I sit steel or does it have any plating? (Pics would help) Plating limits your options a bit, as you don';t want to us electrolysis, (My favored method) and you need to be careful with vinegar as it can also remove plating if left too long.
If the threads are exposed, you can go at them with a wire brush and WD-40 to get the worst off. or a wire wheel, but use a fine wire wheel a heavy braided wheel can leave marks. Once that is off you should be able to get it apart for detailed cleaning.
Again, pics help with giving advise.
