dielectric

A dielectric joint in mechanical system, is a joint between two dissimilar materials usually in process piping, where the dissimilar materials will be subjected to an electrolytic exchange causing corrosion of to the two materials; this is usually accelerated by the substance carried within the piping. Dielectric joints come in various forms from rubber or plastic separators to ceramic coated joints.

And a real good example are the dielectric unions that are commonly used to connect copper water lines to the steel tank of a hot water heater.