Dielectric unions corroded
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? |
Dielectric unions corroded
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." |
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