Have you ever wondered what sacrificial anodes on boats are for and why you have them on your boat? Hopefully, you won’t anymore.
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What are Sacrificial Anodes?
Sacrificial anodes on boats are usually a soft metal block, bar, or ribbon which is attached to the boat to protect heavier metal items like Propellors, shafts, engines, metal thru-hulls or even the hull itself from electrochemical corrosion. The full scientific explanation of how this works is beyond the scope of this particular article but if you are interested then you can find it here.
AB Tools 10oz Zinc Clamp On Sacrificial Anode for 22mm Shaft Saltwater Boat Propeller
What are Sacrificial Anodes made of?
This varies on the purpose of the anode, for instance: Saltwater has less resistivity and therefore Zinc is the main component for this type of anode. Whereas freshwater has a higher resistivity, so Magnesium is the better option for these anodes. The 3rd type is Aluminium. This type of anode is more suited to saltwater and are generally lighter and have a much higher capacity than their zinc counterparts, however, in electrochemical terms, they are somewhat unreliable and therefore not used as often as zinc anodes.
What are Sacrificial Anodes used for?
In simple terms, to protect the harder metals around them from electrochemical corrosion. In most cases, the softer metal, Zinc anodes, Magnesium Anodes, or Aluminium anodes will erode away before the harder metals like iron or steel.
How long do Sacrificial Anodes last?
This is one of those questions that is a bit like asking “How long is a piece of string?”. Its very difficult to answer. My particular boat, when first bought the anodes lasted about a year. Since then its varied substantially. I am aware of a local boat to me who went into the water with brand new anodes on his sail-drive, came out for a scrub 4 months later to find that his anodes had almost completely eroded away.
Its very likely that if you sit in a marina attached to a pontoon and all the boats around you use the shore power, without any galvanic isolation on them, then your anodes probably won’t last very long.
Do Sacrificial Anodes work in air?
The simple answer to this is No! Air is an extremely poor conductor.
Sacrificial Anodes for Engines.
Its fairly safe to say that if you have a marine engine that is saltwater cooled then there very probably is a dedicated engine anode, somewhere within the engine. Check your engine manufacturer’s manual. Buy the anodes and carry a spare or two.