In a corroding pipe, the electrons released at the anode will combine with which species in the pipe at the cathode?

• A. Metal cations in the flowing water at the cathode
• B. Organics in the flowing water
• C. Ferrous hydroxide at the cathode
• D. Two hydrogen ions to form hydrogen gas

Answer: (D)

In a corroding pipe, electrons flow from the anode to the cathode and are used in the reduction reaction that occurs there. In the aqueous environment of the pipe, the easiest and most common species to reduce is the hydrogen ion. Two hydrogen ions gain two electrons and form hydrogen gas: 2 H+ + 2 e− → H2. That’s why hydrogen gas evolution is the typical cathodic process in corrosion.

Ferrous hydroxide can form later from Fe2+ and OH−, but it isn’t the species being reduced at the cathode. Organics aren’t the primary electron acceptors in this corrosion context, and reducing metal cations to metal is not the usual cathodic path here.