I have an object made of what I assume to be brass, i.e. an alloy of copper and zinc.
If I understand correctly, the black layer is tarnish, or patina, and due to oxidation from sulfur dioxide in the air.
But I can't find reliable reports of white corrosion on brass. Could it be wet storage stain, a.k.a. "white rust" ?
Hints on how to remove it are welcome.