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 (SO). 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 (SO), fluorides and iodides. Stagnant water conditions with low concentrations of dissolved oxygen also favor pitting. Thiosulfates are particularly aggressive species and are formed by partial oxidation of pyrite (pyrite|, a ferrous disulfide), or partial sulfate reduction by microorganisms, a.o. by sulfate reducing bacteria (SRB). 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.