, 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 lack of oxygen around a small area. This area becomes anodic
while the area with excess of oxygen becomes cathodic
, leading to very localized galvanic corrosion
. The corrosion penetrates the mass of the metal, with limited diffusion of ions, further pronouncing the localized lack of oxygen. 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 more dense. 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.
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 coating. Polished surfaces display higher resistance to pitting.
Alloys most susceptible to pitting corrosion are usually the ones where corrosion resistance is caused by a passivation layer
: stainless steels
, nickel alloys
, aluminium alloys
. Metals that are susceptible to uniform corrosion in turn do not tend to suffer from pitting. Thus, a regular carbon steel will corrode uniformly in sea water, while stainless steel will pit.
Addition of about 2% of molybdenum
increases pitting resistance of stainless steels.
The presence of chlorides
, eg. in sea water
, significantly aggravates the conditions for formation and growth of the pits through an autocatalytic
process. The pits becomes loaded with positive metal ions through anodic dissociation. The Cl- ions
become concentrated in the pits for charge neutrality and encourage the reaction of positive metal ions with water to form a hydroxide corrosion product and H+
ions. Now, the pits are weakly acidic, which accelerates the process.
Besides chlorides, other anions implicated in pitting include thiosulfates (S2O32–), fluorides and iodides. Stagnant water conditions favour pitting. Thiosulfates are particularly aggressive species and are formed by partial oxidation of pyrite, or partial reduction of sulfate. Thiosulfates are a concern for corrosion in many industries: sulfide ores processing, oil wells and pipelines transporting soured oils, Kraft paper production plants, photographic industry, methionine and lysine factories.
Corrosion inhibitors, when present in sufficient amount, will provide protection against pitting. However, too low level of them can aggravate pitting by forming local anodes.
A single pit in a critical point can cause a great deal of damage. The example is the explosion in Guadalajara
, Mexico on April 22 1992, when gasoline
fumes accumulated in sewers
destroyed kilometers of streets. The vapors originated from a leak of gasoline through a single hole formed by corrosion between a steel
gasoline pipe and a zinc
-plated water pipe.
- Pitting Corrosion - Corrosion Doctors
- Chemical explanation of pitting corrosion - University of Rhode Island, College of Engineering