How do coatings like zinc protect iron from rusting?

Zinc coatings protect iron from rusting by acting as a sacrificial anode, corroding before the iron does.

Zinc is more reactive than iron, which means it will oxidise more readily. When a zinc coating is applied to iron, it acts as a barrier between the iron and the oxygen and water in the air. This is known as a physical barrier. However, the zinc coating also provides a chemical protection. This is because zinc is more reactive than iron and will corrode first, a process known as 'sacrificial protection'.

When the zinc coating starts to corrode, it forms zinc oxide. This zinc oxide layer then acts as an additional barrier, preventing the oxygen and water from reaching the iron. This is why zinc coatings can continue to protect iron even when they have started to corrode themselves.

The process of zinc corroding before iron is also known as 'galvanic protection'. This is because zinc acts as a 'sacrificial anode'. An anode is the electrode where oxidation (loss of electrons) occurs in a galvanic cell (a type of battery). In this case, the zinc coating is the anode and the iron is the cathode (where reduction, or gain of electrons, occurs). The zinc loses electrons more readily than the iron, so it corrodes first. This protects the iron from rusting.

In summary, zinc coatings protect iron from rusting by providing both a physical barrier to oxygen and water, and a chemical protection through sacrificial corrosion. This dual protection makes zinc coatings a very effective way to prevent iron from rusting.