Zincs are used on Naval ships for cathodic protection. Since a ship is a large hunk of iron sitting in saltwater something must be done to protect it from the corrosive nature of that same water. Then add into the mix the ships screws are made of brass and the whole thing gets really important.

I am going to bore you a little bit with some chemistry, but it is needed. Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte. OK, did you get that? Basically if two different types of metal are present in a fluid one will cause the other to corrode. Brass and steel are two types of metal what will do this to one another. One metal is considered the cathode and the other is an anode.

There is some other chemical stuff that happens, but in the end the anode corrodes and this can cause problems. SO, to prevent this the Navy uses ingots of zinc to protect the brass. Zinc is normally a cathode, but when a low voltage current it run to it it then becomes an anode and the brass becomes the cathode. There is a bunch of chemistry involved, but the end result is neither the hull nor the brass screw are corroded because of the use of the zinc bars.

Zinc bars are also used inside of the fuel tanks to prevent a similar action from taking place. Once again brass is the major concern since it is inside a steel fuel tank and filled with a liquid. Oddly enough the lube oil tanks do not have zinc protection in them, neither do the fresh water tanks.

Several other locations which could benefit from zinc protection located all over the ship are the sea water heat exchangers. These use sea water to either cool or heat depending on the end use. These exchangers are made of brass on the inside and steel on the outside. They corrode like mad on a ship and always need to be repaired. It is possible the Navy has fixed this over the years.