Sacrificial Anodes

Whether it’s through direct contact with the water or exposure to salt in the air, nearly everything that comes in contact with the harsh saltwater environment will eventually rust, corrode or deteriorate. There’s simply no way around it, as corrosion is a big part of boating we must accept and learn to overcome. If you enjoy spending time in the brine you need to be fully aware of the signs and symptoms of corrosion so you can thwart accelerated deterioration.


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Corrosion is a powerful enemy, but one that can be defeated with prevention. Photo:

However, there’s one place on your boat where it is actually good to see corrosion, and it’s on the alloy anodes that are strategically placed as sacrificial offerings to the saltwater fish gods. Putting typical rust aside, every minute your boat is in the water it is under attack from two different types of corrosion.

If you notice your anodes are deteriorating faster than normal, then you are likely feeling the effects of a current leak...

Galvanic corrosion occurs when electrically grounded, dissimilar metals are submersed in a conductive liquid—warm saltwater just happens to be an ideal conductor of electricity. Without a complicated chemistry lesson, a reaction occurs that makes both chemical and electrical changes, resulting in electrons flowing from the softer metal to the harder metal. A prime example is an aluminum lower unit (less stable) and stainless steel propeller (more stable). Your stainless propeller is less chemically active than the aluminum lower unit and the resulting reaction is galvanic corrosion. If you’ve hit bottom and scraped off paint from the skeg or lower unit you’re providing an even better platform for the metal to be eaten away. Signs of galvanic corrosion include blistering paint and chalky substances on a lower unit, gear case, hydraulic rams, trim tabs and propeller(s). It’s important to note that metals will only be protected if they are grounded, whether touching or through an electrical connection.

The other cause of corrosion is derived from an electrical current leak from any one of many onboard electronics or nearby vessels. Stray current corrosion can be extremely damaging and cause rapid deterioration by accelerating galvanic corrosion. Whether it’s through a short in your wiring system or an issue with hooking up to shore power, stray current can lead to rapid corrosion.

With stray current corrosion from an internal source you will notice the most extensive damage at the direct spot where the current exits the boat. Even if your boat isn’t leaking electricity you could be susceptible to boats in the general area.

While you can’t ignore current in the water, you can combat the effects of corrosion by outfitting your boat with a galvanic isolator that serves as a filter to block unwanted DC voltage. You should also maintain and inspect your sacrificial anodes on a regular basis. If you notice your anodes are deteriorating faster than normal, then you are likely feeling the effects of a current leak from your boat, or a nearby boat.

To get an expert opinion on protecting against marine corrosion, we spoke with Chris Misorski of Mercury Marine. “Corrosion hours aren’t the same as engine hours, as sacrificial anodes work to protect around the clock. However, in order for sacrificial anodes to work properly they must have a direct electrical connection with the metals they are protecting. A loose anode with loose bolts isn’t making good electrical contact, so it becomes ineffective. You must keep tightening the screw. This is why it is recommended to replace an anode once it has deteriorated by approximately 50 percent. In a typical scenario, your anodes should last a season and if you notice them breaking down much faster you need to do something about the situation.”

Self-sacrificing anodes used to be made exclusively of zinc, which is why many still refer to them as zincs. However, leading manufacturers like Mercury are now relying on aluminum alloy anodes for increased protection in saltwater through a higher galvanic potential. For freshwater anglers, the innovative team at Mercury has recently released magnesium anodes for superior corrosion protection.

If you think something’s up and want to pin down an area of stray current, Mercury also makes a diagnostic test kit with reference electrodes that enables users to test their electronics and check for conductivity to single out the source of a possible leak.

Corrosion is a serious issue boaters in Florida must deal with on a daily basis. Even though the prevention of marine corrosion can be a challenge for boat owners it’s imperative you inspect your anodes to make certain they are protecting your vessel from corrosion. Fortunately, damage is almost always controllable. It simply requires knowledge of what causes corrosion and the proper maintenance steps that must be taken to thwart it. With routine maintenance you will save both time and money in the long run.