Metallurgical question about US "sandwich" coins

The outer layers of US half-dollars, quaters and dimes are cupro-nickel,
I believe 75% copper / 25% nickel. Is the core straight copper? The
US mint website gives an overall composition of 8.33% Ni, balance Cu,
and does not mention any other constituents. I assume they are giving
the total composition of two cupro-nickel layers plus a copper layer.

If this is the case, why doesn't the middle layer of the coin turn green
with verdigris and get corroded away? Cupro-nickel is a very corrosion-
resistent alloy, but straight copper is not.

Similar question: US Dollar coins are supposedly made from copper
sandwiched between two sheets of manganese brass. Why isn't the different
color of the copper layer visible at the edge, the way it is in the half
dollar, quarter, and dime? Granted, manganese brass is not silver in
color, but it still looks rather different from copper. Especially once
the copper would have had a chance to corrode a bit. (The new
presidential dollar coins seem to be holding up to corrosion better
than the Sacajawea ones did.)


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Paul Ciszek wrote:
The outer layers of US half-dollars, quaters and dimes are cupro-nickel,
I believe 75% copper / 25% nickel. Is the core straight copper? The
US mint website gives an overall composition of 8.33% Ni, balance Cu,
and does not mention any other constituents. I assume they are giving
the total composition of two cupro-nickel layers plus a copper layer.

If this is the case, why doesn't the middle layer of the coin turn green
with verdigris and get corroded away? Cupro-nickel is a very corrosion-
resistent alloy, but straight copper is not.

Similar question: US Dollar coins are supposedly made from copper
sandwiched between two sheets of manganese brass. Why isn't the different
color of the copper layer visible at the edge, the way it is in the half
dollar, quarter, and dime? Granted, manganese brass is not silver in
color, but it still looks rather different from copper. Especially once
the copper would have had a chance to corrode a bit. (The new
presidential dollar coins seem to be holding up to corrosion better
than the Sacajawea ones did.)

The visible copper parts of the edges of USA 'sandwich' coins discolor quickly
in circulation. They tone to a dark brown. This also goes for the pure
copper cores on $1 coins - they tone darker than the manganese-brass parts and
can be easily seen on circulated coins.

And yes, the cores of USA 'sandwich' coins are indeed 100% Cu, metallurgically
bonded to outer layers of 75% Cu/25% Ni or manganese-brass.

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Michael G. Koerner May they | | | | | | rise again!
Appleton, Wisconsin USA | | | | | |
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On Oct 9, 11:00 pm, "Michael G. Koerner" wrote:
Paul Ciszek wrote:
The outer layers of US half-dollars, quaters and dimes are cupro-nickel,
I believe 75% copper / 25% nickel. Is the core straight copper? The
US mint website gives an overall composition of 8.33% Ni, balance Cu,
and does not mention any other constituents. I assume they are giving
the total composition of two cupro-nickel layers plus a copper layer.

If this is the case, why doesn't the middle layer of the coin turn green
with verdigris and get corroded away? Cupro-nickel is a very corrosion-
resistent alloy, but straight copper is not.

Similar question: US Dollar coins are supposedly made from copper
sandwiched between two sheets of manganese brass. Why isn't the different
color of the copper layer visible at the edge, the way it is in the half
dollar, quarter, and dime? Granted, manganese brass is not silver in
color, but it still looks rather different from copper. Especially once
the copper would have had a chance to corrode a bit. (The new
presidential dollar coins seem to be holding up to corrosion better
than the Sacajawea ones did.)

The visible copper parts of the edges of USA 'sandwich' coins discolor quickly
in circulation. They tone to a dark brown. This also goes for the pure
copper cores on $1 coins - they tone darker than the manganese-brass parts and
can be easily seen on circulated coins.

And yes, the cores of USA 'sandwich' coins are indeed 100% Cu, metallurgically
bonded to outer layers of 75% Cu/25% Ni or manganese-brass.

--
___________________________________________ ____ _______________
Regards, | |\ ____
| | | | |\
Michael G. Koerner May they | | | | | | rise again!
Appleton, Wisconsin USA | | | | | |
___________________________________________ | | | | | | _______________- Hide quoted text -

- Show quoted text -

I am amazed in recent years how the Red Book has changed the
specifications for the copper-nickel coins. What was the "accepted
wisdom" for years is now greatly refined.

I would think that a well-circulated Sackie or Prezidollar would be
worth much more than an uncirculated example of either. They are
certainly much rarer!

oly

On Wed, 10 Oct 2007 03:32:29 +0000 (UTC), (Paul
Ciszek) wrote:

The outer layers of US half-dollars, quaters and dimes are cupro-nickel,
I believe 75% copper / 25% nickel. Is the core straight copper? The
US mint website gives an overall composition of 8.33% Ni, balance Cu,
and does not mention any other constituents. I assume they are giving
the total composition of two cupro-nickel layers plus a copper layer.

Yep.

If this is the case, why doesn't the middle layer of the coin turn green
with verdigris and get corroded away? Cupro-nickel is a very corrosion-
resistent alloy, but straight copper is not.

For verdigris (copper acetates) to form, moisture is usually required.
One will often find older copper (and even copper-nickel cents) with
verdigris, but if the coin stays dry typically only a mixture of cuprous
and cupric oxide forms.

Similar question: US Dollar coins are supposedly made from copper
sandwiched between two sheets of manganese brass. Why isn't the different
color of the copper layer visible at the edge, the way it is in the half
dollar, quarter, and dime? Granted, manganese brass is not silver in
color, but it still looks rather different from copper. Especially once
the copper would have had a chance to corrode a bit. (The new
presidential dollar coins seem to be holding up to corrosion better
than the Sacajawea ones did.)

It is visible, but some magnification helps. For example, see:
http://wemightneedthat.biz/brassbuckedge.jpg

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