LooseChange said..Ramona said..
There is no Zinc in Bronze unless the Bronze has been made out of non virgin materials with unknown alloys.
It's generally called dezincification and is clearly visible once the prop has been hit with a disc sander. I see it regularly on the slips. One boat lost a blade off his prop just traveling a few hundred yards from his mooring to the slips. The remaining stub of the blade had about 10mm circle of bronze, the rest was like cheese.
If you use a hammer to check the tone just be gentle!