What Are Zinc Anodes for on Boats?
Zinc anodes are “sacrificial” attachments that protect submerged structures from corrosion. They’re most commonly referred to as zincs or anodes. They play a significant role in prolonging the lifespan of your boat and should be considered a regular part of maintenance.
Zincs can be found on all metal surfaces of a boat, depending on their size and propulsion type. On outboard boats, they can be found at multiple places on the outboard, trim tabs, and other metal surfaces. Inboard boats may require more zincs, as the drive shafts, rudders, struts, and even propeller nuts may require anodes. It is generally recommended to have an anode on any metal surface that will be underwater.
Galvanic Corrosion and Electrolysis
There are a few different types of corrosion that are prevalent in the boating world. Oxidation is one type of corrosion that most people are familiar with, resulting in a dull layer on your aluminum or stainless steel. Oxidation isn’t limited to metal, as gelcoat surfaces can oxidize as well.
Galvanic corrosion is another type of corrosion common in the marine environment and is often referred to as bimetallic corrosion. Galvanic corrosion occurs when two dissimilar metals are placed in contact with one another in the presence of an electrolyte (a conductive liquid). A common example of this would be using a stainless steel clamp on an aluminum frame. The proper way to eliminate galvanic corrosion is to isolate the materials from each other or coat them with a protective layer.
Another common type of corrosion is electrolysis. Electrolysis is defined as the “chemical decomposition produced by passing an electric current through a liquid of solution containing ions.” When this occurs, electrons are stripped off of one material and deposited on another. It’s commonly referred to as stray current corrosion, as it can come from faulty wiring discharging its current into the water around it.
How Anodes Protect Boats
Zinc anodes for boats can effectively reduce both galvanic and electrolytic corrosion. Zincs are considered a sacrificial anode, meaning they will deteriorate and corrode instead of the important parts of your boat corroding. In the case of galvanic and electrolytic corrosion, sacrificial anodes work by giving up their electrons before the materials they’re protecting. As a result, they will oxidize and deteriorate while the expensive metal parts of your boat stay intact.
Zinc vs. Aluminum vs. Magnesium Anodes
Sacrificial anodes can be made of different materials for different uses. The three general metals used are magnesium, aluminum, and zinc. These metals all have different levels of reactivity, and therefore, work better for certain environments.
Traditionally zinc has been the most popular metal to use as an anode. It is great for saltwater use but is ineffective at reducing corrosion in freshwater. When used in freshwater, zincs build up a coating that stops them from working. In high concentrations, zinc can be toxic to aquatic invertebrates, fish, and plants.
As a result, aluminum is becoming more popular for use as an anode. Aluminum alloy anodes are a great option, having grown in popularity for boats that are used in both fresh and saltwater, and lasts significantly longer than zinc. In addition to its effectiveness at reducing corrosion, it is not toxic to marine life.
Magnesium is the most active metal used as an anode. Due to this, it is best suited for freshwater use. It is best to avoid using magnesium anodes in salt water as they will not last long at all, and can even damage some paints due to their reactivity.
How Many Zinc Anodes Do You Need?
The amount of sacrificial anodes you need depends on the metal surface areas of your boat. You will typically need roughly one percent of the metal surface area in anodes. Typically you don’t need to do these calculations, as most boat manufacturers will outfit their boat with anodes that will just simply need replacement.
If your anodes are not lasting around a year, you may want to consider adding more. The more anodes you have, the longer they will take to deteriorate. About a year is the minimum time they should last before requiring replacement.
Sacrificial Anode Locations and Shapes
Anodes come in different shapes and sizes for different purposes:
The smallest anodes, and arguably the most important, are in your motor. On inboard motors, they’re small cylinders placed in the heat exchanger. These small cylinders are referred to as pencil anodes. On outboard motors, they’re small cylinders that bolt into the cooling jacket of your outboard.
Propeller anodes can be protected in multiple ways. On large vessels, this is with a propeller nut anode. On outboards, an anode washer can be used, or the anode on the lower unit can be sufficient. Anode specifications for outboards may vary by manufacturer.
The rudder or rudders of large inboard boats or sailboats typically have small anodes that do not cause much drag or turbulence.
Propeller shaft anodes go around the stainless steel shaft and bolt together on either side of the shaft. This design is intended to minimize drag and keep the shaft perfectly balanced. Selecting the perfect size is important as poorly sized anodes can cause balance issues and can result in poor corrosion protection.
Clip-on anodes can supplement a boat's existing anode system to counteract stray currents. They are not intended to replace any permanent anodes. They can be clipped to outdrives or other metal fixtures that need protection. Clip-on anodes can only be used while moored.
Hull anodes can be used to protect any electrically bonded accessories or to protect metal-hulled boats. These are typically much larger than other anodes.
Remember, anything below the waterline needs an anode or to be grounded to an anode! This includes trim tabs and through-hulls. Trim tabs have specifically designed anodes. Through-hulls can be electrically connected by wire to an anode.
There are a few nuances to anodes. They must make electrical contact with the surface they are protecting. Anodes also should not be painted. Paint blocks the anodes from corroding. The surfaces that anodes are applied to also need to be paint-free surfaces. Paint can reduce electrical contact between the anode and the surface it is meant to protect.
When anodes stop working or have reduced effectiveness it is referred to as anode passivation. This can occur to zinc in freshwater when salinity drops too low. Aluminum can passivate if it sits uncleaned for an extended period; however, this can be reversed with a thorough cleaning.
Zinc and other anodes can protect your boat from the effects of corrosion. With minimal maintenance, they can help to provide years of corrosion-free boating.
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