Which statement best describes the drive of pit propagation?

• A. The potential difference between anodic and surrounding cathodic areas
• B. Gravity
• C. External magnetic field
• D. pH alone

Answer: (A)

Pit propagation is driven by the electrochemical potential difference between the anodic region inside the pit and the surrounding cathodic areas on the metal surface. This potential difference creates a localized current that causes metal to dissolve at the pit (the anode) while electrons flow to the nearby metal (the cathode), sustaining and enlarging the pit. The pit environment often becomes acidic as metal ions hydrolyze, which speeds up dissolution, but the fundamental driving force is this electrical potential difference that drives the localized corrosion current. Gravity and external magnetic fields don’t set this drive, and while pH affects the rate once dissolution has started, it isn’t the primary cause of pit growth.