Sacrificial anode

A sacrificial anode is a block of a less-noble metal, zinc, aluminum, or magnesium, bolted or welded to a steel structure so it corrodes in place of the steel it protects. It works by galvanic action: the anode metal sits lower in the galvanic series, so it becomes the anode of the couple and gives up electrons while the protected steel becomes the cathode and stays intact. This is cathodic protection by galvanic anode, used on hulls, rudders, sea chests, and inside ballast tanks. Anodes are consumed over time and renewed at drydock. Impressed-current systems are the powered alternative for large hulls.