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Chemical Damage
Many metals have a thin oxide protective film applied to protect their surface from corrosion. These films are considered passive films, meaning that their chemical structure is stable. When exposed to caustic chemicals, such as chloride, they become active and unstable, allowing corrosion to begin if the film fails to re-passivate itself. When a film re-passivates, it is reforming itself into one piece and blocking the entrance of the chemical into the metal body.
Mechanical Damage
Mechanical damage is considered to be a cause of pitting when the surface of the metal suffers an impact that damages the protective coating on the metal. Small holes or cracks in the surface allow caustic materials to begin corroding the metal.
Environment
The environment that the surface of the metal is exposed to can be a significant source of pitting and corrosion. If the metal is submerged in caustic materials, such as chloride, or if the materials are allowed to pool on the surface, the protective film on the metal can begin to degrade, allowing pitting to begin.
Protective Film Failure
A poorly applied protective film or an application of film that has been chemically compromised during the manufacturing process can lead to pitting. The weakness of the protective film combined with the presence of even minor adverse chemical, mechanical or environmental conditions will kick-start the corrosion process.