Why do my brake discs rust overnight?
Because brake discs are made of cast iron, which is deliberately not corrosion-resistant, and there is nothing protecting the surface between uses.
Cast iron is chosen for discs because it handles heat well, dissipates it quickly, wears predictably and provides a good friction surface with brake pads. It also rusts readily. A stainless or coated surface would resist corrosion but would be worse at every one of the properties that actually matter.
The rusting is fast because each time you brake, the pads scrape the disc face completely clean of any protective oxide layer, leaving bare metal exposed. Add moisture — rain, dew, condensation, a car wash, humid air — and a light orange film can form within hours. It is entirely normal, and it is why cars parked outside overnight often show rusty discs in the morning.
It clears itself. The first few brake applications scrub the surface film off, sometimes with a brief grinding sound. Nothing needs to be done.
What is worth attention is rust that does not clear. Deep pitting on a car that has stood unused for months can leave a permanently rough surface causing vibration under braking. Rust on the edges or in the unswept area at the centre is cosmetic and normal, even on discs in daily use.
A specific risk: applying the handbrake with hot, wet discs can leave pads bonded to the disc surface overnight. On a car being stored, chocking the wheels and leaving the handbrake off avoids it.
Rust visible through alloy wheels is often blamed on poor quality parts. It is not — it is the material working as intended.