Martyr CM43994M, Magnesium Alloy Block Mercury/Mercruiser Anode Cathodic protection for Pleasure craft boats Magnesium Alloy for use only in fresh water Only alloy proven to protect in fresh water Not recommended for use in salt or brackish water High-quality Martyr anodes are pressure diecast and manufactured to strict quality standards through an ISO 9001 Quality Management System Mil Spec Alloy Magnesium Alloy for use only in freshwater