I Have just stripped a set of mc28sp forks with the same problem, as you suggested the alloy part had corroded and caused the adjuster to seize.

I took the fork cap off, took the adjuster apart and cleaned the corrosion in the adjuster hole with wet and dry. Definetely wouldnt hit it with an impact driver, once you have the cap out you can grip the brass adjuster in a vice and wind it out, their is a little ball bearing retained by a spring which you have to remove first though.