Jack wrote:
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.

I'm going to assume that the galvanized piping was grounded and the
installer connected a ground lead to the copper piping and shower
valve/head, or that stuff got connected to ground through some sneak path.

1) Is that your experience too?

Yes, I've seen that happen. The copper and steel pipes act like the
plates of a battery and the (slightly impure) water is the electrolyte.
That "battery" is "short circuited" by the ground connections mentioned
above. The current flow through the electrolyte is strongest near where
the dissimilar metals are closest to each other, namely right next to
the insulators in the dielectric unions.

That's why the corrosion is worst right on the dielectric unions themselves.

2) Could a better dielectric union be used?

Probably not a union, but if you used a "regular" galvanized union
attached to the galvanized pipe and then ran a few inches of PVC between
that and the copper piping, or used one of those plastic lined steel
nipples, you would create a longer water path between the two dissimilar
metals which would have more resistance, so the galvanic current density
, which is what corrodes the metals, would be considerable less than it
is with that very short insulator in a dielectric union, and the
corrosion would take much longer to do the same amount of damage.


3) Or is this corrosion an isolated case?



No, but it will be better or worse depending on the conductivity of the
wayer. If extremely pure "distilled water" were in those pipes, the
corrosion rate would be much less.

HTH,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."