Do copper and galvanized together = corosion?

YES

Just attaching copper to galvanized will not cause corrosion. I've seen very old heat system connections that have no signs of corrosion.

I stand corrected. Are you talking flashing or are you talking water pipes. It depends on application.

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.

I repair machines for a living, and DEFINETELY dissimiliar metals corrode, even moisture in the air is enough to help it along.

What law? Do you mean plumbing code? Definitely this depends on jurisdiction.

My area they used brass fittings between the existing galvanized and the new copper. Passed inspection just fine. Maker of the new stainless tank water heater recommends brass fittings between the tank and copper (and will void the warranty if anything else is used).

sdb