Sacrificial Anodes & Zincs

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Anodes & Zincs How-To's

marine corrosion and boat zincs

How to Test for Boat Corrosion (Video)

Learn how to test for marine corrosion, presented by Boating Magazine. ... read more

Video Transcript

Editor in chief, Kevin Falvey here to talk to you about marine corrosion testing.

Get your multimeter out and you'll also need a silver/silver-chloride reference electrode available at specialty marine retailers.

To begin, set your meter to DC. Connect the electrode to one terminal. Place the electrode overboard close to the fitting you're going to test. Now, connect the other probe to the fitting you want to test. Note the readings and compare against recommended voltages.

Those recommended voltages are available in documentation that can be supplied by your retailer and in books like Nigel Calder's Boat Owner's Mechanical and Electrical Manual.

Corrosion's costly and expensive. Get ahead of it before it gets ahead of you.
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Anodes for an Outboard

How Corrosion Comes About

Corrosion comes as a result of metals being mixed with water. Metals convert from oxides into ... read more pure metals and, energy must be present in order to complete the conversion. Because the energy is there, it makes the metals chemically erratic. When water is present, the metal wants to go back to its original state, and that is an oxide. Oxide is what causes rust. Some metals are more stable than others such as gold and copper, but most, such as steel, will want to degrade, or oxidize. We will share with you why some metals rust and others don't.
corroded Outboard Engine
Taking precautions and maintaining a boat is important. Metal atoms dissolve into the water; this means it's an electrolyte. Electrolyte means it's a liquid that can conduct electricity. These atoms sacrifice electrons which then transform into positively charged ions. Eventually, positive ions flow with negative ions in opposite directions. This part of the process is where it forms back into an oxide. This all means that there is an electric current setup between areas on the metal that result in corrosion.

What makes some metals corrode and not others? Cathodic metals have protection like gold, copper, graphite, etc. Anodic metals corrode first. These are metals like aluminum, zinc, steel, etc. There is a way to measure the voltage of a metal when in seawater. That's what makes up the galvanic series. The more the metal gives up atoms to the water; the more electrons are left over. Metals that have more positive ions corrode much less. When you have two metals competing in water, the anodic metal is going to release positive ions, and remember that the releasing of positive ions is the beginning of corrosion. Electrons flow towards the more positively charged metal such as copper. Now that you have been introduced to Corrosion 101, you now have a sense as to why a sacrificial anode plays a big role in protecting your boat.

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Anode Types

Types of Marine Anodes and Zincs

It's important to check your anodes frequently. Remember even a somewhat corroded anode can ... read more start corroding other places on your boat. So what type is needed? There are three anodes types. We will discuss each one and they're benefits.

Zinc Anode:

This is the most common anode on boats. Do not use boat zincs in freshwater environments though. Zinc anodes are for saltwater. Be sure your zinc anode meets US Military specifications to ensure quality. Be sure to check your boat zincs and anodes regularly, if it's not corroding then it's not working. If it shows corrosion, then it will start to eat away at other boat parts.

Navalloy Anode:

Navalloy anodes (aka Aluminum anodes) are not aluminum; it's an aluminum alloy. This means there is about 5% zinc and a trace of Indium. Aluminum is effective in freshwater and saltwater environments. There is more protection with aluminum, and it will last longer than zinc. Aluminum alloy is the only anode that is safe for all applications.

Magnesium Anode:

Only use this anode in freshwater applications. However, it can over-protect and that is bad with sacrificial anodes. It can mess up aluminum hulls and outdrives in polluted freshwater. Also be aware of salt or brackish water. Magnesium anodes show bad damage after only a few hours in saltwater. So use these with caution, and always double check the anodes.

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