How to prevent secondary oxidation / Rust

[DJthefirst]

I have a smaller one Tesla coil kit and this is the second time the secondary has oxidized it has 5-6 coats of oil based polyuratane yet moisture some how still seeped through. I’ve recently finished winding a larger secondary for a new coil I’m building and fear the same fate. Also buying and winding replacement secondary’s gets pricy. Any suggestions on how to stop coil oxidation?

[MRMILSTAR]

I've never experienced that problem. You are the first person that I've ever heard having that problem. Are you using PVC for the secondary coil form? If so, PVC can absorb moisture. The best practice is to apply a coat of varnish to the inside and outside of the tube before winding the coil. This prevents moisture from getting into the PVC. I also store my TC equipment in an environment that has no more than 50% humidity. Are you using magnet wire for the secondary coil?

[TMaxElectronics]

What exactly does the oxidation look like, can you take a picture?
Copper wire usually does not noticeably rust, even in wet environments. (the generation of the patina (the green layer) on copper statues takes years for example)

Where are you storing your coil? Is there a chance of active chemical corrosion (paints,solvents,cleaners,acids being stored nearby can cause that).

I'd also recommend using clear epoxy for the coating if you can afford it, as it creates a harder and thicker layer in less coates compared to varnish. I use epoxyplast 100 p, but that might be a local product.
Look for some fairly slow curing stuff (~1h), as the fast might harden before you poured it all onto your coil.

[DJthefirst]

The coil came pre-wound with a single coat of spray-on polyurethane upon which I added more layers of polyurethane brushed on heavily. I didn't heat treat either coil to remove humidity before sealing it. My first coil died of internal arching and the second wasn't working though this morning I got it running again. while technically neither coil has died from it yet it still concerns me. The oxidation is a darker brown grayish spotted around the coil. since my first coil was already ruined I sanded some of the oxidized spots to make sure it wasn't just surface discoloration. The second one had a piece of packing tape around the top to protect from the toroid under which the oxidation formed. the second coil is only about 1/2 a year old and I wouldn't think that moisture could perforate the polyurethane. I store my coils in my room which is dry dining winter and gets moderately humid during summer.

[TMaxElectronics]

Odd that the oxidation only occurs in that limited area and with very a sharp cutoff. To mee that would rule out oxidation from the atmosphere, as I'd expect that to happen more or less evenly.
EDIT: maybe i should read the entire text before responding
if it only happened underneath the pacing tape, it might have something to do with the adhesive it uses.

It also seems to go quite deep which is even weirder, as copper oxidation actually protects the metal underneath (like aluminium).

Did you maybe use some kind of silicone sealant somewhere on the secondary? The standard stuff releases acetic acid when it cures (vinnegar), that can attack copper a bit faster than air.

I didn't heat treat either coil to remove humidity before sealing it.

I haven't ever done that and had no issues so far (including my first coil which is now >5yo).

[DJthefirst]

Super glue was used on the end caps but allowed to dry for a day before coming into contact with the coil which had already been coated in polyurethane. Hot glue was used to attach the end caps. I still don’t see how it would have gotten through the polyurethane. And if the discoloration isn't a type of oxidation but some other reaction I have no ideas for what it could be. If it was in-fact a reaction with an acid in the glue it should occur on either end not just the top. And if it was just a reaction with the adhesive it should be only on the surface and not have affected the wire.

[johnf]

Having wound several secondary coils on PVC and having them fail after awhile --tracking / corona on the inside of the pvc which releases hydrochloric acid vapour. Replacement coils on polyethylene formers still gong

[MRMILSTAR]

It doesn't look like copper oxidation. I am guessing that its a degradation of either the varnish or the magnet wire insulation. I have no idea why though.

[Zipdox]

I would think that it's the varnish degrading, not the copper. Do you live in a warm and/or humid part of the world? Where do you store the coil?

[Mads Barnkob]

My bet is also on the varnish cracking, possibly enough to let in oxygen molecules that has resulted in mild copper oxidation.

I have to old spools of enamelled wire that looks the same, usually I can just unwind the outer layers of a spool and it looks fine underneath, makes me think it could be UV from sun light degrading the varnish over time.

Your secondary must surely have had these brown areas from the day you got it?