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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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