I have a metal charm on my keychain that used to be very shiny. Due to wear and daily usage on my keychain on some parts it has gotten a deep black color while on some others it looks copper-y, why is that?
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The keychain probably is made of a dark alloy and only coated with thin copper and chrome layers on the surface. I’ve seen door knobs, bathroom faucets and belt buckles like this. These surface layers can be polished away by abrasion. Keys can also react with dissimilar metals in other keys that they touch through salt and acid from the skin forming an electric battery, which corrodes the more active metal.
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