In cathodic protection, a metal anode that is more reactive to the corrosive environment than the protected metal system is electrically connected to the protected system. Therefore, the metal anode partially corrodes or dissolves instead of the metal system. For example, the hull of a steel ship can be protected with zinc sacrificial anodes, which can be dissolved in seawater to prevent corrosion of the hull. For systems where static electricity is generated by flowing liquids, such as pipelines and ships, sacrificial anodes are especially required.
In 1824, in order to reduce the impact of this destructive electrolysis on the hull and its fasteners and underwater equipment, scientist and engineer Humphrey David developed the first and most widely used marine electrolysis protection system. David installed a sacrificial anode, which is made of a more electrically active (less expensive) metal, connected to the ship’s hull, and used electrical connections to form a cathodic protection circuit. A less obvious example of this protection is the iron plating process. This process covers the iron structure (such as a fence) with a zinc metal coating. As long as the zinc remains intact, the iron will not be corroded. Inevitably, the zinc layer will break due to cracking or physical damage.
Once this happens, the corrosive element acts as the electrolyte and the zinc/iron combination acts as the electrode. The resultant current ensures that the zinc coating is sacrificed, but the basic iron is not corroded. This coating can protect the iron structure for decades, but once the coating is consumed, the iron will corrode rapidly. On the contrary, if tin is used to coat steel, when the coating cracks, it actually accelerates the oxidation of iron.