Well, Gandalf was a wizard, and he was not corrupted. Remember, there is a difference between wizards (istari) and the men who had the 9 rings. Wizards are an entirely different race (Maiar). The Nazgul were mortal men (origionaly at least). There were only 5 wizards, and they could not teach what they knew (except share knowlege of events etc.) since their power came from being Maiar.
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