Zinc sulphate treatment of cast ductile iron

I am seeking a method to protect cast Ductile Iron parts for use in high temperature, high humidity environment, close to the ocean. 316 Stainless is usually specified but I have been told that a treatment with zinc sulphate may be superior to even 316. How could this be so? The items are cast into the outside of concrete walls.

This sounded intriguing, but in searching the Internet and reference books back to the early 1960's, I was unable to find such a treatment. Perhaps someone was thinking of a zinc chromate-containing primer?

If the parts are already installed, and assuming high temperature refers to ambient conditions, I recommend a good cleaning (using a phosphoric acid rust remover if necessary), followed by a zinc-rich epoxy primer and a polyurethane topcoat.