Came across this today. There are a bunch of videos on youtube about using very cheap farm molasses and water to de rust large parts. I like the idea as you can just dump it in the ground when you are done with it and it doesn't attack the metal.
http://www.youtube.com/watch?v=CzTA7u_HIik
http://www.youtube.com/watch?v=b-_z4tinnXk
http://www.youtube.com/watch?v=CzTA7u_HIik
http://www.youtube.com/watch?v=b-_z4tinnXk