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Why do water heaters have a sacrificial anode?
Thursday, June 28, 2018
Here’s the short answer: to reduce the rate of corrosion on the interior surfaces of the water heater tank and thereby extend its life. All tanks eventually corrode through and leak, but a sacrificial anode slows the pace of the corrosion. Premium water heaters with an extended warranty have a bigger anode or two of them.
The long answer requires explaining how they work by a brief lesson in galvanic corrosion, which is an electrochemical disintegration that occurs when two different metals are immersed in an electrically conductive liquid like water. The presence of the metals in water creates a small electrical current—similar to the way a battery works, although the current is barely measurable—and the less “noble” of the metals on the galvanic scale corrodes away.
Multiple dissimilar metals are present on the interior of any tank water heater in the form of heating elements, drain nipples, immersion thermostats, and inlet and outlet nipples, along with the surface of the tank itself. Although the tank has a protective enamel interior coating, the enamel inevitably has microscopic holes and the overall coating will be gradually worn down by the water in the tank. So one of the bottom three “less noble” metals listed on the galvanic scale at right, or a combination of them, is installed as a long rod into the tank. Magnesium is the most common. The sacrificial anode rod literally sacrifices itself to protect the other tank components from corrosion.
The typical sacrificial anode is a straight rod about four feet long and 3/4-inch in diameter. Eventually the anode gets completely eaten away, leaving only a thin steel core with a few bits of the sacrificial metal still clinging to it. The continuing electrochemical reaction then attacks the interior tank surface, speeding up its demise, unless a new sacrificial anode is installed. Shown below is a couple of them pulled from old water heaters by our plumber friend, James Freeman, of J.W. Freeman Plumbing, with almost nothing left below the threaded tank fitting.
Because it can be impossible to position a four-foot long rod over the top of a water heater (where the anode opening is located) when it is installed in a cramped space, segmented replacement sacrificial anodes are available, like the one shown below.
Rheem Manufacturing has an excellent guide to replacing a sacrificial anode that you can download from the link below.
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