Copper Strands Have Discoloured Black?

The black "oxide" layer on un-tinned copper wires is caused by an alkaline content in the water (I deal with equipment used in the food industry and it is a common problem as the cleaning agents used are alkaline based) once it has occured it is virtualy imposible to get rid of and it can re-occur as capilary action takes the moisture up the cable, sometime a very long way, the only sure way of making a good electrical contact is to replace the cable.
sealing the existing cable by coating with vaseline etc will just lock the moisture and the chemicals into the cable!
 
Re: Worse on co-ax cable

Thanks folks,

What an interesting set of responses - thank you to you all for contributing to this discussion. Action plan is to:
1. Replace the one / two wires that should be relatively simple to mouse through
2. Clean the remaining wire, crimp and solder as per suggestion from Pampas - then coat with sealant.
3. Measure the resistance of the wire(s) that wasn't replaced
4. For next 2-3 years, re-measure the resistance of the wire that wasn't replaced to assess rate of degredation

For anyone who is interested / would like to further comment - with the remaining wiring within the boat (that hasn't shown any sign of blackening as far as I can tell) I've been using crimp connectors from RS Components (manufactured by Tyco) and others from Furneax Ridall (manufactured by another company) which have a heatshrink coating - the aim being to make a (near) hermetically sealed joint. They seem to work well, although only time will tell how well!
 
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once it has occured it is virtualy imposible to get rid of

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I have very easely removed the black oxyde by using a rotating steel brush on a drill.. then solded again with tin.. /forums/images/graemlins/wink.gif
 
Re: Worse on co-ax cable

You may well find that measuring the resistance after time will show no degradation at all. The reason is that there will still be enough pure copper to give you a zero resistance reading.

Cleaning the black off and re-using the wire means you will have a thinner wire (less cross section) which in turn means it cannot carry the same current as it was designed to. If you don't replace the wire, the degradtion will continue, removing more copper (the conductor) until such time as not enough copper remains to carry the required current. Result? Failure.
 
Re: Worse on co-ax cable

Everything is relative, careful cleaning shouldn't remove a significant percentage of the conductor. Once the contact and crimp is sealed with solder I can't see how the "black" substance can enter it, I therefore shouldn't need to reclean it unless the resistance increases, in which case its helmet on and strip out time.
 
Re: Worse on co-ax cable

"Everything is relative" - yes, the more money the more relatives....

I agree that if the corrision is only effecting a small length of wire, the damage may well be small and the current carrying ability of the wire not particularly compromised. Hence, re-using the wire might be OK but for a short time only. The reason I say this is that the corrosion will continue untill all the salt ingress has been chemically used. This can go on for years and only noticed when the cable finally fails.

By far the best treatment is to strip the cable back well past where the corrosion has reached. safe is better than sorry.
 
Re: Worse on co-ax cable

If I understand the process correctly, the black substance doesnt 'enter' the joint: it develops there on the surface of the copper, like rust on steel. And like rusting steel, as the black oxide is the product of the breakdown of the copper, the amount of available sound metal to act as a conductor is being eaten away. Cleaning off works, but only if you can arrest the oxidation or 'rusting' process, and enough copper is left to carry the peak current.
 
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