Comparing Anode Types

Anode inspection is most easily performed when a boat is out of the water. 

When replacing anodes, owners should consider the type of water in which they boat to best protect their craft.  The potential environmental impacts of anode materials also come into play, as all anodes are not the same.

Anodes are needed to protect against galvanic corrosion.  Immersing two different metals in water can create a small electrical current.  This steadily corrodes the less noble metal, such as the aluminium outboard housing connected to a steel propellor, or the copper plumbing joined by steel fittings.  Saltwater, which is highly conductive, speeds this process.  So does any stray electrical current, as could be found in a marina or even aboard the owner’s boat.

Sacrificial anodes, made from the least noble metals such as zinc, aluminum and magnesium, attract the electrical current.  Anodes are meant to be destroyed, so expensive equipment isn’t.  They have been used for centuries.

The water in which a boat sits helps determine the best anode type to use.  Zinc and aluminium alloys work well in saltwater.  In brackish waters, aluminum is better, as zinc’s efficiency decreases with reduced salinity.  Freshwater boats cannot use zinc at all – magnesium is the only metal alloy that will offer anode protection in freshwater.

Zinc is one of the most well-known anode materials, a readily available and inexpensive metal.  However, zinc anodes contain small amounts of cadmium, a heavy metal known to be toxic.  When the zinc anode corrodes away, small amounts of cadmium are also released into the water.  The environmental impact of zinc and cadmium is one issue the yacht owner should weigh when selecting anodes.

An alternative is aluminium alloy, which contains nothing environmentally threatening.  Aluminium anodes perform as well as, or better than, zinc.  They have better electro-chemical properties, are lighter, and have become competitively priced.

CMP Global is one US based supplier who offers a broad array of zinc, aluminum and magnesium anodes to protect boats in all types of water.  Its Martyr II aluminum and Martyr III magnesium anodes contain no cadmium.