Last Post: 9:57:17am, 04/26/2023
It depends on what exactly you're looking at. You're probably referring to limescale, which is minerals from the water precipitating onto the metal. That's mostly calcium bicarbonate and magnesium bicarbonate, and rubbing a cloth soaked in vinegar on it will get rid of that.
However, if the metal in question is zinc or a zinc alloy, then it could just be rust. Zinc reacts with hydrogen and oxygen to make white rust (zinc oxide), similar to iron forming brown rust (iron oxide). This is something you really need to prevent in the first place rather than try to clean up after the fact, since the actual metal is being chemically altered, and you'd do so by trying to prevent the metal's exposure to water.
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"That is why war is so tragic. To win means to make victims of your opponents and give birth to hatred." - Kratos Aurion, Tales of Symphonia