Which of the following is an indicator of cathodic protection over-protection?

• A. High corrosion rate
• B. Hydrogen evolution
• C. Insufficient protection potential
• D. Anode depletion

Answer: (B)

Over-protection happens when the protective current drives the metal to potentials that are more negative than needed. At these excessive negative potentials, the water reduction reaction becomes dominant on the metal surface, producing hydrogen gas. Visible or energetic hydrogen evolution indicates the CP system is applying too much current, which can lead to issues like hydrogen embrittlement in susceptible steels and coating damage from gas formation. This is different from under-protection, where the metal still corrodes because the potential isn’t negative enough, or from anode depletion, which points to worn-out anodes and reduced CP current rather than to excessive protection. So hydrogen evolution is the clear sign that cathodic protection is over-protecting.