But the next morning, the brake rotors were covered in orange rust, making a relatively new vehicle look like it had been abandoned outside for years. Brake rotors can begin rusting within hours, and they don’t need to sit outside in a rainstorm for it to happen. Brake Rotors Are Mostly Bare IronMost conventional brake rotors are made from cast iron. So the rotor’s braking surface is left vulnerable to oxygen and moisture—the two ingredients needed to create rust. It Doesn't Have To RainYour brake rotors can rust overnight even when the weather seems completely dry.