Dielectric Unions
Posted by John Scates on February 13, 19101 at 17:31:22:
In response to Dielectric Unions
Galvanic corrosion in piping systems occurs at the joint of two dis-similar metals in contact while in the presence of water. The typical "dielectric connector" used on hot water heaters is just a pipe nipple with a plastic insert. It prevents water contact AT the line of contact between the dis-similar metals, but clearly water contacts the metals on both sides of the fitting. Having some distance-separation helps reduces the galvanic effect, but does not eliminate it completely because there is still metal-to-metal contact around the (dry) outside. The more conductive the water is (more minerals), the more accelerated the erosion will be.

A true dielectric connector would not allow any electrical conduction between the water heater and the piping, thus would have to be plastic. But the harsh environment at the outlet of a hot water heater has largely prevented anyone from coming up with a reliable all-plastic connector. Thus, what we have and use today is actually a comprise, which eventually corrodes as Terry points out.

Brass and Copper are both resistant to galvanic action when compared to steel. The problem with using a brass connector between the copper pipe and steel water heater tank is that you will probably cause the steel tank disintegrate quickly, particularly in the threaded boss. The anode rod in the tank will help some, but I wouldn't bet on it.

Some of the copper flex-line connectors have achieved complete separation of the metals by using the washer and a cylinder-sleeve underneath the nut. Although I see failures of these too (as Terry pointed out), they are generally a good solution for hot water heater connections. There are many causes for corrosion, galvanic-action being only one.

- John Scates

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