brake rotor on 1997 Ford Mustang

why my brake rotor is rusting in the inside surface?
both sides.only lasted 1 year ,and this is the second time.

Asked by for the 1997 Ford Mustang
Do you mean the ventilated brake rotors are rusting in the center ventilation area or are the discs rusting on the in board side of the brake rotor? The brake pads "clean the rotors" as the brakes are applied. The center vent slots will have surface rust in the center any way. Normal brake application should keep the rotors clean.