Heat the pot,,, use regular motor oil on the joint of the pot/pipe,, let the motor oil flow over the joint . Let the oil cool the pot/joint. as the pot cools, the oil will be drawn into the joint... repeat several times.. the oil will fume an burn off it you get the joint too hot,,, it doesn't need to be red.. just hot so it expands. then cool with motor oil, used oil is fine.
With patience, I've removed some really corroded pipes...
If the pipe is toast, and is going to be replaced, cut it off near the joint going into the pot.. then use a hacksaw blade and cut a slot in the aluminum pipe, lengthwise, all the way through the aluminum up to the cast iron of the pot/sleeve.. The gap cut in the aluminum will allow the pipe to shrink/collapse, and it will come right out..
I use a broken hacksaw blade and a pair of visegrips to hold it... take your time.. it will work. don't cut too deep and cut a slot in the pot/sleeve..
Good luck.. Greg L