Zinc anode (sacrificial)

A zinc sacrificial anode is a block of zinc bolted or welded to a steel hull below the waterline to protect it by galvanic corrosion. Zinc sits below steel in the galvanic series, so in seawater it becomes the anode and corrodes preferentially, holding the steel cathodic and shifting its potential below the corrosion threshold, near minus 800 mV against silver/silver-chloride. Anodes are sized from the wetted area, the design current density (roughly 100 to 150 mA/m^2 for bare steel, much less for coated steel), and the interval to the next docking. They appear in the weight estimate as an outfit item and are renewed each drydocking. Aluminum anodes are common too; magnesium is reserved for fresh water.