The rusty crust is galvanic corrosion, an electrochemical disintegration that happens when two different metals are in contact with each other while also in contact with an electrically conductive liquid, typically water. One metal, the less “noble” of the two (see galvanic scale at right), corrodes. In the photo above, the galvanized steel pipe stub, buried under a thick cake of both galvanic corrosion and regular corrosion and no longer visible, is the victim or “sacrificial material” in the electrochemical reaction. The water leakage caused by the galvanic corrosion has caused the surrounding rust. You have probably already seen another example of galvanic corrosion in the powdery crud on the contact pins of an old car battery.

  The top photo above is an example of an earlier stage of galvanic corrosion at the cold-water shut-off valve to a water heater, with the powder falling onto the top of the water heater below. Below it is the same corrosion at the shut-off valve under a bathroom sink.

   Galvanic corrosion can be avoided by electrically separating two dissimilar metals in a piping system using dielectric fittings. See our blog “What is a dielectric union?” to learn more. A second method is to only use metals that are close together on the galvanic scale, so there will be less galvanic electric current between them--the combination of two different metals and water creates a kind of electric battery. And a third solution is to use only one metal throughout the piping system, which is not always easy to accomplish. And finally, replacement of the metal piping with a plastic pipe, like CPVC, also works.

   The water heater itself has a replaceable “sacrificial anode” tube built into it, to resist any galvanic corrosion of the tank. The anode, typically zinc or aluminum, essentially sacrifices itself through corrosion to avoid deterioration of the tank metal.

   The Statue of Liberty is probably America’s most famous example of galvanic corrosion. The contact points between the wrought-iron support structure and the outer copper skin were originally separated with asbestos cloth by the French sculptor, Frederic Bartholdi, who was familiar with the corrosion potential of the two metals. But, over the years, the cloth dried up and became sponge-like, absorbing and holding harbor salt-water spray. This ended up accelerating the galvanic corrosion, instead of preventing it. A major restoration was undertaken by the National Parks Service in 1986, replacing the wrought iron with corrosion-resistant steel.

   Once in while the powdery crust on a pipe is not galvanic corrosion, it’s efflorescence--which is the residual minerals that water leaves behind when it dries up. A very slow leak, one that evaporated at about the same rate that it leached out, created the flaking white powder crust on the CPVC pipe above a water heater shown in the photo below. In this case,  we knew it couldn’t possibly be galvanic corrosion because the pipe is plastic. 

Also, see our blog posts What are the signs it's time to replace my water heater? and What are the most common installation mistakes with water heater replacement?

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home? 

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How do I determine if a water heater is gas or electric?

• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

    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.