Editing Corrosion (section)

=== Pitting corrosion ===
{{Main|Pitting corrosion}}
[[File:Pitting corrosion-scheme.png|thumb|upright|Diagram showing cross-section of pitting corrosion]]
Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which compete as [[anion]]s, can interfere with a given alloy's ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause ''corrosion pits'' of several types, depending upon conditions. While the corrosion pits only [[nucleation|nucleate]] under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an autocatalytic process. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause [[stress concentration]] to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure [[structural failure|fails]]. Pitting remains among the most common and damaging forms of corrosion in passivated alloys,<ref>{{cite web|url= https://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm|title= Different Types of Corrosion: Pitting Corrosion - Causes and Prevention|author= <!--Not stated-->|date= |website= corrosionclinic.com|publisher= WebCorr Corrosion Consulting Services|access-date= 2022-09-13|quote= |archive-date= 2022-09-13|archive-url= https://web.archive.org/web/20220913050419/https://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm|url-status= live}}</ref> but it can be prevented by control of the alloy's environment.

Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area. This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction. The deterioration of this small area penetrates the metal and can lead to failure. This form of corrosion is often difficult to detect because it is usually relatively small and may be covered and hidden by corrosion-produced compounds.