Ehh this is usually not a big problem. I use mild and i only have very little surface rust on mine. Dont forget that the turbo also tends to leach oil into the charge air, not alot (if you do have alot your turbo is fuct) but a very small amount is normal. Compressing the air also changes the water vapors in the air back into denser liquid form at times too, but again, very very small amounts. If you want somthing that has the corrosion resistance usually stainless is much better for charge piping than aluminum, because aluminum conducts heat wayyy too much especially for the piping after the intercooler going to the engine itself, underhood temps can get quite hot, and that will raise your charge air temps, so its better to get piping that isnt as thermally conductive, like mild steel or best yet stainless steel.