Zinc anode block

A zinc anode block is a cast zinc sacrificial anode bolted or welded to a steel hull, rudder, sea chest, or ballast-tank surface for cathodic protection. Zinc sits below steel in the galvanic series, so the block corrodes preferentially and supplies protective current to the steel, keeping it as the cathode. Marine zinc anodes follow alloy specifications such as US MIL-DTL-18001 that limit iron and add aluminum and cadmium or, in newer alloys, indium to keep the anode active in seawater. Zinc suits seawater service; aluminum anodes give more capacity per kilogram and magnesium suits fresh or brackish water. Anodes are renewed at drydock.