How does cathodic protection work to prevent rusting?

Cathodic protection works to prevent rusting by making the metal act as a cathode in an electrochemical cell.

Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. This is achieved by placing a more reactive metal in contact with the metal you want to protect. The more reactive metal, known as the sacrificial anode, will corrode instead of the protected metal, which acts as the cathode.

The principle behind cathodic protection is that it changes the electrochemical potential of the metal surface, making it more negative. This shift in potential reduces the metal's tendency to oxidise, which is the process that leads to rusting. The sacrificial anode, on the other hand, has a more positive potential and therefore readily oxidises, sacrificing itself to protect the cathode.

In a typical setup, the sacrificial anode and the metal to be protected are both immersed in an electrolyte, which could be soil or water. The anode and cathode are connected by a conductive path, allowing electrons to flow from the anode to the cathode. This flow of electrons prevents the cathode from losing electrons and oxidising, thus preventing rust.

For example, zinc is often used as a sacrificial anode to protect iron or steel structures. When both metals are in contact and exposed to an electrolyte, zinc, being more reactive, will corrode first. This protects the iron or steel structure from rusting.

In summary, cathodic protection is a clever method of using the principles of electrochemistry to protect metals from corrosion. By making the metal to be protected act as a cathode and using a more reactive metal as a sacrificial anode, the process of rusting can be effectively prevented.