A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?



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