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Glossary of terms
Pitting of metals is extremely localized corrosion that generally produces sharply defined holes. The attack on the interior walls of the hole is usually reasonably uniform, but may be irregular where the specific conditions introduce a secondary intergranular attack. Every engineering metal or alloy is susceptible to pitting. Pitting occurs when one area of a metal surface becomes anodic with respect to the rest of the surface or when highly localized changes in the corrodent in contact with the metal, as in crevices, cause accelerated localized attack.
In general, when pitting occurs on a freely accessible clean metal surface, a slight increase in corrosivity of the environment will cause general or uniform corrosion. Pitting on clean surfaces ordinarily represents the start of breakdown of passivity or local breakdown of inhibitor-produced protection. When pits are few and widely separated and the metal surface undergoes little or no general corrosion, there is a high ratio of cathode-to-anode area, and penetration progresses more rapidly than when pits are numerous and dose together.
Pitting is one of the most insidious forms of corrosion; it can cause failure by perforation while producing only a small weight loss on the metal. Also, pits are generally small and often remain undetected. A small number of isolated pits on a generally uncorroded surface are easily overlooked. A large number of very small pits on a generally uncorroded surface may not be detected by simple visual examination, or their potential for damage may be underestimated. When pits are accompanied by slight or moderate general corrosion, the corrosion products often mask them.
Some causes of pitting are local inhomogeneity on the metal surface, local loss of passivity, mechanical or chemical rupture of a protective oxide coating, galvanic corrosion from a relatively distant cathode, and the formation of a metal ion or oxygen concentration cell under a solid deposit (crevice corrosion).
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The rate of pitting is related to the aggressíveness of the corrodent at the site of pitting and the electrical conductivity of the solution containing the corrodent. For a given metal, certain specific ions increase the probability of attack from pitting and accelerate that attack once initiated. Pitting is usually associated with metal-environment combinations in which the general corrosion rate is relatively low; for a given combination, the rate of penetration into the metal by pitting can be 10 to 100 times that by general corrosion.
With carbon and low-alloy steels in relatively mild corrodents, pits are often generally distributed over the surface and change locations as they propagate. If they biend together, the individua) pits become virtually indistinguishable, and the fnal effect is a roughened surface but a generally uniform reduction in cross section. If the initial pits on carbon steel do not combine in this way, the result is rapid penetration of the metal at the sites of the pits and little general corrosion.
The most common causes of pitting in steels are surface deposits that set up local concentration cells and dissoled halides that produce local anodes by rupture of the protective oxide film. Anodic corrosion inhibitors, such as chromates, can cause rapid pitting if present in concentrations below a minimum value that depends on the metal-environment combination, temperature, and other factors. Pitting also occurs at mechanical ruptures in protective organic coatings if the external environment is aggressive or if a galvanic cell is active.
Buried pipelines that fail because of corrosion originating on the outside surface usually fail by pitting corrosion. Another form of corrosion affecting pipelines involves a combination of pitting and erosion (erosion-corrosion).
With corrosion-resistant alloys, such as stainless steels, the most common cause of pitting corrosion is highly localized destruction of passivity by contact with moisture that contains halide ions, particularly chlorides. Chloride-induced pitting of stainless steels usually results in undercutting, producing enlarged subsurface cavities or caverns.
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