A chemical/metallurgy question

Mike Harding · Guru

A chemical/metallurgy question

rgren2 · Guru

Ive had a dark discolouration on wires on my boat, its not always overloading, can be a chemical reaction, I scraped the wire to get some bright copper showing, this I dipped in an acid (probably Bakers Soldering Fluid as that would have been the only stuff around), the copper became bright quickly, I slid some glue heatshrink up the wire, twisted and soldered. Slid the heatshrink over the join and heated it to seal the join. There is another school of thought to use crimps and heatshrink.

Tony LEE · Guru

It isn't normally an indication of overloading. I've seen brand new, but long-time stored cable just about black inside) Possibly a reaction with the sheathing. Doesn't seem to happen with tinned conductors Only way I know that doesn't run the risk of even worse corrosion later is to mechanically remove the tarnish with fine steel wool. Years ago in my first job out of uni, an old electrician showed me how it was done - joining a 2" diameter cable. Every strand was separated and polished to a mirror finish before laying it all back neatly to fit into the socket before soldering. No hydraulic crimper available then.

Triggy · Veteran Member

Just a thought is it multi strand all copper or like sat cable steel core copper coat

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Mike Harding · Guru

Triggy wrote:
Just a thought is it multi strand all copper or like sat cable steel core copper coat

It is a multi strand cable and I believe it to be all copper.

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This discolouration is definately from current overload and thus high temperature - I knew I was pushing my luck with cable this small when I chose it :)

Yes, crimping is certainly a possibility but it will change the characteristic impedance of the cable at that point - this may not matter a lot but I'd rather not do it if possible.

It's been in vinegar for about 10 hours and I think it looks more "coppery" but that may be wishful thinking?

Whenarewethere · Guru

A few thoughts.

Marine cable is tinned to minimise corrosion (used to have a yacht).

I have resoldered 40 year old welding cable for jumper leads (brilliant fine copper gauge) cleaned up a bit & used lead/tin solder. Tin only solder is a pain in the neck.

Rather than trying to join the cable directly. Can you add a jumper section with multiple crimped or soldered points for either end. Then heat shrink the overall section.

Mike Harding · Guru

Whenarewethere wrote:
Rather than trying to join the cable directly. Can you add a jumper section with multiple crimped or soldered points for either end. Then heat shrink the overall section.

The issue with using repair methods (that I can think of) other than simple solder joints is that they will likely change the characteristic impedance of the cable this may, or may not, be significant but I would prefer to just solder if possible. As this cable is working at high frequencies things are electrically very different than is the case at DC or 50Hz.

deverall11 · Guru

All that is required is a cleaning of the surfaces. Copper does not change its chemical structure. Actually no metal
does unless you melt it. That's a whole discussion in itself.
Acetic acid is all you need to remove the oxide layer. As soon as it is shinny, it should be ready for soldering.
The issue with soldering is that is a relatively low heat, compared with welding or brazing, and therefore unable to penetrate through the oxide layer.
The oxide is what is preventing the solder from sticking.

KJB · Guru

deverall11 wrote:
All that is required is a cleaning of the surfaces. Copper does not change its chemical structure. Actually no metal
does unless you melt it. That's a whole discussion in itself.
Acetic acid is all you need to remove the oxide layer. As soon as it is shinny, it should be ready for soldering.
The issue with soldering is that is a relatively low heat, compared with welding or brazing, and therefore unable to penetrate through the oxide layer.
The oxide is what is preventing the solder from sticking.

Good, simple explanation...........

Mike Harding · Guru

Forty eight hours in white vinegar brought the copper colour back to the wire and I've now soldered it together.

Many thanks for your responses people :)

bruceg · Veteran Member

To prevent re-occurance of the black oxide coat the first few cm of the copper strands with tin and then use a glue type heat shrink over the soldered joint.

See this video for a cheap DIY tin coating solution. Make sure the solution is rinsed off and the wire strands are dry before sealing with the heat shrink.

https://www.youtube.com/watch?v=Hsw3lOnHaas

As an electrical engineer you may already have a supply of (expensive) proprietry tinning solution ?

bye