ALUMINUM PREMIUM — For use in salt and brackish water. Performs better, protects longer than Zinc. Alloy is manufactured to meet or exceed US Military Specification (MIL-DTL-24779C(SH)). MAGNESIUM FRESH — ONLY for use in fresh water. Not recommended for use in salt or brackish water. The only alloy proven to protect your boat in fresh water. Alloy is manufactured to meet or exceed US Military Specification (AZ91D). ZINC TRADITIONAL— For use in salt water. Not recommended for use in fresh water. Alloy is manufactured to meet or exceed US Military Specification (MILDTL-18001L).