I suspect that the corrosion in your case might not be as bad as with dissimilar metals as with an aluminum stem in a steel steerer tube. I've encountered some steerer tubes in forks that taper to a smaller ID towards the bottom, specially if the fork crown and blades are a different material from the steerer, as it is needed for ovelapping and bonding the two different materials together. It may be that your stem is just jammed in this tapered area and mostly just needs a little physical "persuasion" to remove. Is there a way to tap on it from the bottm upwards with a metal rod without having the wedge inside get in the way?

Chombi