If they are aluminium try putting them into caustic soda for 10 mintues. Dissolves the top layer and removes any anodising. If the caustic mixture is lukewarm it improves the reaction time
I could recommend not doing this! I got the ano off no bother with this method but must have affected the hardness or size or something of the splines. Crank kept creaking and slowing slipping off the axle… Nice shiney crank but useless.
Once I got the ano off I just used polishing mops and paste and got the cranks nice and shiney after a few hours. Had no problems with oxidation after but they were kept in a dry environment.