I had a suspension seatpost that took at least 10 hours to remove(in many pieces).I didn't care much about the frame-hence the heavy handed methods.
Pipe wrench and hammer to finally get it to twist-lots of various penetrating oil-aluminum post-steel frame.
I then twisted it maybe 300-400 full turns-figuring it would loosen up- it didn't!!
3 lb Maul and large cold chisel hitting the top of the seatpost upward-maybe 200 hits-no outward movement.

Soaked several times in full strength HCL-great rust remover.
Rinsed-tried all above-no upward movement.

Pulled guts out of seatpost.
Took hacksaw with just a handle-made two cuts all the way down the seat post- cuts were 100% thru on 4" of the 8" post, and 80% thru the last 4".
Finally twisted it with channel locks after it was weakened by the hacksaw-out it came.

Really a miserable experience. I did have to slightly "adjust" the rear triangle after all this. I bought mainly for the parts-frame was just a bonus I'll sell for $10 or so(Chrome Moly Specialized-probably Katrined judging from the rust/silt)
Luck
Charlie
HCL is a good rust remover-. I suspect the chromo frame is a little more resistant to it than high carbon steel.I wouldn't use it on a frame I actually wanted.Much quicker than Citric or Oxalic acid( dangerous to work with-blind you-hard on lungs , burn skin etc). I had it around to etch concrete before painting.