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Copper Wire Tarnished?
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Nonferrous topics such as copper and aluminum, annealing, etc. go here.
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TOPIC: Copper Wire Tarnished?

Copper Wire Tarnished? 2 years, 3 months ago #1434

I am working for a Cable and wire manufacturing company in Indonesia as a Wire production Engineer. Our company just started our wire production recently and I currently oversee the operation.

Our wire production starts from drawing fine wire from 1mm to 0,26mm. The wires of about 700Kg are then annealed in drum annealer for 4 hrs under vacuum conditions. Temperatures are set at 390C. The wires are taken out only after we left it in the drum for 2 days of cooling in vacuum condition. The wires have a bright, shiny finish when we took it out from the drum.
However, after we store the wire for about 2 months. The wires starts to change to dark reddish color.

The worst is when we have the still bright and shiny wires used for bunching. After stocking the bunched wire for about a month or so. We found the inner layer of the bunched wires changed to black reddish color.

I understand that copper left in normal atmosphere conditions will cause oxidation thus creating a copper oxide film coating over the wire. However, the rate at which our produced wire changes is too fast for me to believe there could be some problem in our manufacturing or handling process. We still stock up wire from our supplier and we can store the wires for more than 5 months with no problem.

Need help desperately, Please advise. Thank you.


Re: Copper Wire Tarnished? 2 years, 3 months ago #1435

Hello Eddy,

First of all, please read the following thread at the Forums so we are all speaking with the same terms:

You seem to be having have a problem with cuprous oxide (brownish reddish color) for some reason and it does indeed have something to do with your manufacturing process because your supplier's wire does not exhibit the same problem.

Perhaps the core temperature of the annealed spools of copper wire is still too warm when removed from your vacuum annealer and oxidizes over a short period of time. Perhaps you haven't purged all the air from the vacuum annealer. (First obtain the vacuum and then let it sit for a period of time to see if the vacuum changes slightly

Peter J. Stewart-Hay
Stewart-Hay Associates

Re: Copper Wire Tarnished? 2 years, 3 months ago #1436

Hello Peter,

Thank you your response. Please advise again with my doubts.

"Perhaps you haven't purged all the air from the vacuum annealer. (First obtain the vacuum and then let it sit for a period of time to see if the vacuum changes slightly"
As the temperature rises, the drum annealer pressure will drop slightly to 60cmHG and we have to re-vacuum the drum to the max of 75cmHG. We have to do it once every time throughout the annealing process and the pressure will maintained at 75cmHG after that.

Is there a specification to as how much pressure has to be reached for a certain amount of load or volume?

I have another question. Please correct me if I am wrong, copper starts to oxidize at around 200C, which is the reason why we need to isolate the wire from oxygen during annealing to prevent the formation of oxides, thus maintaining the shine of the copper. In this case why is it when we remove the wire from the drum annealer, the shine of the copper changes more rapidly if it is still warm? (Say around 50C - 60C)

Is there an ideal temperature that has to be maintained to prevent the formation of copper oxides?

Re: Copper Wire Tarnished? 2 years, 3 months ago #1437

Hello again Eddy,

The formation of cuprous oxide is a function of both temperature and the concentration of oxygen in the atmosphere. The warmer the temperature, the faster the formation of cuprous oxide. At room temperature (20C) and at the standard concentration of oxygen (21% by volume), copper oxide will still form but it does take a longer period of time.

We never removed reels of copper wire from our bell annealers until the copper was back at room temperature, both in the center of the wind as well as at the surface. Saying this however, our schedulers would have been fired if they held any inventory of drawn and annealed copper for a total period longer than say 10 days, including holidays. (Just in time scheduling.)

Smaller wires like strands for bunched wire were always electrically annealed in-line, immediately after drawing.

We strongly recommend that you carefully reread the manual/ operating instructions that came with your vacuum annealer.

Kindest Regards
Peter J. Stewart-Hay
Stewart-Hay Associates

Re: Copper Wire Tarnished? 2 years, 3 months ago #1438

I don't know if it is practical in this case but for a lot of the wire that we would use for magnet wire, we would wrap in foil before putting it into the bell or drum annealer. Sometimes we had to force cool it by spraying water on the bell or drum. At one place I worked we had continuous vacuum pulled on the bell. at another place we drew a vacuum and then put bell into furnace. The foil did help prevent oxidation.

Again most important thing is like you mentioned be cooled within the reel and not just the air in the drum

Re: Copper Wire Tarnished? 2 years, 3 months ago #1439

Dear Spectre and vettecoupe,

Thanks for the suggestion and information. I rechecked my annealed wire for the inner reel temperature and very certain that after 2 days of cooling; the whole reel had properly cooled to the room temperature.
All annealed wires are stored for at least 2 days in vacuum before being used. All are checked visually for color changes.
1. - Wires used for bunching are visually monitored to ensure the shine throughout the reel.
2. - Storage time for the bunched wire <= 2 months.
3. - Inner layer of the bunched wire turns brownish or matte yellow.
a) - If it is oxidation, why the inner layer turns brownish instead of the outer layer?
b) - Is there a problem with our storage? I used a plastic wrap over the bunched wire but it does not help.
We are still monitoring by changing to moisture absorbent paper to wrap the bunched wires for storage. The wires storage lifetime is cut short due to this problem and I am trying to figure out where is the problem because our supplier’s wires can be stored much longer than ours. Even though there is a change in colors, it only changes the outer layer to bright orange while the inner layer still shines as normal.

Appreciate your comments. Thank you.
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