I've seen some pretty nasty corrosion on the outside of engines. I had an engine apart in pieces on the bench. It was several months before I got back to it and the aluminum piston was pretty ugly. As with steels corrosion resistance varies substantially with alloy composition. I've been meaning to buy some Corrosion-X for storing engines, but have not seen it at stores here. I have Boeshield for my machinery, but that puts a thick waxy coating on parts.