The dangers of corrosion are a constant threat to the appearance and operation of marine equipment, especially if you operate your boat in salt or brackish water. Sacrificial anodes, often called “zincs” and usually located on the gearcase, transom bracket or trim rams of an outboard or sterndrive, are designed to protect aluminum components from galvanic corrosion, especially when using a stainless steel propeller. Inspecting and replacing them when needed is a simple maintenance procedure that can help you avoid costly repairs or lost time on the water.

What is Galvanic Corrosion?

Galvanic corrosion is an electrochemical reaction that occurs when two or more dissimilar metals, for example an aluminum lower unit and a stainless-steel propeller, are connected together and immersed in a conductive solution, such as salt or brackish water. Electrons flow from the more chemically active metal directly to the less chemically active metal through the external connection. Positively charged ions move from the anode and negatively charged ions move from the cathode through the electrolyte. The result of this process is the dissolving of the anode material, typically aluminum components such as an outboard gearcase or sterndrive unit. Conductivity increases with water temperature, which is one reason boats in Florida experience more galvanic corrosion than boats in colder climates such as Maine or Alaska.

One of the first signs of galvanic corrosion is paint blistering, usually beginning on sharp edges below the waterline. It appears as a white powdery substance forming on the exposed metal areas. As corrosion advances, exposed metal areas may become deeply pitted, as metal is eaten away. Galvanic corrosion of aluminum outboard gearcases, sterndrive units or any other underwater aluminum on your boat is accelerated by proximity to stainless steel components like propellers, powered trim tabs and aftermarket steering systems.

Sacrificial Anodes

Sacrificial anodes are made of alloys that are more vulnerable to the effects of galvanic corrosion and are intended to corrode more easily, or “sacrifice” themselves, to protect the adjacent aluminum material. Sacrificial anodes are made of materials such as aluminum, magnesium and zinc. Zinc anodes are commonly used on boats operated in saltwater or brackish water. Magnesium anodes are frequently used in freshwater-only applications. Aluminum anodes are a popular all-around choice that can be used effectively in all water conditions. They also tend to be the lightest and least-expensive option for most applications.

A common misconception among some boat owners is that non-corroded anodes are a good sign that corrosion is not occurring. In fact, the opposite is true. If your anodes show no signs of wear or corrosion after extended use in the water, that may be an indication that damaging corrosion is occurring on other more critical components. If this happens to your motor, check the anodes closely. They may be dissipated, coated with scum or marine growth, or otherwise not functioning and should be replaced.

Replacement Tips

• Anodes look like a small gray block and are usually secured with one or more bolts.
• Some anodes are grounded to the engine case with a wire or strap. If yours are grounded, be sure to reattach the ground strap when replacing the anode.
• Anodes may become coated with a thin film of scum in some water conditions. This film can be removed with a stiff brush.
• Anodes must be in direct contact with the water to work properly, so never paint or apply other coatings over them. Either remove or tape over them when painting the surrounding area.
• Replace anodes when about half of the material has been lost to corrosion.

To get the best protection for your boat, always use quality anodes. You can count on Quicksilver replacement anodes to be made with the highest-quality, most-effective alloy for most marine applications.