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Copper Color and Properties
Name: Kamran B.
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: May 2004
Question:
Different colors of copper wire - golden yellow to
reddish black.
During the drawing process of copper wire, if the temperature varies, it
leaves various different shades of colors on the finished wires i.e. from
golden yellow to reddish black. Does this have any impact on the
properties of the wire in terms of electrical characteristics,
conductivity, mechanical properties, elongation etc.?
Replies:
From the description of the drawing process, it sounds as if the different
colors are due to oxidation or corrosion on the surface of the wire as it
exits the die. Typically, the outer surface of a wire has little or no
effect on the properties of the wire itself. However, the heat treatment
that the wire undergoes while being drawn has an influence on the properties
of the wire. For example, if the wire comes out hot it could both oxidize
badly and be mechanically soft.
Sometimes hardening elements are added to the copper to strengthen it (Cr,
Be, Cd, Zr, others) and the presence of these elements may influence not
only the properties, but the color too.
Bob Erck
The colors at the surface are just differing thicknesses of oxidation, thin films of
copper oxide.
The conductivity will not be much affected by a little oxidation at the surface.
It might affect the hardness or softness of the copper a little.
It can't affect the elastic constants of the metal (but then elasticity is hard to see in
soft copper).
But "elongation", the percentage of stretch before breakage occurs, is sensitive to
surface condition.
A stiff non-metallic surface film will crack rather than stretch,
and then the crack in the surface film will start a deeper crack into the soft metallic
copper, making breakage happen sooner.
In effect, the yield strength is the same, but the final breaking strength is lower. I
think.
A darkened wire may become more likely to break at little scratches after some time in
slight tension or vibration.
I don't know how inevitable this is.
These surface oxides can be stripped off with acid + oxidizer. Possibly dilute nitric
acid.
A solution of ammonia and hydrogen peroxide can work too. Then you have two
concentrations to play with.
The new surface is usually a little grainy, rougher in a way,
but the color will be light and any remaining stiff film on the soft copper will be very
thin.
With some art and experimentation the surface is not too rough.
So could re-finish your copper this way, if you want to try it.
Jim Swenson
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Update: June 2012
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