Marine zincs for salt water immersion should have a MIL SPEC number stamped on them to assure the proper metal alloy. I see them on shaft and rudder zincs, maybe not pencil size for engines. I posted that spec number a few years ago but do not have it with me. Anybody know what that is and can post it?? There may be different alloys for industrial versus marine applications. For example, underground storage tanks and connecting plumbing should be protected via zinc anodes, but the metal composition may not be suitable for salt water immersion. Rob Brueckner Hatteras YF