Rusty Parts

Here is the way Dragon #346 describes the physiology of the rust monster. Apparently the antennae have tiny little hair like appendages that have tiny little capillaries that run all the way to the to the rust monsters stomach. The rust monster breathes in carbon dioxide and exhales oxygen and other caustic chemicals that travel those tubes and start a very aggressive oxidation process that rusts metals instantaneously. These same hairs exist on the body of the rust monster hence when attacked the body damages the metal that attacked it. So the way I see it, no breathey, no rusty. Once the beast is dead, his ability to rust metals dies with it.