Martyr™ III Cadmium-Free Magnesium Anodes for Freshwater ONLY. Use Cadmium-free, magnesium anodes if you only use your boat in freshwater. (Zincanodes can “passivate” and stop working in freshwater.) Magnesium anodes should notbe used in any other water type. If you trailer your boat, and use it in a variety of saltwater,brackish and freshwater locations, aluminum anodes are the best all-around choice forprotecting your outboard or outdrive.