I believe that the question should be re-phrased: which electronic components should be protected against corrosion?
The short answer is: depends on the operating environment.
For instance, an electronic device designed to be used indoors, in a protected environment like an office, won't have any or very little corrosion protection.
A device designed to be outdoors, will have a layer of conformal coating which will protect it against humidity and dirt.
A device to be used in an automobile engine bay, should be protected not only against water but oil, battery acid, fuel, brake fluid, etc, and will be fully potted.