Identify indicators that a CP system is under-protected or over-protected.

• A. Under-protected: high corrosion rate, insufficient protection potential, anode depletion; Over-protected: hydrogen evolution, coating disbondment, hydrogen-induced cracking.
• B. Under-protected: coating discoloration; Over-protected: very low current density.
• C. Under-protected: low corrosion rate; Over-protected: no hydrogen evolution.
• D. Under-protected: adequate protection; Over-protected: improved coating adherence.

Answer: (A)

The key idea is how the CP system affects the metal surface: under-protection means the surface isn’t being driven enough into a protective cathodic state, while over-protection means it’s driven too far, causing hydrogen-related damage.

Under-protection shows up as a high corrosion rate because the metal surface isn’t sufficiently polarized to suppress corrosion reactions. If the protection potential isn’t negative enough, the driving force for anodic corrosion remains too large. Anode depletion signals that the sacrificial anodes have been consumed or exhausted, reducing the available current to maintain adequate protection.

Over-protection emerges when the surface becomes too negatively polarized, pushing reactions that produce hydrogen. Hydrogen evolution can blister or lift coatings (disbondment) and, in susceptible materials, cause hydrogen-induced cracking.

Other options don’t fit as well because coating discoloration isn’t a reliable, direct indicator of CP level, very low current density isn’t a clear sign of over-protection, and statements like no hydrogen evolution or adequate protection describe non-protected or properly protected conditions rather than the issues asked.