I think he may be talking about the inner part of the disc ie the bit the brake pads dont touch.

I got a new disc for my XTR after i had an off and bust the old one and after about 2k miles from sept onwards on all the salty roads its shown more corrosion on it than the standard one which was 2 and a bit years old!!! Nothing to make it fall to pieces but signs of furryness and a few orangey metal bits

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Plough on