If you use your boat in saltwater, these zinc mil-spec anodes are a good choice for protecting metal rudders and trim tabs against galvanic corrosion.The zinc alloy material of these anodes makes them a good (see note below) choice for use in saltwater. Designed to meet stringent military standards (Mil-Spec MIL-A-18001) for sacrificial anodes, the alloy is mostly zinc, but with measurable concentrations of cadmium (~0.1%) and aluminum (~0.25%). The additional cadmium causes the anode to erode from the surface inward, which causes a sandblasted” appearance to the surface. When the anode is almost dissolved, it needs to be replaced.NOTE: Zinc alloy anodes should be used in saltwater only. Used in brackish water or freshwater they can "passivate" and cease to provide protection. If you use your boat in brackish water, aluminum alloy anodes are your best choice. If you use you boat in freshwater, we suggest magnesium alloy anodes.Key FeaturesZinc alloy for use in saltwaterMeet stringent Mil-Spec MIL-A-18001 for sacrificial anodesSlot-head machine screw fasteners