Final answer:
In a closed system, pitting is most commonly caused by corrosion, particularly when the protective barriers of metals such as iron are compromised, allowing for oxidation and rust formation.
Step-by-step explanation:
In a closed system, the most likely culprit for pitting is corrosion. Corrosion involves the disintegration of materials due to chemical reactions with substances in the environment. Rust formation on iron is a common example of corrosion, typically involving the creation of a galvanic cell at an iron surface. When iron is exposed to oxygen and water, rust forms through redox reactions where iron is oxidized to Fe2+ (aq) and oxygen is reduced, leading to the formation of pits, especially at places where protective coatings have been breached.