Zinc anodes, also known as sacrificial anodes, play a key role in the maintenance of your boat.
The term “zincs” has become synonymous with sacrificial anodes, being that zinc was the original material used for this purpose. However, other metals, such as aluminum and magnesium, also work as sacrificial anodes.
A sacrificial anode is made of a relatively inexpensive metal that will corrode in the place of the more expensive metal components of your boat or yacht – your shaft, propeller, rudders, stern drive and other vital parts. The idea is that the metal in the anode is more “noble” and will “sacrifice” itself, in order to corrode first, keeping your metal components free of corrosion when in the water for long periods of time.
There’s a place in corrosion protection for all three anode types – zinc, aluminum or magnesium – depending on various factors, explained here.
The Advantages of Aluminum Anodes
There are a few reasons why many manufacturers and boaters use aluminum over zinc or magnesium anodes.
- In recent years, aluminum has dropped in price, making it a more attractive choice for manufacturers when installing sacrificial anodes onto their new vessels, engines, strainers, heat exchangers, etc.
- Zinc protects well in salt, fair in brackish and a little in fresh. Aluminum protects well in salt, good in the upper levels of brackish waters, but not so good in the lower levels of brackish, and decent in fresh. However, aluminum passifies (films over with oxide coating) quicker than zinc. If cleaning occurs on a regular basis, aluminum will work. If not, the aluminum anode can become “passified”, rendering it useless.
- EPA reports have suggested that sacrificial anodes using magnesium, and secondly, aluminum, are more environmentally-friendly than zinc, however any environmental effect is minimal even worst case scenario with zinc. (Note: This is referring to National Pollutant Discharge Elimination System (NPDES) Vessel General Permit (VGP) with vessels over 79′ long*).
With that being said, if you are a boater who frequents brackish waters, or switches often between fresh and saltwater, you should consider an aluminum anode as your sacrificial anode. It is cheaper, lives a longer life, and can handle the switch between different water types better than the other metals.
If keeping your boat in saltwater only, take note of your location: in very warm tropic waters, higher water temperatures create more dissolved oxygen which increases corrosion rates and anodes have more demand on them, resulting in a much shorter life expectancy.
When Magnesium Sacrificial Anodes Should Be Used
While aluminum and zinc anodes can be used in freshwater, both aluminum and zinc anodes bow to magnesium (MG) anodes in freshwater.
The high current output of MG is needed in the high resistivity of fresh waters. While more expensive than zinc and aluminum, and live shorter lives, magnesium anodes are your best choice for freshwater, especially if your vessel will be in the water for long periods of time.
Magnesium’s high current output in saltwater runs the risk of over-voltage (hydrogen release) which can be remedied with either smaller and/or less Mg anodes, however a magnesium anode’s life in saltwater is typically too short to be a good choice there.
Where Zinc Anodes Still Reign Supreme
There comes a time and place where no other material beats zinc as a sacrificial anode – when it is docked or anchored in seawater for long periods of time.
Zinc anodes are better than other metals at “sloughing” off any film buildup, exposing fresh zinc alloy to the water, allowing the anode to be continuously electro-chemically active. Without this improved ability to slough, the consumed metal in aluminum anodes can film over and form a crust/barrier which will passify the anode.
The longer the vessel sits, the higher the chance of anode passivation.
However, when a vessel is underway, the moving water current creates electrical current which puts a demand on anodes, thereby reactivating them. Cutting through the water also cleans the oxide film off of the anode, whatever the metal type.
If a vessel remains active in saltwater on a regular basis, both zinc and aluminum anodes will work. If sitting for periods of time in saltwater, zinc is the way to go.
How Do Anodes Work?
Whenever there are two different metals physically or electrically connected and in water, they technically become a battery. There will be some amount of current flowing between both metals.
The electrons that make up the current are supplied by one of the metals giving up bits of itself – in the form of metal ions – to the seawater. This is called corrosion, and if left unchecked, will slowly destroy underwater metals on your boat.
The most common victim of corrosion is an aluminum prop on a stainless steel shaft, but struts, rudders, outboards and stern drives are also at risk.
By adding a second metal (an anode which is more noble), an electrical current is established towards the less noble metal which will receive the current allowing the anode to sacrifice itself by an ion transfer.
When To Change Your Sacrificial Anode and Other Tips
Regardless of the material you choose for your sacrificial anode, there are certain guidelines you should follow to keep your anodes in good shape and the vital components of your boat protected and corrosion-free:
- Replace the anode when about half of its size has been lost to corrosion. If this occurs in less than a year, you may want to increase the size / weight of the anode.
- For a zinc anode to work, it must have electrical contact with the metal you are trying to protect. For instance, on the prop shaft right ahead of the prop. If not able to form direct contact, it can be connected by a wire as well.
- Never coat an anode with paint or anything else that will cover it; this will render it useless.
- Different types of anodes may be used, just don’t intermingle them. For example: Shaft and prop protection is somewhat isolated so a change of anode type is acceptable.
- It is essential to make sure the surface under the zinc is bare and bright before the anode is installed. This is to ensure good electrical contact.
Moral of the anode story? It is better to spend the little time and money required to replace the anodes on a regular basis, rather than spending thousands on a new propeller shaft or stern drive.