The rust on that servo in the link is due to it rusting inside out!
The air inside has moisture in it and over time it corrodes. If you notice the metal is so thin it eats from the inside as the external surface had a good paint seal on.
Another thing that proves this is the rust is at the bottom of the servo and not anywhere else, where the condensation collects.
Everytime you brake a gulp of air goes in to make the servo work and so therefore so does the moisture in it.
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