Galvanic anode

A galvanic anode, also called a sacrificial anode, is a block of a less noble metal bolted or welded to a hull or submerged steel structure so it corrodes preferentially and holds the steel cathodic. The two marine metals are zinc and aluminum: cast and wrought zinc anodes are specified to ASTM B418 (Type I and Type II compositions), while aluminum anodes are aluminum-zinc-indium alloys covered by DNV and US military specifications. The anode supplies current with no external power, unlike an ICCP system. Anode mass is sized to the bare steel area, the design current density, and the docking interval; depletion to roughly 80% consumed is the usual replacement trigger.