Aluminum vs. Zinc Anodes for Boats: Saltwater and Freshwater

Most boaters tend to notice and address the corroded metal without realizing that, somewhere nearby, another metal is being protected. The metal that gives up electrons to “protect” the other metal is referred to as being less “noble,” having a more negative charge than the other metal. For example, the aluminum used on a lower unit is less noble (the anode) than your stainless steel propeller (cathode) and will corrode, while your propeller remains untouched.

That simple fact is the key to protecting your boat from unwanted corrosion. Introduce a separate “sacrificial” metal into the equation that is less “noble” than the metal you want to protect, and you can redirect the corrosion to something you don’t mind losing. In effect, the sacrificial anode creates an invisible, electrochemical shield for the critical metal components of your boat. 

Sacrificial anodes are small pieces of zinc, magnesium or aluminum that are attached directly, or via an insulated wire, to the metal they are designed to protect. These metals have a much more negative charge than the metals they are attached to. When exposed to an electrolyte (water), they “sacrifice” their electrons in the inevitable electrochemical process and keep the other, more noble, metal intact. 

• Outboard engines/outdrives — Sacrificial anodes can usually be found on the bottom of the transom bracket and on the cavitation plate of the gearcase, usually used as a trim tab just above the prop.

• Inboard engines — Inboard engine systems may have multiple anodes to protect the various components. Generally, you will find sacrificial anodes on the propeller shaft and rudder, as well as within an engine itself if it uses a raw water cooling system.

• Trim tabs — Because trim tabs are usually independent of other metal components of a boat, they may have their own sacrificial anodes to protect them. These will be attached in a manner to ensure the anode is submerged and will benefit from a constant flow of water to help keep them clean and active.

• Metal hulls — Aluminum and other metal hulls are subject to the same electrochemical process that any other metal components are and need to be protected. In most cases, sacrificial anodes will be attached to the hull in areas that are constantly submerged. Depending on the size of the hull, multiple anodes may be required at various points along the hull, especially around “hot spots” of galvanic activity.

Zinc anodes are most commonly used for outboards and smaller boat applications. They corrode more evenly and can better withstand impact. Due to the unique properties of zinc, zinc anodes are often the best choice for boats that remain still (docked) in saltwater for extended periods of time. Movement helps “clean” these anodes and keep them active.

In order for an anode to work, the raw metal in the anode must be in direct contact with both the metal it is protecting and the electrolyte (water) it is submerged in. Anodes can become filmed over or “passified,” reducing their exposure to the water around them and impacting their ability to “sacrifice” electrons and protect the more noble metal. Generally, simple water movement around the anode will keep it clean, but it may require periodic bushing.

Avoid using a metal wire brush, as this can impact the chemical reaction by introducing a more noble metal into the anode itself. Plastic brushes, Scotchbrite pads or other non-metallic abrasives can be used. Avoid using vinegar, acid or other chemicals to clean your anodes.

Some anodes are connected to the metal they protect with insulated wiring. Ensuring that this wire has a strong mechanical connection to the anode and that the wire insulation is in good condition is essential to ensuring the anode protects the metal component it is attached to.

The general rule of thumb is to replace your anodes when they are half gone. For outboards and outdrives, the zinc trim tab above the propeller will generally be used up before the one on the transom bracket, but both should be inspected regularly. Anodes are relatively inexpensive and far less expensive than the parts they are designed to protect, so when in doubt, replace them.

Zinc, aluminum and magnesium anodes should last at least a full year before needing to be replaced. If they need to be replaced more often, you may need a larger anode, or you may need to explore the possibility of stray currents that may be accelerating corrosion. This is particularly true in marinas and with metal boat hulls. Poorly maintained wiring on a boat or in a marina basin can accelerate the electrochemical reaction and corrode anodes at a very high rate.

Water temperature will also impact the lifespan of your anodes. Warm water environments have higher oxygen content, which accelerates corrosion, placing a higher demand on anodes and wearing them out more frequently. In cooler water, anodes will generally last longer.

Beware of passification — filming over. Anodes must allow fresh material to be exposed to the electrolyte solution (water).

It is important to ensure that your anodes are well connected to the metal they is designed to protect. This may be accomplished by directly attaching anodes to the protected metal surface, or by properly wiring to the metal using an insulated wire. Keep in mind that as an anode corrodes its contact to the metal may be reduced.

Anodes should be positioned close to the area being protected. Remember these anodes “sacrifice” electrons to protect other metals, so proximity does matter. On metal hulls, multiple anodes may be required at various points along the hull.

Anodes generally do not require much maintenance, but if they appear to be filmed over or coated with organic material, a brushing may be required. Do not clean with a wire brush.

Any coating that keeps the metal anode from direct contact with water and/or the metal it is designed to protect will render it useless. Some boat owners inadvertently paint over their anodes with anti-fouling (bottom) paint, completely eliminating their effectiveness. Do not paint over or coat an anode with anything, and ensure that there is no paint or coating between the anode and the metal it is protecting.