Corrosion can be a big headache in terms of vessel maintenance. Galvanic corrosion occurs when two dissimilar metals are connected in seawater. The electrons from the base metal attach themselves to the more noble metal. The result is that the base metal eventually corrodes away. In order to prevent this corrosion, sacrificial anodes must be attached to vessel components made of base metals in order to prevent this corrosion from occurring.
The chart above lists the various metals commonly used in marine manufacturing. Note that Zinc is at the bottom of the list. The nearer to the top of the list, the more “noble” a metal is. Therefore, anodes are usually manufactured from materials near the bottom. Hence, the common use of zinc in anodes. It is base, and usually cheap.
An alternative solution is electro-cathodic protection of the entire vessel. These systems measure the current level of the seawater, and produce an opposing current, thus preventing corrosion by keeping the elements from moving. These systems are often quite good at preventing corrosion, though they can often become expensive. It is also important to note that salinity, water temperature, and other factors impact the current level in the water, and such systems nearly always require monitoring.