1991 Toyota Corolla rear wheel eloaded

Brake dust from the rear shoes will accumulate on the inside of the rear wheel, black and sooty. Corrosion, or rust is simply water reacting to the steel wheels.
If rusted, remove wheel use a cone shaped steel brush on a drill and remove as much rust as you can, then repaint with a rust inhibiting paint.

Also check for brake fluid leaks, as brake fluid will damage the paint on the wheels.

The front of the wheel is not as exposed to brake dust and generally is covered with a wheel cover, but they will rust too.