Car exhaust is a weird case though. The water vapor in the exhaust condenses on the cold metal. If you drive far enough, you add enough heat to evaporate the water back out. Else, you end up with a bunch of water in the exhaust facilitating the rusting.
In general, you have this problem with cold metal when there's enough humidity to cause condensation. Bare cast iron in an unconditioned space under cover will definitely rust from condensation.
