ES345 stain on pickup covers - what is it ?

[WhiteCatzs]

I’ve had this 1964 reissue for about 6 months and love it- but I noticed these weird marks forming on the pickup covers under the strings.  Almost like corrosion or something- it cannot be wiped off with a rag.

Something is happening to the metal covers.

Has anyone seen this before, know what it is and if it can be removed?

[rct]

I can't explain it.  Seen it a zillion times.  It's why I prefer uncovered buckers.

rct

[Dub-T-123]

It’s just the way nickel tarnishes. If you put chrome covers on it won’t do that