Pitting corrosion

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 (oxidation reaction) while an unknown but potentially vast area becomes cathodic (reduction reaction), leading very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions.

Severe pitting corrosion problems caused by chloride ions on a truss beam of the Nandu River Iron Bridge (Hainan Province, China) leading to the complete rupture of a metallic element.

Mechanism
Diagram showing a mechanism of localized corrosion developing on metal in a solution containing oxygen

The more conventional explanation for pitting corrosion is that it is an autocatalytic process. Metal oxidation results in localized acidity that 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] For example, when a metal is present in an oxygenated sodium-chloride electrolyte, the pit acts as anode and the metal surface acts as cathode. The localized production of positive metal ions in the pit gives a local excess of positive charge which attracts the negative chlorine ions from the electrolyte to produce charge neutrality. The pit contains a high concentration of metal (Me) chloride (MeCln) which hydrolyzes with water to produce the corresponding metal hydroxide (Me(OH)n), and n H+ and n Cl– ions, accelerating the corrosion process.[2] In the pit, the oxygen concentration is essentially zero and all of the cathodic oxygen reactions take place on the metal surface outside the pit. The pit is anodic and the locus of rapid dissolution of the metal.[3] The metal corrosion initiation is autocatalytic in nature however its propagation is not.

This kind of corrosion is often difficult to detect and so is extremely insidious, as it causes little loss of material with the 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 the alloy composition (or local impurities, e.g. metallic sulfide inclusions such as MnS or NiS), or a damage to the protective coating. Polished surfaces display a higher resistance to pitting.

Susceptible alloys and environment combinations

Pitting corrosion is defined by localized attack, ranging from microns to millimeters in diameter, in an otherwise passive surface and only occurs for specific alloy and environmental combinations. Thus, this type of corrosion typically occurs in alloys that are protected by a tenacious (passivating) oxide film such as stainless steels, nickel alloys, aluminum alloys in environments that contain an aggressive species such as chlorides (Cl–) or thiosulfates (S2O32–). In contrast, alloy/environment combinations where the passive film is not very protective usually will not produce pitting corrosion. A good example of the importance of alloy/environment combinations is carbon steel. In environments where the pH value is lower than 10, carbon steel does not form a passivating oxide film and the addition of chloride results in uniform attack over the entire surface. However, at pH greater than 10 (alkaline) the oxide is protective and the addition of chloride results in pitting corrosion.

Besides chlorides, other anions implicated in pitting include thiosulfates (S2O32−), fluorides and iodides. Stagnant water conditions favor 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 handling sulfur-derived compounds: 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.

Engineering failures due to pitting corrosion
A corrosion pit on the outside wall of a pipeline at a coating defect before and after abrasive blasting.
The Silver Bridge collapsed into the Ohio River as a result of stress corrosion cracking.

A single pit in a critical point can cause a great deal of damage. One example is the explosion in Guadalajara, Mexico on 22 April 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.[4]

Firearms can also suffer from pitting, most notably in the bore of the barrel when corrosive ammunition is used and the barrel is not cleaned soon afterwards. Deformities in the bore caused by pitting can greatly reduce the firearm's accuracy. To prevent pitting in firearm bores, most modern firearms have a bore lined with chromium.

Pitting corrosion can also help initiate stress corrosion cracking, as happened when a single eyebar on the Silver Bridge in West Virginia, United States failed and killed 46 people on the bridge in December 1967.[5]

See also

References
  1. ASM Handbook, Volume 13, "Corrosion", ISBN 0-87170-007-7, ASM International, 1987
  2. "Pitting corrosion". substech.com. 21 July 2015. Retrieved 4 December 2020.
  3. Princeton.edu, pitcorrosion.
  4. "Sewer Explosion due to Corrosion". Corrosion Doctors.
  5. Silver Bridge Collapse, Corrosion Doctors, read May 13, 2016


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