My plumber installed couple of dielectric unions (one for hot and one for
cold) for my shower from galvanized pipe (house) to copper (shower) about 5
years ago. I took the unions apart as I'm re-piping my house with all
copper. The funny thing is the copper and galvanized pipes looks pretty
clean but the unions on both the copper (brass) and galvanized (steel) side
were corroded from the inside. Looks like it was installed correctly without
the dielectric union metal components contacting each other. So water itself
acts as a bridge and short circuited across the dielectric unions and cause
the corrosion.

1) Is that your experience too?

2) Could a better dielectric union be used?

3) Or is this corrosion an isolated case?