Any boat with a metal hull, or a structure that is immersed or partially immersed in salt water, requires a system of some kind to protect it from corrosion. Salt water can corrode metal in a big, bad way, completely compromising the structural integrity and resulting in failure of a boat’s hull or an important structural element if it isn’t properly prevented. There are two primary tools used for this prevention: sacrificial anodes and cathodic protection systems.
Sacrificial anodes are little more than pieces of zinc affixed to a boat’s or other metal object. Understanding why this helps requires some understanding of how sea water corrodes metal. When salt water and metal surfaces come into contact, and either the water or the metal is moving, the interaction of substances causes an electrical current that gradually bleeds electrons and causes the metal to degrade. This electrical current is very similar in the way it is generated to how a battery functions. In lieu of motion, the use of plural metals in a single component can produce a similar effect. A sacrificial anode assists this process by controlling where the electrical current originates from and terminates, causing all degradation to occur to the zinc before it can reach anywhere else.
The zinc anodes need to be replaced regularly, hence their “sacrificial” title, but they are quite effective at balancing the charge and preventing corrosion. However, they are a more archaic solution, and can be ineffective for very large objects. While any quantity of zinc will help, smaller quantities will corrode more quickly when attached to larger objects, and the difference in mass can result in a geometric decrease in efficiency. For this reason, active cathodic protection systems were developed.
Active cathodic protection systems work on the same principle as sacrificial anodes. They alter the charged current induced by the salt water on the metal components of a hull or ocean-anchored object to prevent corrosion. However, they do not do this by balancing the charge with a piece of metal, but rather by pumping a countering current into the hull. This allows for a number of advantages that sacrificial anodes cannot provide.
First of all, this allows for the cathodic protection of much larger objects. The largest ships in the sea use cathodic protection for this purpose. This is also better for extended and infrastructural solutions where it simply wouldn’t be feasible to utilize items that need to be replaced regularly. Active cathodic protection is usually brought in alongside the use of waterproof paint and other solutions to produce a composite defense against corrosion. Cathodic protection systems like this use circuits to determine how much voltage needs to be put out, allowing them to compensate for damage to passive solutions. The only disadvantage is the power requirement.
All in all, cathodic protection is a necessary component of anything that will be in extended contact with salt water. Corrosion can destroy infrastructure and ruin investments, but cathodic protection allows many aspects of our modern world to be possible. If you would like to know more specifics about different kinds of cathodic protection systems, you can learn more here.