BIL's 2001 2wd was pulling erratically to side when brakes were applied. I figured warped rotors from lot of city driving...
This is what I found when I pulled the rotors off.
Both side rotors had this corrosion, but only on the inside surface of the disc. The outside surfaces were normal.
Any ideas what would cause this?

 

That's pretty bad
It's just rust from where the brake pad was sitting for a period of time
Replace both rotors and the pads and it will most likely still pull right
Have the alignment checked and possibly cross rotate the front tires to verify or eliminate tire pull
Those rotors look as old as the truck (original) and would still be okay to run for some more time IMO if they are not warped

 

Already replaced rotors, pads and wheel bearings.
Didn't pull much after replacement (was pulling to left), and had alignment done. Perfect now.

Posted pictures because in over 40 years of doing general auto maintenance/repairs I've never seen that kind of corrosion damage on brake rotors.
All better now.

 

It is common look of worn-out rotor. My at small car looked exactly same a few months go.

 

Agree with all the above....... rotors turned or worn beyond spec, rust formed, rotor fell apart.