Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the creation
of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small
area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very
localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of
ions. The mechanism of pitting corrosion is probably the same as crevice corrosion. It is supposed by some
that gravitation causes downward-oriented concentration gradient of the dissolved ions in the hole caused
by the corrosion, as the concentrated solution is denser. This however is unlikely. The more conventional
explanation is that the acidity inside the pit is maintained by the spatial separation of the cathodic and
anodic half-reactions, which creates a potential gradient and electromigration of aggressive anions into
the pit.[1] This kind of corrosion is extremely insidious, as it causes little loss of material with small
effect on its surface, while it damages the deep structures of the metal. The pits on the surface are often
obscured by corrosion products. Pitting can be initiated by a small surface defect, being a scratch or a
local change in composition, or a damage to protective coatings. Polished surfaces display higher resistance to pitting.
