Which factor drives corrosion during the propagation step of pitting?

• A. The potential difference between the anodic area inside the pit and the surrounding cathodic area
• B. Ambient temperature increase
• C. Surface roughness
• D. Humidity level

Answer: (C)

During the propagation step, pits grow because the interior of the pit becomes the active anodic region while the surrounding surface acts as the cathode. Surface roughness plays a crucial role here because rough features and micro-crevices trap chloride-rich, acidic solution and create stagnation zones. This environment keeps the protective film from reforming easily, lowers the local pH, and maintains high local dissolution at the pit mouth. The rough surface also increases the real interfacial area and can sustain higher local current density, promoting continued metal loss inside the pit and deeper penetration. In smoother areas, repassivation is more readily achieved, which slows or halts pit growth, whereas roughness helps sustain propagation.