3e- STD electrode potential, E= +0.04 V
Net reaction: 12H2O + 3O2 + 12e- + 4Fe -> 4Fe3+ + 12e- + 12OH- E=+0.44 V
Free energy of reaction = -nFE
A reaction is spontaneous if Delta G is -ve

So I've just proved that rusting should take place. Don't shoot me...I'm a chemist but not an electrochemist.

Yeah, I knew my highschool chemistry knowledge is a bit rusty (too).

Thanks for the illumination!

 

[mjnansen]

I'm probably way wrong here, but- these look a lot like the wrappers of chocolate coins- some of them are usually based on a similar type of coin. Just a thought!

 

[mikerault]

In the Navy we used sacrificial anodes to prevent hull corrosion, these where chunks of zinc that were bolted to the hull. Same thing here I will bet, where it touched the other coin an electrical potential was established that preferentially ate the more reactive metal. I believe it is a form of galvanic corrosion.

Mike