Nor will using a dielectric union necessarily stop corrosion from
occuring, unless the two metal systems seperated at the union do not
make electrical connection again at some point. A classic example is
an electric hot water heater, which has an electrical ground. If you
use a dielectric union on that, the water heater and the other side of
the cold water pipe going into it are joined back together again by the
grounding of the electrical system. That puts two dissimilar metals
in water connected by a wire.
I have no dielectrics on my gas hot water heater and have never had a
corrosion problem. I've also seen reports here from folks who have
used dielectrics and had severe corrosion problems. I think unless you
know the whole system and what you are doing, it's a crap shoot.