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My friend is installing copper pipes in his house, which is going to be in a humid area, and he says that copper will only be oxidized by the air and the excess water vapor in the humid air won't affect it. However, it is to my understanding that since water vapor is $\ce{H2O}$, which contains oxygen, there will be more oxidation occurring, and thus more corrosion to the pipes because of more oxygen present in the air. Is this correct?

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    $\begingroup$ Though copper may have a very thin oxide layer after exposure in air, the green color that will eventually appear is verdigris, "usually a basic copper carbonate, but near the sea will be a basic copper chloride" [Wikipedia]. Yes, water will speed the corrosion (not simple oxidation) of copper. $\endgroup$ – DrMoishe Pippik May 21 '15 at 0:53
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Although $\ce{H_2O}$ contains oxygen, it is there in the most reduced state $\ce{O^{2-}}$ and therefore cannot act as oxidizer. It is the oxygen in the air $\ce{O_{2}}$, which is the main and strongest oxidizing agent.

On the other hand, presence of water as a solvent can speed up some reactions. For a short overview of copper corrosion, see e.g. Why does copper oxidize. As the corrosion products form compact (or even protecting) layer, the corrosion itself is not a problem for copper, as compared to e.g. steel corrosion.

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