Quote:

help me learn...

if the corrosion on the rods is only on the surface, and does not affect the structural properties of the metal and can be cleaned off and used, why does the same surface corrosion on the bolts make them junk?



If the same corrosion is on a rod , I'd do more than clean the rust affected area. Like stated above , polish or shot peen to restore a " non directional " finish , leaving no place for a crack to start.
Not an option with a bolt......at least not one that doesn't contain a fair amount of risk. For the money , it is a no brainer.