Boat Anodes

Corrosion is one of the major enemies of all marine equipment. Since various metal parts of a ship, boat or other vessel are in constant contact with salt water, they can degrade very quickly due to natural electrochemical reactions. These reactions occur because many boat parts are made from two or more types of metal. And when two dissimilar metals, for instance, a bronze propeller and a stainless steel propeller shaft, are physically connected and submerged in water, they technically start functioning as a battery. The water acting as an electrolyte transfers electrons from a more active metal (anode) to a less active one (cathode), thus producing current. To create electrons the more active metal dissolves, giving up pieces of itself (metal ions). This process is called galvanic corrosion.

To protect vital boat parts from galvanic corrosion you have to prevent the active metals from deteriorating due to electrical activity. The only way to do this is to add a metal that is more electrically active than both original metals. And that’s when sacrificial anodes come into play. These pieces of an active metal are specifically designed to give up electrons and deteriorate instead of boat metals. This means they sacrifice themselves to the electrical current, leaving original metals unaffected by the process of corrosion. Anodes are usually installed on such underwater parts as propellers, shafts, stern drivers, fittings, struts, etc. The main benefits of boat anodes are that they are inexpensive and don’t require any internal power source. Plus, they are easy to install following the instructions.

Sacrificial anodes have originally been manufactured from zinc alloy which was extremely easy to obtain. But nowadays boat anodes are available in three metals: zinc, aluminum, and magnesium, all having a different electrical potential and current capacity. Magnesium produces -1.6 Volts, aluminum – -1.1 Volts, and zinc -1.05 Volts. If, for instance, zinc is attached to the steel propeller shaft that generates only -0.6 Volts, you’ll get protecting voltage of –0.45 Volts. The higher the voltage difference the better the protection. But remember that using too many anodes can result in overprotection, which can cause the coating on the aluminum to disbond or even lift the paint off the steel hull. To ensure adequate protection boat anodes should be replaced when a half of the anode is eaten away by corrosion.

Zinc anodes are designed to work well in salt water but aren’t much effective if you’re boating in brackish or freshwater. Although they have been traditional for so many years, these anodes have recently become an environmental concern because they contain cadmium and are toxic to the marine environment. Aluminum anodes are becoming increasingly popular these days since they offer many benefits over traditional zinc anodes. First of all, they are more electrically active and deliver exceptional protection in both salt and brackish water, as well as work adequately in freshwater. Aluminum anodes last up to 50% longer and weigh much less than zinc ones. On top of this, they are completely safe for the environment. Magnesium anodes are perfect for use in freshwater where the electrolytic conductivity is low. However, they should not be used in salt or brackish water because magnesium is a very active metal and will dissolve very fast without you even noticing it. Just like aluminum, magnesium is cadmium-free and harmless to the aquatic life.