AC receiver/dryer to evaporator connection. Stuck!

[Konstantin Bogach]

Hi.
I had to perform lobotomy on receiver/dryer to remove it cause I could
not undo the brass nut (not actually a nut but I don't know proper term
in English.  I would call it threaded insert. ).  So, I removed
receiver/dryer cutting off it top and separating threaded aluminum
flange from the body of receiver.  This flage then came off easily from
the brass nut.  But I can not rotate that brass thing on the copper
pipe.  It corroded well.
What can I do to release   this brass thing on the pipe?  Pipe is soft
and I am afraid to do something silly.

Thank you.
Konstantin.