DIY Network

Sacrificial Anodes: Overview

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Trim Tab Anode Corrosion Control Anode When compared to a new anode, it's easy to see how corroded the anode can become.

Host John Greviskis demonstrates the replacement of engine anodes, as well as prop maintenance and selection.

Greviskis is working on a Model 2001, 13-foot Boston Whaler Sport. The craft is being powered by a 40-horsepower, four-stroke outboard engine, with zinc anodes that have been partially destroyed and need replacing. Deterioration of sacrificial anodes is a good thing: the anodes are protecting the engine from corrosion.

• An ordinary voltmeter can indicate how much voltage or electricity different conductors have running through them -- in this case, a body of water. To test the water, Greviskis uses a siver, silver chloride and an alligator clip that is attached to a ground. According to the meter, 1100 milliamps of electric current is flowing through the water of the Intercoastal Water Way in Palm Beach, Florida.

• Different metals on a boat have varying amounts of potential, or varying amounts of current drawn to the metal when submerged. If the metal aren't protected from the electricity in the water, then electrolysis take places -- a corrosion that can change and weaken the molecular structure of the metal. To remedy this, a sacrificial metal (the anode) is built into different metal components on the boat and around the engine. With sacrificial anodes, one metal is essentially sacrificed for another, and these anodes need to be examined and periodically replaced.

• Tip: Anodes should come into contact with the metal part it will protect, and the potential of any sacrificial anode needs to be at least 200 milliamps higher than the other metal components.

• The type of water you'll be boating in -- and the combination of metal found on your boat -- determines the type of sacrificial anode you will need. For freshwater boating, chances are you'll need magnesium or aluminum alloy anodes. Zinc will not protect anything in fresh water because there it has a potential of about 900 milliamps; whereas, a magnesium in freshwater will have the 200 milliamp spread needed. In saltwater, zinc will have a potential of about 1200 milliamps.