"But it just fits so perfectly all around, no gaps visible, it looks like it's literally welded, not corrosion welded." AlanHK

That's exactly what galvanic corrosion is. The two dissimilar metals merge to a degree with electrons migrating from one to the other.

The alu. oxide can be "dissolved' away with ammonia, Drano and Easy Off oven cleaner also attack alu.

When i goggled "Removing stuck seat posts" this was the top link. http://sheldonbrown.com/stuck-seatposts.html Andy.