Anodes protect the underwater metals on your boat from galvanic corrosion or electrolysis. But what causes galvanic corrosion? And how do you prevent it from eating away at your expensive propeller or outdrive?
Batteries, anodes and galvanic corrosion
In the 1800s, Alessandro Volta made the first battery using alternating plates of silver and zinc separated by salt water-soaked paper. Essentially, he discovered that you can generate an electrical current using two dissimilar metals connected electrically and surrounded by electrolytes (salt water or acid).
It turns out that galvanic corrosion, the destroyer of your boat’s underwater metal, is simply a giant battery cell on your boat’s hull.
Imagine your boat moored at sea: a stainless-steel prop connected to an aluminum outdrive floating in the world’s biggest electrolyte. It’s a giant battery, albeit one that doesn’t generate enough energy to do anything useful like brew your morning coffee.
The problem comes from your boat’s dissimilar metals. In the example below, the aluminum outdrive (the battery’s anode) has a negative charge compared to the stainless-steel prop (cathode). As in a household battery, the anodic metal gets used up (destroyed) in the process of generating electricity.
We call this galvanic corrosion. Unfortunately, on your boat, the used-up metal could be your expensive bronze propeller or aluminum outdrive.
Use anodes to prevent galvanic corrosion
The most practical way to prevent or reduce galvanic corrosion is to use anodes to actively or passively interfere with your floating battery’s ability to generate electricity. To do this, you introduce a new less-noble metal, such as aluminum, zinc or magnesium, into the cell to protect your more noble bronze propeller.
When you attach a sacrificial metal, usually zinc, near your expensive underwater hardware, it becomes the anode, or negative part, of the battery cell, disintegrating first and protecting your expensive hardware.
You can also accomplish this by impressing a reverse blocking current into the galvanic cell to stop the destructive flow of galvanic currents. This process, called “impressed current cathodic protection,” works on the same principle as sacrificial anode protection, except the protective current is monitored and adjusted by the system and a non-corroding material is used for the anode.
How and where anodes protect your boat
Most boats have factory-installed sacrificial anodes. Inboard-outboards and outboard-powered boats usually have several installed at different locations on the lower unit. Inboard-powered boats have them on the prop shaft, rudders and trim tabs and sometimes on the transom. A fiberglass sailboat will usually have a collar anode on its prop shaft and one on its rudder shaft. As mentioned above, boats with impressed current systems use permanent anodes.
When to replace anodes
Sacrificial anodes protect your boat as they wear out. As they corrode, the surface area wears away, and they become less effective. So, it’s important to replace them at least every year or when they’re half consumed. In salt water, anodes should be replaced every six months. Other conditions can cause anodes to wear away more quickly, so check them often to protect your boat.
Different anode metals
Anodes can be made of zinc, aluminum or magnesium, and each metal has different uses.
In general, zinc anodes should be used only in salt or brackish water. Aluminum anodes can be used for salt, fresh and brackish water. And magnesium anodes should be used only in freshwater.
Aluminum anodes create a higher voltage (driving force) and have a much higher capacity (useful life) for the same weight, making them more effective than zinc. Although they cost more, they do a better job of protecting boats used in both freshwater and seawater.
More expensive, magnesium anodes are often used for outdrives used in freshwater. Check your owner’s manual. Since magnesium anodes can “over protect,” causing paint blistering and other problems, they’re often used under the advice of a specialist.
Caring for your anodes
To ensure that the anode has good electrical contact, the attached surface should be clean and unpainted. For effective connection, make sure the anode fasteners are tight.
- Never paint over an anode. This turns off the anode and turns on your expensive and less noble underwater metals.
- Pay attention to your anodes. If they start to corrode faster, you could have a stray current corrosion problem in your boat or marina. If this happens, have the problem checked by a specialist.
- Use aluminum anodes for the best protection if you boat in both freshwater and seawater. Aluminum is also good for boats only used in seawater. If you only boat in freshwater, use magnesium anodes as recommended by the manufacturer.
- Don’t use an anti-fouling paint containing copper on an aluminum outboard or outdrive; instead, use a paint specifically formulated for outdrives.
- Be extra-vigilant about the anodes on bonded boats, which have their through-hull fittings electrically connected to underwater machinery. The failure of these anodes could cause galvanic corrosion on your boat’s through-hull.
- Replace your anodes every year or when they are half gone—even if they look pretty good. Replace anodes used in seawater every six months.