Pitting corrosion
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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 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 area tends to bury into 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 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 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. Eg, 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.
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 in Mexico, 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. [1]
