Choosing a Sacrificial Anode

What is a sacrificial anode, and how does it work?

A sacrificial anode is a metal rod or electrode that is made from a material that is more easily corroded than the metal it is protecting. When the anode is connected to the metal object, it attracts corrosion away from the object, sacrificing itself in the process and extending the lifespan of the protected metal. The current will flow from the newly introduced anode, and the protected metal becomes cathodic, creating a galvanic cell. The oxidation reactions are transferred from the metal surface to the galvanic anode and will be sacrificed in favor of the protected metal structure. This process occurs via "galvanic corrosion". Galvanic corrosion occurs when two metals with different levels of electrical activity, say a bronze propeller and a stainless steel prop shaft, are immersed in the same conductive liquid (like seawater). The liquid allows a weak electric current (electrons) to flow from the more active metal (the anode) to the less active one (the cathode). This is the same type of chemical reaction that occurs in a battery circuit. As the electric current flows, the more active metal (the anode) gives up electrons to the other and slowly dissolves in the process – not good if the anode happens to be your propeller!

A sacrificial anode is a third type of metal, say aluminum or zinc, which is installed because it is even more electrically active than either of the two original metals. When electrically connected to them by seawater, it becomes the material that gives up electrons and dissolves – thus sacrificing itself while preserving the original two metals. As long as you keep replacing the sacrificial anode, such as an aluminum anode rod before it has dissolved, the other less active metal components of your boat remain protected.

All Types of Anodes and ZincsThese days, anodes don't have to be "zincs." Sacrificial anodes are now commonly available in aluminum and magnesium in addition to zinc.

Often, non-zinc anodes are actually the better choice. Here's a quick summary, followed by a more in-depth look at this topic:

The Metals

Which Metal Should I Choose?

Additional FAQs

A Boat with New Anodes

Every boat is different, and while the above information is typical, with so many variables, it may not apply in all situations. When buying a new boat, it's important to keep in mind that what worked in the old marina might not be right for your boat's new home as needs can change from marina to marina (or even dock to dock!). To protect your investment, it may be best to consult with your local experts to make sure your current anode coverage is sufficient for the warranty of your water heater or other marine appliances.

Fisheries Supply carries all types of sacrificial anodes from all the best manufacturers - including Martyr and Sea Shield - so we've got everything you need to protect your boat. We hope you've enjoyed this Navigator, but if you have any further questions, please don't hesitate to contact our product experts at (800) 426-6930.