back when i did locksmithing we got broken keys out of locks by using a very thin (needle thin) drill bit looking thing that you screwed down alongside the end of the key, and then pulled it out, bringing the key with it. but if it's really that rusted, you'd be ebtter off buying a new lock and replacing it, because since locks work on prety precise parts, a new key probably won't be able to work the lock.
