reporting on corrosion

Looking for a resource to report on corrosion, if anyone has one? How much corrosion is OK? Should all corrosion be replaced? What resource can research to learn more about it.

Corrosion noted at the ###. Recommend the corrosion be removed and using dielectric unions where dissimilar metals are joined (i.e. copper to galvanized) as to avoid this type of corrosion.

Looks like a weep or leakage.