Copper Strands Have Discoloured Black?

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SOLENTSPRAY

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I'm tidying up the wiring on a Jeanneau yacht I recently purchased and have found three wires where all the copper stands have turned black inside the insulation / sleaving. The boat was first sold in 1988 and I believe this is the original wiring. The copper strands haven't been tinned and the wires in question go to internal and external devices (lights).

Has anyone out there any ideas regarding what may have caused the copper strands to turn black?
What does it indicate - should I replace the wires?
What should I use to clean the strands at the terminal points if I don't change the wires entirely?

I've stripped back the wires about 30cm and found two out of three wires still have black strands. On the wire that feeds the internal lights the stands are still bright copper in colour - no black discolouration nor any sign of oxidation. The colour of the insulation is white on one wire, brown on another and black on the other.

Any ideas / advice? /forums/images/graemlins/smile.gif
 
The black layer is Copper Oxide. It is quite common where non-tinned wire has been used and is due to corrosion in a salt-water environment

As for cleaning the strands, you only need clean the part which will come into contact with a terminal etc. What I do is splay out the wires and give then a light rub with fine sandpaper. This will remove the black layer and the shiny copper will be revealed. Then, twist up the wires again and fix. It may help if you solder them at this point but others may differ.

Hope this helps
 
Liberal application of vaseline or, even better, electrical protection spray, will reduce how quick it goes black.

As mentioned, bare copper cables are much cheaper than tinned copper. IMO use normal copper cable, and replace as necessary. Not much consolation though if your boat is a production boat with 'loomed' cable assemblies :-(
 
Thanks

What confused me is that I expected the salt water effect to create a white / grey oxide powder. Any idea why the copper hasn't produced a white powder oxide and instead turned black?
 
The clean option sounds preferable to re-wiring - removal of glued healinings isn't very appealing! Shall try the fine sand paper approach.

Many thanks again.
 
I would have expected wiring to turn black due to heat rather than seawater which would cause copper to corrode to green copper chloride.

Whether that copper oxide blackness is a problem depends on the extent of it. A lighting load will potentially warm wires as it may be on for a long period. It may be necessary to increase the wire size if it is too close to the wire's capacity and is overheating. Excluding oxygen by using Vaseline or similar may prevent the blackening around terminals.

Tin the exposed ends with solder to protect from salt.
 
The heat theory is one that did occur to me as a possible reason but the insulation shows no sign of having been excessively hot in the past.
 
Just found a website that discusses reactions of copper and it mentions copper turing black without application of heat. I'm inclined to believe the black discolouring is a chemical rather than a thermal effect - just not sure what chemical reaction is taking place and its effect on the conductivity and insulation properties of the wire?
 
The black oxide isn't specific to seawater. Motorcycles of a similar age to the boat mentioned often suffer from the same problem.
I'm no chemist, and other forumites will doubtless have more authoritative views, but a bit of googling sugests that the black oxide is copper oxide (CuO) which is formed when cuprous oxide (Cu2 O) is heated -- is it might be when it's asked to perform as an (inefficient) conductor. Cuprous oxide is red -- sometimes called the red plague.
Similarly, copper carbonate (the familiar green stuff) decomposes to the black oxide when heated.
As well as embrittling the cable, the oxide is highly resistive relative to elemental copper and also prevents tinning.
I've used plenty of tinned cable and, to be honest, it's far from immune from oxidisation. In fact -- and this is an utter no-no for boat use -- the least oxidation I've encountered on old cable was on domestic twin-and-earth (installed by a domestic electrician, tight sod!). The thick strands seem far more resistant to corrosion.
The lesson may be that starting off with a good crimp or tinning (most authorities seem to favour crimping over tinning) reduces local hot-spots which might encourage oxidation. Be interesting to hear from anyone with expertise in this field.
 
in reply to 2Tizwoz,
are you bonkers, if i had electric circuitry heating to the extent of it blackening i would be seriously worried.
i think you can happily believe it is oxidation
 
Calm down Sean!

I believe most people accept that the black discolouration is caused by oxidation. Certainly on all the boats I've had there has never been any overheating and all the wiring ends up black.

The only way of stopping it is to use tinned wire which is so expensive and to my mind unnecessary
 
Thanks folks.

Sounds like this is the norm that many tolerate without rewiring.

With all the other wires I've simply been crimping, but for the blackened wires I was thinking I may clean then tin the stripped wire ends and then crimp - will this work or is tinning and crimping a no-no?
 
Tinning and crimping would probably result in lest contact area between copper and crimp, and introduce another (corosion-inducing) metal into the equation.

It is a good idea to clean up the cores before crimping, but then afterwards liberal vaseline or other sealer to seal the join and prevent further ingress of humid air.
 
i agree with your last comments as I know that in my radio gear for my R/C models after a while one of the wires from the Rx battery goes black (cant remember which one ) & has to be replaced - sorry I cant remember all the tech info for this - seem to remember it has something to do with letting battery level go low - but not 100% sure

cheers
 
It is called Negative corrosion, is somewhat slowed down if the conductors carry an AC supply. salt atmosphere does not help, it pays when using untinned conductors to crimp and fill crimp with solder, this method ensures that minimun resistance at the connection. Once sailed on a DC ship that had a fire in a winch house, first port two winches failed due to this problem,
 
An unfortunate side effect of blackening is that it has an insulating effect, and on some non marine type connectors , such as the chocolate box type so beloved by amateurs, will result in failure of the connection. It LOOKs fine, but the current 'aint going nowhere!

As has been suggested, plenty of protective vaseline or battery temrinal jelly is the simple way to stop it. A less messy way is to spraya couple of coats of PCB Lacquer from Maplins over all the connectors to seal them. The only problem then is if you want to modify the wiring it doesnt come off easily except with special solvent or a soldering iron!

http://www.maplin.co.uk/module.aspx?ModuleNo=29005&doy=2m12
 
As well as providing an insulating layer, the oxide coating has no strength, and on thin wire this will mean bad news as there isn't much else left of the wire. Would not consider remedying but replace.
 
I agree that this is a chemical process and oxidation. My experience is that heat promotes the oxidation process. I have noticed similar oxidation for instance where a domestic hot water pipe turns black whilst the cold water pipe running alongside remains copper coloured.

I am not suggesting that the wiring is necessarily overheating to the extent of damaging the insulation or becoming excessively hot but rather that cabling running warm may be affected in this manner.

What others have said concerning a terminal being a point at which overheating is liable to occur is my experience also even to the extent of melting the terminals and cables.

Tight clean joints making good contact and protected in the various ways described above from corrosion problems will prolong the life of any wiring.

Moisture ingress will create conditions which will promote corrosion.
 
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