Copper Wire Tarnished?

Hello Eddy,

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

www.wireassociation.com/forum/viewmessag...m?Forum=12&Topic=387

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

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

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?

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
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

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

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.

Hello again Eddy,

OK well you have just provided us with some new and quite interesting information in that your supplier's wire changes color in your plant as well, at least on the outer surface layer of their reel. (bright orange.) We believe that is cuprous oxide and your stored copper wire may also be showing this effect as well, but......

It is difficult to compare the effects on the two separately manufactured wires because of different drawing lubricants, the different age of the drawing lubricant solutions, different contaminants in the drawing lubricant solutions, different annealing equipment and perhaps if electric annealing, the application of a lubricant in the annealer cooling water. For those reasons, the effects witnessed on the two different supplies of wire may be confusing the issue and thus clouding the logic. We recommend that you focus on wire manufactured your facility and ignore the secondary supply of wire until your problem is cleared or at least well understood.

With regard to your question (a) above, we believed the oxidation on the inner layers to be the result of removing the reels from the vacuum annealer while they were still too hot although the outer layers were say at room temperature. We cannot see any other explanation at this time with the information we presently have. You are going to have to run a series of well controlled experiments to really understand what is going on. Assume all of your present data is bad.

With regard to your question (b), we are not prepared to say that you have a problem with your storage at this point time.

Let us know how you make out with your tightly controlled experiments.

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