About Freshwater Anodes

Fresh Water Protection!

Yes, corrosion does happen in fresh water but generally at a slower rate. For many years, boaters had a problem protecting their hulls and/or drives against stray current in fresh water. An alloy much more active than zinc or aluminum is needed. Magnesium is the answer.

A magnesium anode is a super activated metal which means it will protect more efficiently in fresh water. Traditional zinc or aluminum anodes are just not effective as they do not produce the voltage necessary to work properly.


The salt content found in brackish or salt water dramatically increases the rate of corrosion.  If a magnesium anode is used in salt/brackish water it will corrode very quickly, possibly lasting only a very short time thus leaving your hull and drive completely unprotected.

WHY? Let the experts tell you:

The common anode for fresh water is magnesium. Aluminum is a good “catch all” anode meaning a boat that frequents both fresh and salt has decent protection in either environment. The quality of the anode and the purity of the aluminum or magnesium is also important. On this topic, experience (water type, boat location) is your best reference.
- John Adey

Technical Director, ABYC Master Technician

The reason why Mg anodes work better than Zn or Al in fresh water has to do with the higher driving voltage available when using Mg anodes. Fresh water is much less conductive to flow of ionic current, i.e. it has notably higher resistivity than salt water. Thus, the higher driving voltage of Mg is able to deliver more current than Zn.  In other words, it is easier to overcome the resistance of the current path in fresh water with Mg anodes than with the same size, shape, and number of Zn anodes. Furthermore, with Zn anodes there is a risk of potential reversal at elevated temperatures in certain waters.  With Al anodes, there is a risk of “passivation” in fresh waters, i.e. if the driving voltage is adversely reduced, insufficient current will be delivered for complete cathodic protection.

- NACE (National Association of Corrosion Engineers)

Material Engineer

Composition - Martyr III Magnesium Anodes

Aluminum8.3 - 9.7 %
Zinc0.35 - 1.0 %
Manganese0.15 - 0.50 % *
Silicon0.10 % Max.
Iron0.005 % Max. *
Copper0.030 % Max.
Nickel0.002% Max.
Total Others* 0.02 % Max. Total
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