Seatpost stuck - where would I get Caustic soda from?

Well, except in the unlikely event that it's aluminium inside steel, because aluminium expands more than steel, so heat would only make the problem worse. I can't think of anywhere that might happen. Maybe some kind of aluminium pole, or post, inside some kind of steel tube...? No, I agree, seems pretty unlikely.

But steel expands at a lower temp, hence why heating works