For future reference...
I may be offering this advice a bit late now that you've broken the stub. But, for future reference, here is a reliable old trick that I learned many years ago. In the interlude between graduating from college and starting my post-graduate work I had to do a bit of work helping a steamfitter on the job I was working at. They were recycling old 3" steam pipe to use in fitting the "new" second-hand boiler and retorts being installed as part of an expansion. Breaking down the corroded joints was a bear of a job and as I was a big boy, the task fell to me. As I was struggling with the job, the old fitter taught me to first tighten the joint before attempting to loosen it. Heat and hammers were part of the process if that failed, but rarely did we have to resort to them and when we did, it was still in the tightening mode.I don't know what the physics of the technique are, but I suspect that the ramp effect of a threaded union must be more efficient in tightening a pipe/bolt than in loosening one. In most cases, you would put pressure on the wrench (with a big 'cheater' extension for leverage)and there would be a little sound like a creak or snap and then you would instantly feel the joint move as the corrosion broke free. Stop turning as soon as that occurs and the joint can be unscrewed with little difficulty. I have learned to my regret that you should try this FIRST. Often, the various efforts to unscrew you intuitively take seem to bind the metal worse. I now try to notice if there is corrosion and if so, tighten 1/4 turn first. While a bolt or pipe can be so corroded that this won't work, I have found that to be extremely rare when I remember to tighten first.