Copper discoloration

We fabricate wire that is subsequently insulated on continuous vulcanization (CV) lines. The bare copper wire going into the CV lines will appear bright and shiny. When it comes out of the CV lines, it is still bright and shiny. However, after 1-2 weeks, the wire will be discolored - reds, purples, greens.

One of the obvious culprits that we have identified in the past is the wire going on the reel being too hot. However, now we observe the discoloration on samples of a few feet in length that we pull and place on our desks. The wire will be look great after one day. But by the end of a week, oxidation sufficient to interfere in ultrasonic welding process.

Thoughts?

Hello wire_drawer,

Copper combines with oxygen naturally at normal indoor temperatures but this happens over fairly long periods of time. Any old copper pipe displays this brownish corrosion. Perhaps this is what you are seeing at your desks. Perspiration and oils on your hands also contain compounds that will quickly corrode bright copper. Perhaps this happened.

Cuprous Oxide can be formed quickly at high temperatures in a wire plant or by contact with high oxygen pressure. The resulting compound is called Copper(1) Oxide, Cuprous Oxide, Cuprite or Red Copper Oxide. It is a brownish-red solid. Its chemical formula is Cu2O meaning two oxygen atoms combine with one copper atom. (Cu + O2 = CuO2) On copper wire, you can see the cuprous oxide layer in full color as various shades of reds, oranges, pinks, and purples.

A second form of oxide can be made when copper is heated in air and it is called Copper(II) Oxide or Cupric Oxide. Its chemical formula is CuO (2Cu +O2 = 2CuO). It is a black solid.

Copper is also slowly corroded by the combined action of oxygen, moisture and the oxides of sulfur (Generally from the products of combustion.) to form that blue-green or green basic copper sulfate film on copper flashing that we all have seen many times. The copper sulfate film adheres tightly to the copper and this prevents prevents further corrosion.

Regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Hello wire_drawer,

The colours that you describe are more typical of some form of sulphur compounds. - but not impossible to get with thin layers of oxide. The colouration is due to the thickness of the layer rather than the actual colour of the contaminating material.

You did not mention what material you are insulating with in your CV line. If it contains sulphur, then this may be the source.

The first step in tracking down the source is to identify the discolouring film. The most likely candidates are sulphides, closely followed by oxides. A dilute solution (say 10%) of hydrochloric acid in water will easily remove most oxides, but not sulphides. If the film is not removed by hydrochloric acid, then try the same with nitric acid while observing the wire under a binocular microscope. If the film is not dissolved by hydrochloric, but is dissolved by nitric, then it is most likely a sulphide. If it floats off as dark particles in the nitric acid, then it is most likely carbonized organic matter. Of course, if you have access to a SEM with appropriate nalysis equipment then you can get a much more definite identification of the film.

From the behavior you describe, it is unlikely to be organic as carbonized materials should be inert to copper and not change over a few weeks. The insulation material should exclude contact with the air and prevent further oxidation after the wire leaves the CV line. The most likely culprit would be sulfur in your insulating material - but without knowing what you are insulating with, this conclusion may be way off beam.