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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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?
• Can you wire a 240-volt water heater with 120 volts?
• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?
• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater?
• What is required clearance for access and working space in front of an electric water heater?
• Why is the water heater older than the house?
• Does a water heater need a shut-off valve?
• Why should a tankless water heater have an isolator/service valve kit installed?
• When was a gas water heater first required to be elevated 18 inches above a garage floor?
• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home?
• What is the purpose of a thermostatic mixing valve above a water heater?
• Does a tankless gas water heater still work with no electricity during a power outage?
• What is the difference between a manufactured/mobile home water heater and a regular water heater?
• Does a tankless water heater require a pressure relief valve?
• 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?
• Does a tankless water heater in an attic require a drain pan?
• Does an electric water heater require a disconnect?
• Is a catch pan and drain piping required for a replacement water heater?
• What is the difference between a single element and dual element electric water heater?
• What is an FVIR water heater?
• What is a heat pump water heater?
• What is a dielectric union?
• What's that powdery crust on the pipe connections at the water heater?
• What are the most common installation mistakes with water heater replacement?
• 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 does a hydronic heating system work?
• What is the difference between a regular water heater and a direct vent water heater?
• What is the difference between a regular water heater and a power vent water heater?
• What is backdrafting at a gas water heater?
• 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 WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.