why would disc brakes corrode on the INSIDE only?
 

Brakes are 'juddering' - almost like the ABS kicking in under gentle braking. Mechanic pal says the discs are corroded on the inside where you cannot normally see. What could be wrong? My car is a Cherokee 2014 2.0 l Diesel 4 x 4

Hopefully he means that the rotors are warped, which would cause a pulsation that is felt in the brake pedal while braking. If he said corroded pack your **** and find another mechanic. You will probably need new front brakes discs and pads, also check the calipers for proper operation.