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How to Replace Sacrificial Anodes on an Outboard Motor

Don’t overlook this easy but essential task, especially if you boat in salt water.

How-To Gear & Tech
How to Replace Sacrificial Anodes on an Outboard Motor

Inspecting and replacing the sacrificial anodes on an outboard or sterndrive is an important maintenance task that is often overlooked by boat owners. It’s a great DIY task, and Mercury offers replacement anode kits for every popular engine brand, all of which meet OEM specifications. Your owner’s manual should help you locate the anodes for your particular engine model as well as the specific OEM part number a Mercury Anode kit will replace.

What are anodes used for?

The anodes are designed to protect the outboard or sterndrive from galvanic corrosion, which occurs when two dissimilar metals are immersed in an electrolyte. On your boat, the dissimilar metals could be the aluminum gearcase and the stainless steel propeller, or the aluminum propeller and the stainless steel propeller shaft. Immerse these dissimilar metals in a good electrolyte, like salt water, and you have created a low-voltage battery. The aluminum component is the battery anode and will have a negative charge in relation to the stainless-steel, the battery’s cathode. The aluminum anode will be consumed in the process of generating electricity. This is the process of galvanic corrosion, which in some situations can cause significant damage rather quickly, especially when the boat is moored in the water.


Protect your drive

To protect the aluminum parts of the outboard or sterndrive, a sacrificial anode made of a more-noble metal – zinc, magnesium or aluminum, or an alloy of the three – is attached to the motor, usually on the gearcase and transom bracket of an outboard. Anodes may also be found on the propeller shaft, or on the rudder of an inboard-powered boat. This anode corrodes, or “sacrifices” itself, and protects the aluminum of the outboard or drive. Because many sacrificial anodes were once made of zinc, they are simply called “zincs” by many boaters.

Replacing anodes

Anodes should be replaced when about half of the anode has been lost to corrosion. On many outboards one anode doubles as a small trim tab on the underside of the anti-ventilation plate, a good location because it’s near the propeller and is easy to see and inspect. When replacing this anode it’s important to align the new anode at the same angle as the old anode. To work well, the anode also needs to have good contact with the metal it’s attached to, so make sure the mounting surface is free of corrosion and other material build-up. It may be helpful to use a fine-grit sand paper to clean the area before installing the new anode.

Mercury anode kits are available for many Mercury outboard and sterndrive applications. These anode kits are manufactured from cadmium-free, environmentally safe, aluminum. Each kit includes stainless steel mounting hardware, and may be purchased from an authorized Mercury dealer.

Help from your local dealer

Need help replacing the sacrificial anodes for your Outboard motor? DIY maintenance is not for everyone and an authorized Mercury dealer will be happy to help you with this task. You can find a dealer close to you by going to our Dealer locator

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