All About Anodes

3 Core Insights

• When two different types of metals are connected, a voltage is created between them and the metal that is the most anodic will corrode. Electrons flow from the most anodic metal, degrading the structure of the metal. Other metals that are electrically connected to the most anodic one will be protected.

• The rate of metal corrosion depends on the environment — air, freshwater, or salt water — and the proximity of nearby metals or electrical charges. Salt water is highly conductive, inducing the quickest corrosion for boat parts. Dissimilar metals that are touching, or bonded together with an electrical wire, will also induce anodic corrosion.

• Zinc, aluminum, and magnesium are anodic — they have the chemical property of corroding faster than other metals and are therefore commonly used as anodes. When zinc, for example, is connected to other metals on your boat, the electrical reaction will cause the zinc to corrode instead of your steel prop shaft or bronze seacock, protecting those critical parts. When the zinc degrades, it is working. They are called “sacrificial anodes” for a reason. You want to see it corrode so that your other metals stay intact.

3 Practical Tips

• Materials to use: While zinc has long been synonymous with anode materials, anodes made of aluminum alloys have been developed to work in fresh and salt water and are less expensive than zinc. Aluminum is also more environmentally friendly. Magnesium can be used in fresh water but will corrode too quickly in brackish or salt water.

• Installation: Connect the anode as close as possible to the metal component being protected. The electrical connection between the metals directly touching is far greater than the electricity conducted by water. If there is no electrical connection — physical or via wire — the anode will do nothing.

• Selection: The anode must be sized correctly. A small anode connected to a large metal part is likely to corrode too quickly, requiring frequent replacement. Use a larger anode or two anodes close to each other and the part being protected. If more than half of an anode has corroded away, replace it.

3 Resources

• An excellent source to buy anodes is boatzincs.com (no affiliation to Fathom Bay — just superb selection, price, and service). You can find anodes to protect any boat components from corrosion: props, prop shafts, outboards engines, rudders, bow thrusters, and more.

• “Boat anodes: a practical guide for sailors” (via Practical Boat Owner)

• A detailed test of anode materials from Practical Sailor

3 Helpful Videos

• How to check and replace an anode - Yachting Monthly

• Anode resistance check: after installing an anode (a) use a multi-meter on the resistance setting (usually indicated as Ω), (b) touch the probes to the anode and the metal that it is protecting, and (c) make sure the reading is nearly zero (0.2 or fewer Ohms Ω).

• A straightforward description of chemistry, metal types, anode types, various uses, and maintenance. This one is 17 minutes long and I suggest watching it at 1.5x speed.