why is rust sometimes considered bad and sometimes good? isn’t all rust eventually bad?

Question

why is rust sometimes considered bad and sometimes good? isn’t all rust eventually bad?

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Answer 1

Sometimes rusting is done on purpose in a controlled way that only affects a microscopic layer of the metal, which protects the metal from rusting in the bad way. This is most notably used on older guns, and is called “bluing” due to the slight blue tint given to the metal.