How To Look At A House
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What's that powdery crust on the pipe connections at the water heater?
Saturday, July 28, 2018
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.
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