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Sacrificial Anodes

Sacrificial anodes save your boat from galvanic corrosion but are often forgotten. These anodes protect valuable outdrives or outboards that are suspended in the water we boat in. The general recommendation is that sacrificial anodes be replaced when they are less than 50% of their original weight. Recognizing when a "zinc" has reached that point may not be obvious.

These two zincs, from a Volvo Penta outdrive, were made by Volvo Penta and had the same part number. It was surprising to see how much weight the old zinc had lost even though its apparent size was not all that different. The old zinc weighed 14 1/2 ounces while the new zinc weighed 37 3/4 ounces. This represents a 62% loss from the original weight of the zinc anode!

There were some tip offs that the old zinc should have been replaced sooner. The external surface of the zinc did not appear smooth and metallic, but spongy with definite loss of mass from the body of the zinc.

Sacrificial anodes are made from one of three materials. The most commonly used material is zinc which is suitable for saltwater use. Magnesium is recommended for boats that are exclusively used in fresh water. Aluminum alloy anodes can be used in either fresh or saltwater and are becoming the recommended anode for most boaters. It may seem paradoxical that an aluminum anode is being used to protect aluminum outdrives since there would be no difference in nobility of the anode, but "aluminum" anodes are alloys that are less noble than the aluminum used in an outboard or outdrive. Thus, these anodes will protect a typical outboard or outdrive from galvanic corrosion.

To keep your outdrive or outboard protected, change those anodes before they appear to need it! Make sure that you have identified all of the anodes that are present on your outboard or outdrive since they may not be readily visible at first glance.

Old Zinc AnodeNew Zinc Anode