Does a dielectric union electrically isolate or avoid corrosion?

[sduel]

We purchased dielectric unions to help avoid galvanic corrosion between a copper nipple and steel pipe. Problem is, the unions are steel on both sides of the isolator. So now we have a copper nipple threaded to the steel union. Are there different uses of dielectric unions? Or are we using the wrong terminology? Do they electrically isolate (to avoid static charges) or do they physically isolate (to avoid corrosion)?

 

[KenRad]

The dielectric union that should be used in your case has copper on one side, and steel on the other, with plastic isolating the halves. There are different kinds made for mating different materials.

It will isolate the two sides electrically, ie, it will not allow current flow through the piping from one side to the other, and thereby prevent galvanic corrosion.

---KenRad

 

[sduel]

Thank you. I've read that corrosion of steel pipe( the anode) is not an issue when it is relatively large compared to the size of the copper cathode. But in our case, we've electrically isolated the steel pipe in the system from the copper pipe. So the anode (instead of being the entire piping system) is just the small union itself. Sounds like we've made the situation worse and that we can expect severe corrosion between the copper threads of the nipple and the steel threads of the existing union. Am I on the right track? I'm wondering if we had simply welded the copper headder to a steel pipe connection if we would have been fine??

 

[walkes]

I think you may need a brass dielectric union, where the brass isolates the copper from the steel with an insulator between the brass and steel components.

 

[ChasBean1]

sduel, no. For galvanic corrosion, you need dissimilar metals in an electrolytic solution. The contact point of the steel nipple to both pipes (copper and steel) is on the non-wetted side (the outside). The plastic sleeve on the interior separates the wetted portion of copper from the wetted portion of steel to prevent galvanic action. If you run the pipe underwater or if it is frequently wetted on the exterior, your right... sorry for the late input. -CB