What is corrosion, and what is a corrosion cell?

Corrosion is commonly observed as the rusting of iron or tarnishing of silver, but it encompasses a broad spectrum of metal degradation processes. A corrosion cell is fundamental to this process, playing the role similar to a battery. It consists of an anode and a cathode, submerged in an electrolyte, facilitating the movement of ions. The anode undergoes oxidation, losing electrons, while the cathode experiences reduction, gaining electrons.

In maritime environments, for instance, the saltwater serves as the electrolyte, and different regions of a metal structure can act as the anode or cathode based on their relative potential, material, or even impurities present on the surface. Corrosion cells are not created by design, but rather form spontaneously when metals are exposed to corrosive environments.

An understanding of corrosion cells is critical in many industries, as it helps in taking preventive measures, such as galvanization or the application of protective coatings, to extend the lifetime of metal structures.

Electrochemical cells are the fundamental units of batteries and the basis of understanding corrosion cells. An electrochemical cell comprises two electrodes—the anode and the cathode—immersed in an electrolyte solution that contains ions. These cells operate on the principles of redox reactions, where electrons are transferred from one substance to another.

In a typical setup, the anode experiences oxidation by shedding electrons, and these free electrons travel through an external circuit to the cathode, where reduction takes place. The electrolyte ensures ion balance by allowing the transfer of ions to maintain neutrality in the half-cell solutions. The electron flow from the anode to the cathode through the external circuit is what we harness as electrical energy in batteries or observe as metal deterioration in corrosion cells.

Oxidation and reduction reactions, or redox reactions, are a matched set—the occurrence of one is dependent on the other. Oxidation involves the loss of electrons, whereas reduction involves the gain. In the context of corrosion cells, the metal that becomes oxidized acts as the anode, deteriorating over time as it loses electrons.

Common Examples

• The rusting of iron: Iron atoms lose electrons and form iron oxide.
• The tarnishing of silver: Silver atoms react with sulfur in the air to form silver sulfide.

Understanding the role of these reactions allows students to grasp why certain metals are more prone to corrosion and how protective measures can prevent oxidation and thus, reduce the rate of corrosion. For instance, preventing the completion of the electrical circuit by physically separating the anode and cathode can inhibit the progression of corrosion.