Anodic protection is used in which scenario?

• A. To lower the metal potential to promote passive film formation.
• B. To shift the potential cathodically to suppress dissolution of steel in all environments.
• C. To replace coatings in mildly corrosive environments.
• D. To raise the metal potential to form or maintain a passive film in highly acidic environments or when passive films are unstable under CP alone.

Answer: (D)

Anodic protection works by applying an anodic potential to push the metal surface into its passive region, promoting the formation or maintenance of a protective oxide film. This approach is used in highly acidic environments where the passive film would be unstable or dissolve, and where cathodic protection alone cannot keep the surface passive. By raising the potential, the surface becomes oxide-rich and resistant to dissolution, reducing corrosion.

This is not achieved by lowering the potential or shifting cathodic direction, and it isn’t about replacing coatings. It’s specifically chosen for scenarios where passivity is achievable only if the surface is held in an anodic, protective range.