It is true that electrolysis,( the decompostion of an eloctrolyte by the action of an electric current passing through it)can deteriorate dissimilar metals. Two of these metals are copper and galvanized steel. Very recently it was assumed that dielectric unions between the two dissimilar metals would slow down this deterioration. In most recent codes dielectric unions or pipe extenders have been eliminated. There use has been known to increase the decomposition action between the two metals. Other code jurisdictions are just slow to follow. The best transition between two dissimilar metals is a brass fitting or brass pipe extender. Good Luck, Bill The Local Plumber Tustin, California http://www.TheLocalPlumber.com
Regarding die-electric fittings: they also break the electrical continuity through the water heater. The cold water line would have been required to be bonded to the main electrical system ground. This continuity was assumed to be continuous through the water heater to the hot water piping. The die-electric unions break this continutiy and now the hot water piping needs to be bonded to the cold water piping (which we will assumed is already bonded).
This should be done by a qualified and licensed electrical contractor who should also verify that the cold water lines are properly bonded.