The dielectric couplings for use between copper and galv pipes have proved to be almost useless. It is best not to mix copper and galv pipes, but when it needs to be done, the codes have allowed the dielectric couplings as a "next best" thing. In the real world, using an anode like Bud mentions helps the piping system. Water heaters come standard with them. The brass nipples that some of us use, will slow down the corrosion better than the dielectric unions. When the dielectric unions are used, they are the first thing that fails. Most electical codes require that the two pipes be "bonded" with a copper strap to ensure grounding. So much for breaking the current.