Originally Posted by joeswamp

Freeze the post as cold as possible, as the aluminum will shrink twice as much as the steel. Consider using dry ice. Maybe after freezing it for hours put a wet boiling water washcloth on the steel around the post, to maximize the size difference just before hammering. The corrosion is very strong but also very brittle so you want to have lots of sharp blows, a large slide hammer works well (when it does).