I understand that when copper is in contact with galvanized steel, and an electrolyte present, the zinc acts as an anode, as it wants to ‘give’ its electrons over to the copper. But brass already has zinc present and is stable, so why would galvanic action occur between these two metals?
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A layer of oxides forms on the surface of the brass that protects the metal below it from rusting, but for this to work the oxide layer has to effectively block the oxygen and moisture from contacting the un-oxidized metal. If the oxides flake off, like iron oxide flaking off of steel, it exposes fresh metal that can continue to corrode.
Also, both iron and zinc will be an anode for copper.
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