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