Most keys I've seen (not sure about yours) have a semi-circular key sitting in a semi-circular groove. The key is shaped like a shallow "D", and the groove is as if it was cut by placing a cutting wheel on the shaft, rotation parallel to the shaft.
You most likely won't be able to remove the key until you get the wheel off.
Try using a bunch of penetrating solvent (Break Free, Kroil, PB Blaster, etc.) and a pulley remover. Gentle heat application might help, too (be careful if you've used an oil-based penetrating solvent, or have recently applied *any* penetrating material.) A couple of whacks with a hammer may also help to break the corrosion bond, but may mar things a bit.