To save your steel.
First you can wash and wax it- carnuba paste floor wax. I might add that the important removal of salt as BDAHUNTER explained is an important first step. A bath of vinegar will neutralize salt(s) during the last bath given.
The other way I know is to take one canon ball or chunk of rusted iron, wash and put it in something plastic with warm water. Do this for about an hour if it has mass. What you are doing is warming it up. Use hot water that you can put your hands in. Now you can add some of that rust treatment stuff that changes the nature of rust, like Rust Mort to the last bath. Let it sit for a while and let the chemicals do their thing. Remove it and shake it dry, using a sponge to get the drips off. Let 'er dry. If you don't like the results and want a black- black finish like in a museum then apply the rust mort full strength with a sponge on the item. When dry and cured it will be smooth to the touch, black and shiney-- Arrrs.
WARNING: DO NOT HANDLE LIVE ORDNANCE NO MATTER HOW OLD> enjoy life
jim
PS helped a diver find some old canon balls ( not 'armed') using the magnetic anomoly thingy ar ar ar! Keep an eye out for the goonies.