It's only about 5% acidic but you could try cider vinegar.....inexpensive! Maybe too mild...IDK.
Thanks for the suggestion! In the past I've used vinegar to clean tools and assorted rusted metal objects but I doubt that it would have gotten far enough up in the threads to do any good. I switched to electrolysis and have found it smells MUCH better and the metal items are a lot easier to clean up after the rust is gone, but in this case I don't think that would have worked either. As the bodies of these are made of malleable iron I don't think the amount of heat I used will affect the strength much if any.
On the 3 that were locked up solid, after heating and letting cool, then beating on a rod put through the head with the body of the jack in the vise, it took between 3 and 5 minutes of beating back and forth to get any movement in any of the 3. Once broken loose and out, the screw threads were completely dry from about a half inch in from each end (where the oil had penetrated), so there was an inch and a half of solid rust holding them in place.
Will sand blast and paint them when I get the barn somewhat finished as I need them for quite a few future projects.
Put one in place and used it to jack a 2x4 into place so I could screw it to 2 others. The jack worked much easier than the other ones I have been using. Got to get 3 more posts cut to length and will put them about 6 feet apart and lift the whole side one turn at a time. Or that's the plan.
Dave