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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:
Diagram of Galvanic Corrosion
Two basic types of anode materials are used to combat corrosion:
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.
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:
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: