H20/o2 has a standard cell potitial higher then all metals except gold and platnum. If any other metal is exposed to water it will eventually be completely dissolved.
However for our purposes this reaction occurs so slowly at room temperature in pure water that its not important. Adding a corrosion inhibitor further slows this process down.
If you add another metal, then entirely new reactions become possible, many of which can occur quite rapidly. Thats why we don't mix metals.
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