Before I would use steel wool or 1000 grit sandpaper I'd use a high quality cream silver polish like Wenol or Maas.

It will take off the corrosion but only a micro-miniscule amount of metal like all polishes do. Polish is way less aggresive than steel wool or sandpapaer and the trick is to go slow with metal removal when working with ferrules.

Unfortunately you may find the ferrules will be loose after they are clean but don't despair; a rub with beeswax will usually tighten up brass ferrules to fishable condition.