It's worse than that (water) the by-products of combustion are effectively a mild acid, that's why it gets to the exhaust steel so easily.
And it's even worse than that, when you see rust spots on chrome-plate it's not the chrome that's rusting. Chromium does't rust, that's what makes stainless steel "stainless" as it's alloyed with steel in various percentages to gives different levels of resistance. The rust is from the underlying steel that is corroding and passing through pores in the chrome so that you can see them.....that's why Bryan's local 500 guy has got his pipes to last so long by nuetralising the acid and excluding the oxygen.
The best you can do when in use is to run it as close to lean as reasonably possible for less unburnt fuel and never switch off and park it unless the pipes are really hot.....rain on the outside doesn't reall do much except cool the pipes and help cool the gases inside to leave the pipe's condensation inside the pipes and dripping out the drain holes.
