Paul:
I wish I had some scecret recipet for soaking threaded materials in to loosen them, but I don't. I simply use heat on the male piece and that seems to break the bond of corrosion that is holding it tight. It is opposite to my logic. I thought that heating the male piece would cause it to expand and hold even tighter, but this is not the case. Maybe we are talking molecular bonding that the heat upsets.
Now, if both parts are iron, I use a commercial off the shelf penetrating oil in a spray can. The label is dark red and black. It is called BC or something like that, I forget. I let the penetrating oil work for a couple of time units depending upon how rusty the stuff looks, but then I use heat before attempting to loosen. We are in a salt environment here and the heat seems to be the trick to unlocking the salty corrosion espceially on stainless steel. Not on aluminum or zinc. I heat first, and usually I just twist 'em off and throw 'em away and drill out and refrabricate.
Jerry What's that? The grey poupon mustard? I use that on my sandwiches I eat while I thinking of the next project to mess up!