Sacrificial Anode on the Hull of a Tug / Ship

Steel hulls of ships, which come into contact with seawater (an electrolyte) and consequently corrode (rust) are directly connected with base metals (e.g. zinc) to provide protection against corrosion.
Through the transfer of electrons from the zinc to the iron atoms of the steel, corrosion is inhibited until the zinc has been completely dissolved by the electrochemical reaction. The sacrificed pieces of metal are known as "sacrificial anodes".

Source: http://www.tis-gdv.de/tis_e/misc/elektro.htm