Copper wire gone "green"

Most of what you see is surface oxidation only and it seals the rest of the metal much as aluminum oxide does. Thus, conductivity will not be greatly affected, provided the surface oxidation is not disturbed and provided all connections are protected from oxidation. You do have to guard against galvanic action however. I'm guessing these are overhead wires that you are referring to?

This almost looks like a trick question. I don't think you need to be a scientist to figure this out, though it does bring up some other interesting questions.

First let's tackle the green. (Are you just asking because it's St. Patty's day?) That color is an effect of oxidation. Oxygen molecules combine with copper molecules and form a new compound on the surface of the copper. Because the copper molecules (conductive) are interspersed with oxygen molecules (non-conductive) the current (flow of electrons) is interupted or reduced depending on the voltage (pressure behind that flow). NOTE: I may have interchanged volts and amps. I often confuse the two.

You may know as a matter of practicle experience that if you brush the green off at the point of attachment the flow can be restored. This then brings up one of those other questions. If electricity flows on the surface of the wire, why does it work to just clean the point of attachment? Silly of me to ask but it's something I've never had a satisfying answer to.

Electricity does not flow just on the surface unless it is very high frequency. For dc and 60 Hz ac, you should just use the whole wire cross-section (there's a small reduction for 60 Hz but it's not worth worrying about).

Friend,

The "skin effect" pushes the electron flow towards the surface of the conducting material. If you have an insulating material on the surface, regardless of whether this is a completely different material such as a plastic coating or a chemical reaction product such as copper oxide (any oxidation state will do), the skin effect is pushing the electron flow towards that surface. But, it doesn't push it onto the surface. Therefore, the only effect the insulating material will have is on the dissipation of the heat build-up due to the I²R losses. This will indirectly affect the current flow because of a change in resistance as the conductor temperature changes.

Now, when you are terminating the conductor, the current flow must not only get onto the surface of the conductor, but leave it where the surface is intimately attached to another conducting material. In this location the oxide coating is definitely harmful.

Does this explanation help? --JMM

OK...

What you all are telling me is that;

1. the green color is OK along the wire itself
2. however, I must make sure that where the wire ends and is attached to any other conducting surface such as a switch or motor the wire must be cleaned of any oxidation so that it conducts properly

OK?

It would be helpful to know the diameter of the wire in question. Assuming it is sufficiently large in diameter to carry the voltage and current it was designed for, one can also assume that any surface oxidation is of no consequence, unless of course, the oxidation has caused excessive reduction in the diameter of the wire. Surface oxidation between the wire and a connector is quite another matter, as has previously been addressed (galvanic action). Oxidation at the wire-to-connector location (as in crimp-on connectors, etc.) can result in an degradation of conductivity, due to an actual insulation effect, occurring at the surface interface, because of the oxidation. This is one reason that microwave ovens mysteriously fail to fire the magnetron after several years of normal operation. After testing components in the high voltage circuit, and finding they are all good... then, one needs to re-crimp or replace the spade connectors, and voila, the oven heats like new.

The green color is never ok because it shows you that you
1. didn`t enough to prevent corrosion or
2. didn`t stop corrosion if detected.

Its truely essential to clean the surface before connecting two conductors. But greenspan id the copper salt of acetic acid and a lightly toxic substance used e.d. for fungicidal applications.
Together with (salty) humidity your corrosion will not stop.

Independant of whether you are at the coast or not, corrosion endangered connections should always be greased as known from the battery contacts of your car. When you are crimping cables pls. have in mind that your connection can never be better than the tools you used - so throw away the supermarket scrap and buy a good one which you can use for many years and which will not deform the first time you want to make a gas-tight crimping connection.

Good luck
Uwe

Hi Uwe This is a constant problem here in Bali not just with cables but also with PCBs. Are you suggesting cleaning/dipping the copper in acetic acid? Mark

Hi Mark,

no - just not !
Acetic acid is an unwanted substance forcing surface corrosion and creeping everywhere.

There are a lot of products available which can clean/isolate/lubricate contacts as well as PCBs. For use under hard environmental conditions (as humidity and temperature changes in e.g. railway applications) we use sprays for PCB covering which protects from unwanted current paths or corrosion.

Have a look at wacker.com - thats a big german company specialised on products like the above. But there are others, too, depending on your possibilities.

Regards Uwe

Hi Uwe, Yes, Wacker has 1000s of chemicals :-) . Can u point me to the specific 1s ur talking of here? MT

Hi Mark,

we use products of CRC Industries who belong (or formerly belonged) to Wacker.
There is a "CLEANER 601" for stopping corrosion and a "PLASTIK 70" for covering PCBs.
You get some info here:

http://www.crcind.com/wwwcrc/msds/AB27430-3.htm

Best regards Uwe

Hi Uwe, Many thanks, I'll check it out. Mark

Spot on. We have picked up two problems with bare copper transmission lines (actually 3 as you'll see)- they are at 11kV, around 25-30 years old. and railway catenary wires (up to 60 years old):

1. We have experienced no reduction in current capacity. The oxidation only occurs at the surface. We have found that removing clamps and re-positioning them led to failures where the old clamp used to be - it seems as if the old clamps assisted in mechanically holding the wire together. In all these cases, the Cu wire had overheated at the joint, and it was clear that the joint was sslightly dirty. Infra red scanning assisted in pinpointing these pro-actively.
2. The transmission lines also experienced mechanical fatigue, possibly due to wind. They have no dampers. This would probably not be related to oxidation, but I'm just assuming so, never analysed it properly, due to (3) below.
3. In South Africa, copper lines don't really get old enough to evaluate anymore, due to theft. In virtually all cases we have to replace with ACSR.
   

Except for superconducting materials, electricity flows through the cross section of the wire by electrons transferring from crystal to crystal on the lattice. It is my understanding (comment based on information from a cryogenics course back in the 1960's) that superconductors circulate current only along the surface atoms and without resistance; this is why it is said to be possible to make a superconducting loop in which current flows nearly perpetually around the loop.

In vehicles, they use silicon grease on junctions of male and female terminals to avoid entry of oxygen.

Metal varnish would also surely help retard the on-set of corrosion.

I warp my connections around salt or anywater for that matter to keep down maintance. Heat sealing connection covers work great.

My personal experience is that the oxidation will never stop untill the entire wire is consumed. The oxidation is a poor conductor, and the wire will pass current only untill it can no longer carry the current load upon it. Then it will burn up and fail. I've seen this happen in the field, but pehaps there are other things going on here too. There's a reason we try to keep all our electical connections dry, as even condensation can be a big problem.

What kind of lugs are the conductors landed on? If they are aluminum and the wire is copper, then you have the dis-similar metal problems and need to coat the copper with an agent that is designed for stopping this problem. I have seen it quite a few times that this hasn't been and the effects are ugly...

Are they a mechanical connection of sorts? Like a kearny? There are packets that can be bought to cover the connection inorder to protect it... Lastly, therer is always the good ol' rubber tape wrapped around the joint covered by a bunch of scotch 33 (again ONLY if it is a mechanical sort of connection - not a lug, panel, or gear type of connection)...