Know Your Anodes, Boat Owners!

Are you a boat owner who needs things like marine anodes? If you’ve ever shopped for these items, you’ve probably noticed that you have to buy these products by the type of metal they are made of. These devices tend to be made out of a variety of different metals. Which one is best for your boat motor? It depends on a variety of factors, including which type of boat motor you have and what type of waters your craft travels. To help you develop a better understanding about marine anodes, Solid Signal shares this important information about marine electrical accessories.

Marine Sacrificial Anodes

To understand the metals used in marine electrical accessories, you first have to understand what sacrificial anodes are. These devices are made from aluminum, aluminum-zinc, magnesium, or zinc for a reason. A sacrificial anode is made from a highly active metal and is used to prevent a less active material surface from corroding. In a marine application, the anode is sacrificed by allowing it to corrode, which protects the hull of the ship from something called galvanic corrosion. These sacrificial anodes are made from a metal alloy that has a high negative electrochemical potential than the other metals the anode is designed to protect.

What is Galvanic Corrosion?

Galvanic corrosion is one of the biggest enemies to your boat’s hull, which is why the sacrificial anodes are so important. This form of corrosion happens when metals of different potentials are attached and submerged in water that carries an electrical current. Different metals corrode at different rates, so a current flow is created from the faster corroding metal, the anode, to the slower corroding metal, aka the cathode. For your anode to be reliable, it must be capable of continuously supplying electrical current throughout its life.

Anode Metals for Marine Electrical Installations

Marine anodes are made of one of four types of metals: aluminum, aluminum-zinc, magnesium, and zinc. While these metals are all used to make a variety of anodes, each of these types of anodes have different uses and applications. The key to choosing the right anode exists in knowing the differences between these metals, and choosing the right one for your boat. To help you, we’ve include this breakdown of the various marine metals, properties, and applications:

  1. Aluminum Anodes: These anodes provide more protection and tend to last longer than zinc, but the tradeoff is that aluminum’s electromechanical behavior (-1.1 Volts) isn’t considered as reliable as zinc. This means boat owners must take greater care of their aluminum anodes. Aluminum tends to work best in salt water and brackish water with higher levels of salinity. In low-salt brackish and fresh water, aluminum anodes tend to film over rather than shedding its consumed metal, which leads to a loss of galvanic protection.
  2. Magnesium Anodes: This metal has the most negative electric potential of all the marine metals (-1.6 Volts), and it’s best suited in water where the electrolyte resistance is higher. This means it’s best suited on boats that sail in fresh water, though it can be used in brackish water with lower levels of salinity. While zinc is reliable, high temperatures tend to make zinc anodes less negatively-charged, which prevents the current to flow.
  3. Zinc Anodes: Zinc has a relatively low-driving voltage (-1.05 Volts) which means that it might not be able to provide sufficient current in water that is resistive. Zinc anodes tend to work best in salt water and brackish water with higher salinity levels. In fresh water and lower salinity brackish waters, this metal tends to not have the current output to offer full protection from corrosion.
  4. Zinc-Aluminum Anodes: These two metals can be used to form an alloy that’s used in marine electrical accessories such as anodes. Zinc and aluminum anodes are typically used in salt water, where the resistivity is generally lower than fresh water and some brackish water. If you need to protect the hull of your craft, a zinc/aluminum anode might be the marine equipment you need. These anodes are also used in and/or your salt water-cooled boat motors, small boat propellers, and rudders.

Which Anode to Use and Where…

While knowing your anode metals is helpful, it’s also important to understand their applications in different boat installations. Whether you have an inboard boat motor or a stern drive and outboard motor will determine what type of anode you use. The waters your boat normally travels in also factor into the equation. Here’s a quick breakdown of the best types of anodes to use, based on engine type and waters travelled:

  • Inboard boat motors is any marine propulsion system that is mounted inside the hull of the boat. These boat motors are usually protected with zinc or aluminum anodes. The voltage generated by these metals usually won’t cause damage to the engine. Boat owners can also use magnesium anodes in freshwater locations as long as the boat’s hull is made of fiberglass. Magnesium anodes aren’t recommended for wooden- or steel-hulled boats.
  • Stern drives are marine propulsion systems that combine inboard power with an outboard drive. These boat motors sit forward of the transom while the drive unit is inside the hull. Zinc anodes were initially used with stern drive motors, but this led to corrosion problems. Since the 1990s, aluminum anodes were used due to the increase in protective voltage this metal provides. Using magnesium anodes with a stern drive boat motor runs the risk of overprotection, which causes blistering or cathodic disbondment.
  • Outboard boat motors are a self-contained marine propulsion system that includes an engine, gearbox, and propeller or jet drive. These boat engines are designed to set outside the transom. Like stern drives, outboard motors used to use zinc anodes, which led to corrosion issues. Most manufacturers recommend using aluminum anodes in outboard motors. (Zinc anodes might invalidate your warranty!) Like stern drives, magnesium anodes can lead to overprotection when used with outboard boat motors.

Your Marine Equipment Expert

When it comes to marine electrical accessories, there’s a lot for a boat owner to know in order to safely enjoy their hobby. Do you need a replacement anode for your vessel? Solid Signal carries a wide selection of Tecnoseal anodes, a top-selling marine products manufacturer. We also carry a huge variety of other marine equipment in our online inventory. While this availability is great, Solid Signal is more than just a bunch of well-stocked shelves. Solid Signal has a team of product experts who can answer your questions and even make product recommendations. To get everything you need to help keep you sailing straight, call us at 888-233-7563.

About the Author

Jake Buckler
Jake Buckler is a cord-cutter, consumer electronics geek, and Celtic folk music fan. Those qualities, and his writing experience, helped him land a copywriting gig at Signal Group, LLC. He also contributes to The Solid Signal Blog.