An 8" steel wire wheel in a polishing motor will clean that right up. Then you will be able to see what's up with it. Chances are that it's perfectly good. You could try using a bench grinder, but the problem is that they do not have long arbors like polishing motors do, so the motor gets in the way as you are manipulating the work piece. You could also use smaller steel wire wheels in a hand held drill, but the drill needs to be capable of achieving a lot of torque at a high speed, and it will be a bit ungainly and a whole lot more work than using a bench-mounted motor.
Alternatively, you could blast it with a non-aggressive media such as soda.
