Savvy's armor is bare aluminum. I personally like when it's powder coated more. However, if you plan on scratching it up a lot, leading it bare would be the better option.
Aluminum corrodes but it does not rust. Rust refers only to iron and steel corrosion. Aluminum is actually very prone to corrosion. However, aluminum corrosion is aluminum oxide, a very hard material that actually protects the aluminum from further corrosion.
As for the aluminum causing moisture to get trapped between the body and the aluminum part, I wouldn't worry about it. If that was the case, the same thing would happen with your stock plastic fender flare and the body (since plastic doesn't rust either).
