Brass tub overflow pipe stuck in brass tee. How can I remove it?
Heat both parts moderately and safely (ie, don't set the house on fire) concentrating on the Tee or outside part.

Then apply ice to the pipe (ie, the "inside part" of the joint.) 

This is usually a more practical (and less frustrating) approach than trying to heat one and cool the other at the same time.

The idea in play is to expand the outer part and shrink the inner part to aid in separating them. It won't always work, but it's what I'd try first when simple disassembly fails.

Heat may also help to break up the corrosion more directly by driving water out of it.