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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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house? 

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?

• What are the abandoned pipes sticking out of the wall in my house?  

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains? 

• What is a "combination waste and vent" in a plumbing system? 

• What is a building trap?  

• What is a galvanized nipple?

• What are the pipes sticking out near my water valves?

• How do you accurately find a broken water pipe leak under the floor slab?

• What is the difference between water pipe and sewage (waste) pipe? 

• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

• Is a hot water faucet handle required to be on the left? 

• If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?  

• What is a P-trap?

• Why is old galvanized steel water pipe a problem for homebuyers?

• What does polybutylene pipe look like? Why is it a problem? 

• Which water pipes are an insurance problem and possibly uninsurable?

• Can you connect CPVC pipe directly to a gas water heater?  

    Visit our PLUMBING and WATER HEATERS pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.