You can clean the contact or in this case Wire with some very fine sand paper - like 400 or 600 grit. I've even done it by scraping with my pocket knife.
Though probably better to just cut 1" off the cable and strip the ends. Once the Copper is exposed, you could use a very thin layer of
Dielectric Grease or
Anti-seize compound. Wipe a bit on, give it a second or two, then wipe it off. Even if you wipe it off there will still be a thin coat on the wire.
I would suspect you could even use something like
WD-40 assuming you wipe it off after you spray it.
I did find a copper based Anti-Seize Compound, massively more than you need, but still tolerably priced.
Amazon product ASIN B000LDDOXG
Let's see if I can find that in the UK -
Amazon product ASIN B001HWBSII
This is enough to last more than a THOUSAND Lifetimes, so perhaps you can find it in smaller quantities for less money.
I suspect general
Electrical Contact Cleaner with Lubricant would also protect the metal over time. Spray it on, then wipe it off, even after wiping there will still be a thin layer of Lubricant. This is pretty much any Contact Cleaner that claims to Lubricate and/or Protect.
If you use something like Banana Plugs, these hold the wire with Screws, and under the clamping pressure of the screw, it is not likely to oxidize even if the wire exposed to air is oxidizing.
Curious are you near the ocean/sea? It seems as if your wire is oxidizing pretty quickly.
I was able to find something called
Dioxit SN5 Shield that is specifically meant to coat metal to prevent corrosion. That is, I found in the USA, but not in the UK. I suspect it is just a matter of finding the right phrase to search for though -
Amazon product ASIN B005S3JW44
Typical Dioxit is th
e D-Series. The
Dioxit Sheild is the S-Series, and also comes in a variety of types assuming you can find it in the UK/EU -
Lubricant & Protectant for Better Metal Electronic Connections & Contacts. Recommended after DeoxIT® D-Series contact/connector treatment for best p ...
caig.com
Many of the
Dioxit products are on the expensive side, but you don't really need that much so these small amounts should be more than enough for your needs, and at a more reasonable price -
DeoxIT D-Series D100L Squeeze Tube, 100% solution, 2 ml, great for oems - include with your equipment! part no. d100l-2c (2 mL)
caig.com
DeoxIT D-Series D100L Mini-Brush Applicator, 100% solution, 1.6 mL, (applications = 100 +/- brush strokes, 10mm long) part no. d100l-16bx (1.6 ml)
caig.com
Just a few thoughts.
Steve/bluewizard