Glorian

I have quite a few discs, and I've noticed over time that some of those discs have what might be called 'burn lines': a thin, dark line on the edge of the disc's metal layer (almost looking as if the metal was singed or burned). It can be on the outer edge of the disc (in which case it's along part or all of the disc circumference), or the inner edge of the metal layer, near where the disc is gripped by the case hub (in which case the line circles the inside perimeter and looks like a burnt ring). If these 'burn lines' are on the inner perimeter, the line is usually very regular, while lines on the discs' outer perimeter are usually irregular.

I haven't found any mention on the internet about these lines (maybe because I don't know what they're called), but other people must have seen this, because I've seen it myself on a number of discs (including discs owned by other people).

I don't know what it is, but I'm guessing that it's oxidation of the edges of the aluminum layer because of a problem with the material used to seal the inner and outer edges of the disc from the outside air. Can anyone tell me if that's correct, or, if not, what the actual cause is?

If that is the cause, is it something to worry about? My guess would be no, because passivation of the aluminum should prevent further oxidation, but I'm not sure.