If you use your boat in saltwater or in brackish water, these aluminum mil-spec anodes are your best choice for protecting your prop shaft and prop against galvanic corrosion.Used in saltwater, aluminum anodes protect better and last longer than zinc. Used in brackish water, aluminum anodes are superior to zinc which can passivate” and not work at all or magnesium anodes which corrode too rapidly. Aluminum anodes are also lighter than zinc and contain no cadmium which is a substance of environmental concern. Offered with hex-head stainless steel machine screws.Key FeaturesAluminum alloy for use in saltwater or brackish waterProtect better than zinc anodes when used in saltwaterMeet stringent military specifications for sacrificial anodesInterlocking halves with hex-head machine screw fastenersExtra alloy around fasteners