Trailer springs will rust, since they are tempered steel. I like to wash them well with fresh water after each dunking. Some folks spray or pour (have to catch the residue) transmission fluid on them to help prevent rust. Others use corrosion X, some use tar based under coating or grease. All of the above work--before use clean off with a good stiff wire brush (wear face and eye protection). You can also clean the steel with phosphoric acid or "naval Jelly". I have an abalone iron I made from an old Chevy leaf spring (at least 60 years old). By coating it with grease after use, there has been virtually no further corrosion.
With leaf springs I always carry an extra set incase one breaks. However my current trailers use torsion axels, which seem less prone to rust problems.
