Which mechanism primarily affects the activity of cathodic reactions involved in corrosion?

• A. Filiform corrosion
• B. Uniform corrosion
• C. Oxygen concentration cell corrosion
• D. Galvanic corrosion

Answer: (C)

The amount of oxidant available at the metal surface controls how fast the cathodic reactions can proceed. In aqueous corrosion, that oxidant is typically dissolved oxygen. When there’s a difference in oxygen concentration across the surface, an oxygen concentration cell forms: regions with more oxygen become cathodic and drive faster reduction of O2, while nearby areas with less oxygen become anodic and dissolve. The cathodic reaction rate is effectively set by how much oxygen can reach the surface, so this gradient directly governs the activity of the cathodic half-reaction.

Other patterns aren’t about this specific control mechanism. Filiform corrosion is a coating-initiated pattern that advances beneath a film, not the local regulation of cathodic reaction rate. Uniform corrosion describes general, even material loss rather than cathodic-site activity driven by oxygen supply. Galvanic corrosion involves current flow between different metals or alloys, changing the distribution of anodic and cathodic areas, but the primary impact on cathodic reaction activity in this question is the oxygen concentration gradient.