I've noted the same thing. Mfrs say "silver nitride" for anything that looks silver it seems. IMO, the Citori graying that I've seen on the bottom edges of the frame are where the clearcoat and/or silver metal (whatever it is) has worn thru at various levels and usually you're seeing bare steel in the worst spots with some graying from corrosion. Machinery's Handbook has a description of "silver nitride" and one method on how it's done.