I was doing some electrical wiring on a trailer today and was surprised to see black instead of shiny copper when I cut and stripped a section from the middle of a long section of uninterrupted wire. There was literally at least 6m of uninterrupted wire on either end from where I cut!

My question is: can wire oxidize INSIDE the protective rubber? And if so, is the wire still good to use or should I be ripping it out and redoing all the wiring on my trailer?

For context: the wire is 18 AWG copper; it simply used to run current from the vehicle 4-pin connector to the trailer lights.

Here are a few pictures:

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You can see two of the oxidized wires at the top; I included a new/fresh wire for comparison.

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The oxidation can be rubbed off with the edge of a knife, but it's not ideal - it seems to be really easy to damage/break off the copper strands.

  • 2
    \$\begingroup\$ if it happens, it must be true! \$\endgroup\$
    – Neil_UK
    Nov 22, 2021 at 11:06
  • \$\begingroup\$ It will eventually rust thru from absorption and harsh exterior climate from poor moisture hydroscopic characteristics (H2O absorption). Time to rewire with proper rated wire \$\endgroup\$ Nov 22, 2021 at 11:23
  • \$\begingroup\$ Cross posted here: mechanics.stackexchange.com/q/86108/10976 \$\endgroup\$
    – Solar Mike
    Nov 22, 2021 at 13:50
  • \$\begingroup\$ That was the negative lead was it not? Common on boats too. Tinned copper wire helps somewhat. \$\endgroup\$
    – user16324
    Nov 22, 2021 at 15:23
  • 1
    \$\begingroup\$ I had some speaker wires with clear insulation, that were in a humid environment during the summer months. After a few years, the green corrosion was over 12 inches into the cable length! It happens! \$\endgroup\$
    – Aaron
    Nov 22, 2021 at 18:16

3 Answers 3


This is a common sight in old wires on trailers, machines etc. Indeed, the wire can oxidize down its entire length. All it takes is some oxygen and time, and wires typically are not air tight. Also if an open end of a wire is exposed to liquid, the wire can suck up liquid by capillary force increasing the rate of oxidation.

This isn't that much of a problem in itself, only the fact that the copper has turned black doesn't make it much worse of a conductor. The issue is if you need to put new connectors on a wire like this.

If you clean it thoroughly before applying the new connector, it can work pretty well. I usually clean the end with fine sand paper, if you are careful and take your time it can be done without any damage to the wire.

  • \$\begingroup\$ The previous owner did use those twist-type wire cap connectors on some of the lights; I understand these are not remotely insulated and should not be used for anything automotive/outside! I'm guessing that because the end of the wire was exposed, that sped up the oxidation of the entire length? I'm guessing that ensuring that any connections are correctly sealed off would reduce the chance of this, along with using thicker gauge wire with more rubber insulation? \$\endgroup\$
    – wild coast
    Nov 22, 2021 at 11:08

Copper is among the few metals whose natural oxide doesn't passivate the surface from further oxidation at typical athmospheric conditions. Given enough time and potentially slightly elevated temperature, it will oxidize very deeply.

Also while the rubber might be rather airtight, it can also outgas oxidizing gases and of course, there are gaps between the strands.

Aluminum wiring in contrast will remain shiny virtually forever.

  • \$\begingroup\$ What aluminum wiring have any disadvantages? I wonder why all trailers don't use it? \$\endgroup\$
    – wild coast
    Nov 22, 2021 at 17:41
  • \$\begingroup\$ @wildcoast aluminum wire has a higher resistance, and thus power losses. Stick with copper for power distribution! \$\endgroup\$
    – Aaron
    Nov 22, 2021 at 18:13
  • \$\begingroup\$ @wildcoast Aluminum has several disadvantages, and is best avoided for most purposes. It corrodes on contact with brass or copper terminals. It's weak and can fracture more easily than copper. It's easily crushed if you use it in a screw terminal. It naturally develops an insulating oxide layer, and only looks shiny because that oxide is transparent. \$\endgroup\$
    – Simon B
    Nov 25, 2021 at 9:10

Talked to a rep from PICO Canada (automotive wiring/electrical manufacturer). They had the following to say: "The cause of the corrosion is the T connectors. Anytime those are used, and not properly sealed the wire corrodes within a couple of years. The thickness of the jacket means nothing towards the protection of the wire if it’s just going to be penetrated anyways."

The person who wired the trailer used those stupid T connectors in a few places. They also used insulated but not waterproof ring terminals for the ground with the exposed wire end showing. Sounds like in wet/corrosive environment applications like trailers EVERYTHING needs to be sealed, otherwise wires just oxidize...

Sounds like these T connectors are literally the devil in terms of quality wiring. I knew they caused intermittent connectivity issues, damaged the wire strands and reduced the gauge essentially, now sounds like they also cause wire corrosion! Horrible... not sure why they even sell them!


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