What is a common cathodic reactant?

The common cathodic reactant in cathodic protection systems is oxygen. In these systems, a reduction process occurs at the cathode, where electrons from the metal structure are consumed primarily by oxygen dissolved in the electrolyte. This reaction is crucial for the effectiveness of cathodic protection, as it helps to reduce corrosion by providing a mechanism for preventing the flow of electrons that otherwise would lead to metal loss.

Hydrogen, while it can be produced as a result of cathodic reactions (especially in the electrolysis of water), is not a common reactant in the context of cathodic protection systems. Rather, it is often a by-product in certain conditions, such as in the cathodic reaction when protecting steel in high resistance environments.

Nitrogen does not participate in cathodic reactions in a way that would affect corrosion processes in cathodic protection systems. It is largely inert under the typical conditions found in such environments.

Carbon dioxide may play a role in some specific corrosion processes, such as those occurring in water, but it is not considered a primary cathodic reactant in the context of cathodic protection.

Therefore, oxygen is recognized as the primary and most common cathodic reactant among the choices provided,