Dielectric unions

Entry in Fittings and valves · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

A dielectric union is a pipe union with an insulating gasket between its two halves, used to join pipes of dissimilar metals, typically copper and steel, so that electric current cannot pass through the joint and speed galvanic corrosion.^[1][2] One common style has a male pipe thread on the steel side and a solder cup on the copper side.^[2]

The fitting is most often seen where copper water tube meets galvanized steel pipe or a steel water heater or tank connection. Makers describe it as protection against both galvanic and stray-current corrosion.^[2]

1 Why dissimilar metals corrode

Galvanic corrosion is an electrochemical reaction between two metals of different electrochemical character in contact with each other and with an electrolyte, which in a water line is the water itself.^[1] How fast it proceeds depends on the potential difference between the metals, the relative surface areas, and the electrical resistance of the connection and the electrolyte.^[1]

All three conditions must be present for the reaction to occur, so removing the metal-to-metal contact is what the union does.^[1]

2 Construction

A dielectric union has a union nut, two tailpieces and a gasket; the gasket separates the tailpieces so current cannot flow from one to the other.^[2] One maker rates its lead-free threaded-by-solder model at 250 psi maximum.^[2] Unions and fittings sold for drinking water must meet the federal lead-free limit of a weighted average of 0.25% lead on wetted surfaces.^[3] Because one end is soldered, the soldered joint and the threaded joint rules both apply to installing one.

3 Problems

A US National Institutes of Health facilities bulletin notes that dielectric unions are commonly used as a galvanic break but can leak under some circumstances, recreating the conditions for corrosion; such corrosion may be hidden inside a pipe that looks normal outside.^[1] The same bulletin advises that heat from welding can damage the isolating part of a dielectric fitting, so heat work is done before the fitting is installed.^[1] Selection should account for the metals, the fluid, and pipe vibration and movement.^[1]

4 Code status

Code wording on copper-to-steel joints differs by code and by state. Massachusetts, for example, lists dielectric-type unions for dissimilar metals among the unions allowed in water supply piping, and allows dielectric unions and fittings at water heaters unless the water heater maker prohibits them.^[4] The same code requires joints from copper tube to threaded pipe to be made with brass or wrought copper adapter fittings.^[4] Other states follow the IPC, UPC or IRC with their own amendments, so the locally adopted edition governs (see plumbing codes). Brass has its own corrosion issues, covered under brass fittings and dezincification.

Supply houses list dielectric styles alongside ordinary unions; PlumbingSupplyAndMore's union fittings category (PlumbingSupplyAndMore has the same owners as this reference) is one example.