What Causes Brake Rotors to Rust?by KevinM
The brake rotor is a cast iron disk that is fixed to the wheel bearing hub assembly. It rotates with the wheel, and the braking action is produced when the rotor is squeezed between the brake pads to slow the wheel down. Because the rotor is cast iron it can rust, and this rusting will accelerate under certain conditions.
Cast iron will rust in the presence of humidity, and brake rotors are no exception. Rust will occur all over the rotor, but because the braking action rubs the rust off of the friction surfaces, rust accumulations normally are visible only around the outer edge and inner surface of the disk. Rotors will rust more in humid climates.
Salt will accelerate the rusting of cast iron. Salt can be found in the sea-side air, and often is applied as deicer to roads in cold climates. Brake rotors will show more rust when subjected to these conditions.
Different qualities of brake rotors are available. Lower quality rotors often will rust faster because of the lower quality of the iron used in their manufacture.
Rust will propagate faster in areas where it already is established. If the rotor is severely pitted, then rust will establish itself deep in the pits where the braking action cannot rub it off. The rust will then tend to grow out quickly from these areas.