The two parts are corroded together, the stainless pipe is not the best quality stainless. I think I would try some brick cleaner (hydrochloric acid I think) trickled into the joint, it should not damage the stainless but will attack the corrosion.
If you are unsure try dropping a rusty bolt into some brick cleaner and after about 15 minutes it comes up like new. Obviously you should also try a stainless bolt to see if it damages it.
I think the acid will dissolve the corrosion and then with some diesel/acetone mix or plus gas or similar that should lubricate it sufficiently for it to move.
Also I have found using rotational force initially is more likely to break the corrosion bond.
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