An often overlooked maintenance item on many boats are the sacrificial anodes commonly referred to as “Zincs.” They are the only line of defense between the metal parts of your boat and the water that is trying to dissolve them.
When two dissimilar metals, like a bronze prop and a stainless steel prop shaft, are in direct contact with one another in the presence of an electrolyte (like water), the most reactive (less noble) metal becomes an anode and tends to dissolve more quickly than the more noble metal through the process of galvanic corrosion. This problem is not unique to boats and you may have experienced preventing this problem with your home water heater which is commonly connected to the copper plumbing with di-electric unions to isolate its metal from the copper, and is further protected with a magnesium anode rod inside the tank.
How Do We Prevent This On Boats?
Boat and marine component manufacturers recognized this problem long ago and figured out that they could prolong the life of the metal components by introducing a third and least noble metal into the equation. The addition of a sacrificial anode made from zinc placed in contact with the other components, made it the part that was going to dissolve first, saving the next more reactive metal from damage.
Not All Zincs Are Created Equal.
When shopping for new anodes for your boat, you will no doubt see that there are anodes made from different metals to choose from. While we may refer to the anodes as zinc in conversation, they can also be made from aluminum or magnesium. Part of the reason for using different metals is economic; aluminum and magnesium are generally less expensive than zinc, but there is more to it than that. The other reason for the different materials has to do with the electrical conductivity of the water that the boat is going to be used in.
So How Do I Choose Anodes For My Boat?
If your boat is always used in fresh water, you should choose a magnesium anode set. Fresh water has a higher electrical resistance than both salt water and brackish water, so magnesium as the least noble of all of the galvanic metals, will give the greatest protection against corrosion. If your boat sits in salt water or brackish water, or even visits them occasionally, you’re going to want to choose either aluminum or zinc. While aluminum is lighter and cheaper than zinc, it does have a tendency to passify, or get a coat of corrosion on its surface while sitting, but that layer does slough off while going through the water. So, if your boat gets used often, you may be safe to use aluminum anodes in those situations.
What Am I Looking For?
The life expectancy of your zincs can vary, not only for the reasons we’ve discussed, but also due to the duration of the exposure to the water and the electrical current found in it. If yours is a trailer boat that comes out of the water at the end of every use, the anodes could last for many years. If your boat sits in a marina, the water composition and the condition of the electrical systems on your boat and the other boats in the marina will have an effect on the life of your zincs as they can leak voltage into the water and dissolve your zincs faster than would otherwise be expected. It’s a good idea to check them every time the boat is out of the water and maybe every few months – especially if the conditions change, like a new boat gets docked near yours. Many owners whose boats are slipped, change them every year, but a good rule of thumb is to change them when they are 50% of their original size.
Fair Winds and Following Seas!