A Quick Guide to Boat Anodes

Author: Boat Accessories Australia Date Posted: 7 July 2015

A Guide to Anodes

Corrosion is an unfortunate part of boating and marine life. That’s why sacrificial anodes are used where corrosion eats the anode instead of your sterndrive, prop shaft or raw water through-hull.

There are two types of corrosion:

Electrolytic Corrosion – Damaged appliances or wiring that is leaking current to ground via the hull (metallic hulls) or via common earth to submerged metal causes what's known as electrolytic corrosion. This can also rot bearings, washers, oil and water coolers and cause major engine damage quickly.

Galvanic Corrosion – Relies on two dissimilar metals connected and in the same electrolyte (sea water). The two metals act like a battery, with some amount of electrons flowing between the two that leads to one of them giving up metal ions to the seawater.

Galvanic Corrosion Example Diagram

Diagram of Galvanic Corrosion

Sacrificial Anodes

Two basic types of anode materials are used to combat corrosion:

Zinc Anodes – Zinc is traditionally cheaper and more commonly used than aluminium used for sacrificial anodes that have lower electrons to dissolve to protect the more valuable metal parts of a boat.
Aluminium Anodes – Suitable for outboard motors due to its ability to form a protective oxide surface film with good corrosion resistance but also prone to galvanic corrosion when in contact with more noble metals like bronze, brass or nickel.

Boat & Marine Material

Boat Hull Materials differ in their corrosiveness based on the materials. Fibreglass and wood hulls are not susceptible to corrosion however the equipment attached to it are, such as propellers and outboard equipment.

Aluminium and steel hulls are susceptible to galvanic corrosion. Aluminium on its own does quite well in both fresh and salt water but when combined with other metals it will start to corrode. Usually a protective paint coating can provide a high electrical resistance barrier between the aluminium and water although in some cases employing dedicated sacrificial anodes can be useful.

Boat Size – How Many Anodes to Use

Hull anodes are fitted for boats which stay in the water. Usually the amount placed will be in relation to how much of the hull stays below the water line.

Smaller boats that are trailered in and out of the water only usually require outboard anodes that are built to last longer. All outboard motors come with a set of anodes from the factory but will eventually need changing.

Water Type

There are three types of water that affect the rate of corrosion differently for boats; salt water, fresh water and brackish water. As mentioned before, aluminium performs quite well in salt water so aluminium anodes will work well too, as long as they’re not combined with stronger materials i.e. things that aren’t zinc or magnesium. Brackish water is a mix of salt and fresh water that can be harmful to zinc, where a calcareous coating can build up on the anode that essentially prevents it from corroding.

Here’s a handy chart to help you in choosing the right metals for the right water and boat type:

	Inboard				Outboard
Hull	Wood	Fibreglass	Aluminium	Steel	All
Freshwater (Pure)	Aluminium	Aluminium/Magnesium	Aluminium	Aluminium/Magnesium	Aluminium/Magnesium
Freshwater (Polluted)	Aluminium	Aluminium	Aluminium	Aluminium	Aluminium
Brackish	Zinc	Zinc	Zinc	Zinc	Aluminium
Salt	Zinc	Zinc	Zinc	Zinc	Aluminium

There are other considerations. For example, if your boat is connected to shore power, and thus connected to all the other boats at the dock, you may choose “weaker” zinc over aluminium to minimize the electrical activity.

But if your boat has a galvanic isolator, or you’re not plugged in, the more active anode might be a better choice.

For more information, check out our range of anodes including:

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A Quick Guide to Boat Anodes

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