Using a slight variant of Philbert's chain cleaning method, I just saved two long chains that a logger threw away that had severe rust but almost unused cutters. I soaked mine in vinegar for 36 hours in a shallow pan, threw that out, washed them up with a laundry detergent solution, and then used EvapoRust to finish them off. I also tried next soaking in a concentrated degreaser, and that worked a little, but I doubt it was much improvement. After some final sharpening and local spray lube these two chain loops are ready to mount.
I have run into some rusted chains with good cutters that cannot even be unclogged or unwrapped from a tangle. Those are trashed.