I was doing some electrical wiring on a snowmobile 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 5m of uninterrupted wire on either end of where I cut!

My questions are as follows:

  • Can wire oxidize INSIDE the protective rubber? I know wires exposed to air oxidize, but this is literally meters down a rubber-coat wire...
  • 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?
  • I'm assuming simply crimping it without at least cleaning it will introduce a lot of electrical resistance, and potentially melt my crimp connector?

For context: wire is 18 AWG copper; used for simple trailer wiring - running current from the vehicle 4-pin connector to the trailer lights. I'm from the PNW, so it is pretty wet around here, and the trailer does see a lot of winter driving on salty roads. 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 - seems to be really easy to damage/brake off the copper strands.

Any thoughts? Suggestions?


3 Answers 3


This pretty much happens. Sometimes in hard-to-replace wires.

Things that promote this type of corrosion:

  1. Positive potential for prolonged periods (e.g. connected to the battery plus or otherwise constantly powered in a heavy use car.

  2. Proximity to the battery. The sulfuric acid likes to crawl in a tiny layer over metal surfaces.

  3. (related to 2) the battery being old, bad or overcharged (these factors get more acid out of the battery).

  4. Moist (for prolonged periods)

  5. Salt (nearby sea or heavy winters with overly salt-happy road administration)

  6. Low-quality cabling. Moist, salt and acid can penetrate and soak the pores in the plastics and get to the wire not only from the stripped ends, but also directly thru the insulation. Some cables are better than others in this regard and no plastic can be considered gas-tight or water-tight when long timespans are considered.

  7. Agressive flux used for soldering. Some people like very much the acid-based fluxes that work well everywhere (including wires already oxidized like this). These fluxes are not used in electric/electronic work for a reason - they cannot be washed away.

This kind of corrosion is usually slow enough, but it gradually increases the wire resistance, sometimes all the way to the point where the voltage on the other end being unacceptably low or the crimped contacts becoming losy. At this point, nothing short of replacing the wire helps.

If the situation is not this bad, the wires can be soldered by cleaning them mechanincally first. The oxide layer is harder than the copper itself and easilly flakes off.


Yes, the oxidation can travel the full length of the wire given time.

Used to have to clean the strands and use flux to solder joints on wires like that, patience to scrape them was needed.

When I fitted trailer sockets we used to fill the socket with a waterproof insulating grease to reduce the possibility of corrosion and we used to do the same when using crimp terminals on the other ends in the lamps - made such a difference as we rarely saw cars come back that we had done. Many cars that we had not done though...

  • I suspect the problem is that the type of wire insulation is not appropriate for the very harsh environment under a trailer. It's exposed to all manner of the elements and if you live in an area where roads are salted, then you have that as well.
    – jwh20
    Nov 22, 2021 at 13:55
  • @jwh20 car manufacturers use the wires than they source in bulk according to the price point as well as the markets they plan to sell in so they also know the climates and temperatures etc the cars will be used in.
    – Solar Mike
    Nov 22, 2021 at 13:59
  • 1
    This is a trailer which in the USA at least, are usually made by smaller manufacturers. It would not surprise me to see inadequate wire used.
    – jwh20
    Nov 22, 2021 at 14:24
  • Any wire recommendations if I were to rip it all out and replace it. I'd rather pay a bit more and not have to spend another day rewiring everything in a few years time!
    – wild.coast
    Nov 22, 2021 at 19:34

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...

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