Dielectric Fittings and Galvanic Corrosion

[PEDARRIN2]

It is common practice to install a dielectric fitting when you have to connect a copper pipe to a steel pipe in building systems design, especially at water heaters, boilers, and sprinkler systems.

A common fitting used for smaller pipe connections where flange fittings are not appropriate is dielectric waterway by Victaulic (

http://static.victaulic.com/assets/uploads/literature/09.07.pdf)

which appears to be a carbon steel outer portion and a thermoplastic lining. Other fittings use a similar construction.

My question is how well this would actually work. It doesn't appear the galvanic circuit has been broken. While the distance from the dissimilar materials is increased on the interior of the pipe, the metal outer shell would still conduct the current of the galvanic circuit. Maybe the resistance of the outer shell is enough to minimize the current flow to levels where corrosion is minimal.

Any opinions or thoughts would be appreciated.

 

[moltenmetal]

I'm anxious to hear others explain how this could work well. It seems to be depending on the water not being particularly conductive, because those painted Victaulic hubs aren't going to offer much resistance on the exterior after tightening. It would seem to me that all these devices would do is move the corrosion location, reducing the rate a bit by reducing but not eliminating the current flow.

 

[CoryPad]

You are correct moltenmetal, it depends on the water having low conductivity. Neither the waterway nor traditional dielectric fittings are very good for long.

 

[PEDARRIN2]

The water would be domestic tap water which flows and fire sprinkler water which remains fairly stagnant except for required flow tests and of course a fire event. TDS is normally limited to less than 500, but hardness, alkalinity, etc. can vary.

 

[Compositepro]

I agree with the comments above. All metallic parts parts of a plumbing system should be grounded and therefore they are electrically connected. The important fact to bear in mind is that current flow through an electrolyte is by movements of ions. This requires a threshold voltage to be exceeded. The ion flow is greatly reduced by making the path longer and will also be affected by fluid flow. If the dissimilar metal pipes are truly electrically insulated from each other, then the unions would work according to theory. But in most real world piping that does not seem possible. In that case the unions probably slow down corrosion. However, it may also happen that the corrosion occurs in a thinner area of pipe or fitting than without the union.