The Role of the Sacrificial Anode

By: Martin Wigg and Paul Fleury


Determining which sacrificial anode will fully and safely protect a boat depends on a number of factors. As previously discussed, it is important to understand the physical properties of each anode material. In addition, boaters should also understand how each anode works with different hull compositions, sterndrives and types of water.

The best way to determine whether a boat is fully protected is by measuring its cathodic protection voltage or hull potential using a voltmeter. But, if one isn't available, here are some simple guidelines for selecting the right anode.
The hull material of a boat determines, in part, which anode material to use. A fiberglass boat having an inboard engine with bronze and stainless metal parts needs less protection than an aluminum hull or a boat with an aluminum sterndrive. Zinc or aluminum alloy anodes will work well for these types of boats. The voltage generated by these anodes cannot overprotect, i.e., they cannot cause any damage no matter how much anode material is added. The maximum voltage generated is the voltage of the anode itself. Magnesium would also work with a fiberglass boat but only in freshwater. However, aluminum or wooden hulled boats can be overprotected by very active magnesium. Steel hulls can also be overprotected and the excessive protection voltage will rapidly lift the paint off the hull.
Sterndrives and outboard motors, because of their very active aluminum assembly, are hard to protect. Initially, the anodes for these units were made of zinc. But corrosion problems in the early 1990s sparked the major engine manufacturers to start selling aluminum alternatives. The increase in protective voltage ensures that the sterndrive is protected. Today, in some cases, using zinc may invalidate an engine's warranty. Again, caution is needed when using magnesium anodes as they can overprotect.
Water type is the final influential factor when choosing an anode. When used in freshwater, a zinc anode forms a coating of zinc hydroxide that insulates it and stops it from working. Magnesium used in saltwater can disappear very quickly, and if it is used on an aluminum sterndrive or outboard motor, it can be very dangerous. If piloting a boat with an aluminum hull or sterndrive down river and into the ocean, the overprotection of the magnesium anodes would cause hydrogen bubbles to form under the paint on the hull or drive resulting in it being literally blown off. Acid rain and pollution can also increase the conductivity of freshwater to the point where the same thing will happen.
Aluminum alloy is the only anode material that is safe for use in all types of water and accepted by the major sterndrive manufacturers as the best material to use. It is lighter and protects better than zinc and is not so active that it becomes dangerous like magnesium. Aluminum alloy is also environmentally friendly, unlike zinc, which is considered a pollutant.
Editor's note: Martin Wigg is the president and founder of Performance Metals. He holds an MBA and a Bachelor of Science degree in mechanical engineering. Paul Fleury is certified by the American Boat and Yacht Council as an Electrical Technician and the National Association of Corrosion Engineers as a Corrosion Technologist.
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