What is a dielectric union?
Saturday, July 28, 2018
When copper and steel pipes are connected, which happens often at the fittings near a water heater, an electrolytic reaction begins. Water is the electrolyte, and the combination of two metals and water essentially becomes a battery. It happens slowly over many months. A small amount of electric current starts flowing—nowhere near enough to shock you—and atoms from one metal begin to break off and cling to the other metal.
The result is a crumbling, white powder buildup on the galvanized steel pipe shown in the photo below, which is called galvanic corrosion. To learn more about galvanic corrosion, see our blog post “What is that powdery crust on the the pipe connections at the water heater?”
To stop this type of corrosion, a fitting called a dielectric union is used to connect the two different metal pipes. An example is shown at the top of the page. There is an electrically non-conductive washer, usually rubber, between the two sides of the fitting which prevents the two metals from reacting. Actually, in some cases it just reduces the pace of the galvanic corrosion, so there are contractors will tell you they are not worth the trouble.
But it’s better than nothing at all, and required by the building code, so plumbers install them—with the exception that many jurisdictions allow a brass fitting as an alternative. See our blog post Should I use a dielectric union or brass nipple to connect copper tubing and galvanized steel pipe? to find out more about using brass.
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