maninthesea

not sure how to explain the question so here go's

When a larger copper wire (say 4AWG or thicker) has a loose connection going to a motor a motor starter and "Cooks"(my term) I have noticed it turns a maroonish red color and you can scrape it with a knife or file and it does not give that bright shiney copper color. Also over here it will usually develope a green patina inbetween the strands. I have allways told customers they need to either cut that run back to good copper or replace it but I was wondering if anyone could tell me what was really happening to the copper.
Is it changing molecular structure kind of like tempering steel? is something being cooked out of an alloy like when overworked electrical solder starts to get less shiny and melts at higher temp. Is it caused by long exposure to heat? Becuase if just the short term exposure changed the structure then braising copper pipe should do the same but it does not, you can still sand scrape or polish off the discolored metal and a braised pipe will shine.
I am not sure how this knowlage will ever benifit me but I have often wondered.

woodchuck2

I am sure someone will come along with more info but it seems to me that the copper wire has a coating put on it during the manufacturing process to ward off corrosion. The overheating of the wire causes this coating to be burned off as well as copper(II) chloride to be burned out of the copper itself making it brittle. The green tint you are seeing is corrosion starting to form. I too prefer to strip the wire back into fresh copper as a repair, if fresh copper cannot be found then the wire is replaced.

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maninthesea

thanks woodchuck
If there is copper chloride burning out that would explain the different mechanical properties. Yes the green is corosion but it seems to form from exposure to air after a wire has been cooked while I have other wires that have been hanging around the workshop that the ends are not green and one solid wire that was hammered flat(which would have removed any coating for sure) then shelved and its still the dull copper color. Maybe the accelerated corrosion is caused by humid air and a chemical that is released when the insolation Breaks down.

Inner10

The heat could be releasing a corrosive agent. Have a look at an old dry cleaner or other hazerdous chemical environoments, copper pipes will turn black and green from corrosive chemicals.

Grab that piece of wire you have, toss is on the ground, take a piss on it and wait a few weeks; theres your corrosion.

When I was younger I would take these questions to my grandfather who was a chemist; he would answer by drawing molecular diagrams I never understood...sadly he passed away some time ago and I never retained the information.

If I see a wire like that I trim em back and give it a good brushing with my serrated scisors; if it doesn't clean up them I replace as much as I can.

LJSMITH1

Simply put its accelerated oxidation brought on by the overheating of the copper. Exposed copper will always tarnish when exposed to air and moisture (humidity). Adding heat to the equation causes rapid oxidation. The green is a more severe form of corrosion, brought on by the chlorine in the decomposing insulation due to overheating. Also, the heat causes a metallurgical change - like annealing, which could make the copper even softer and have higher internal resistance.

Best practice would be to cut back that damaged portion of conductor and get to some fresh copper for best conductivity and current carrying capability.

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Immanuel

The biggest problem with burnt wire though is not the wire, its the damage to the insulation. Hence .. thats why we have Meggers

maninthesea

Thansk to all and of course that wire would be replaced.
I keep remembering (no cooked wire handy) that the cooked wire seems harder/stiffer not softer like anealing would do. But yes it seems that metelugicaly it is different. Does it change electrical charecteristics when it changes metalurgicaly? Like is it more resistive per foot so it heats up faster?

maninthesea

I decided to do a quick backyard expement. I took about a foot of #6 wire and stripped back half of it and then heated it up with mapp gas to the point it was cherry red. (melted a few strands) I even heated it up right next to the insulation. I am going to leave it in my storeroom which is open to outside air which pretty humid(I think we are getting about an inch of rain a day this week!) Tommorrow I will check how the ductility changes and how much corrosion. But mostly I am wondering about the metalurgical changes.
This is a loose experement becuase I only breifly heated the wire instead of long heat times so the results may not tell me much but what the heck