Pitting corrosion, pitting.

DIN 50 900, Part 1 (April 1982) gives this definition and explanation:
"2.1.3 Pitting corrosion. Corrosion in which electrolytic erosion of the metal affects only small areas of the surface and produces pitting (see Section 3.3). Pitting corrosion is caused by the presence of corrosion cells (see DIN 50 900 Part 2)... 3.3 Pitting. A form of corrosion characterized by crater-like depressions, surface excavations or pinprick-like depressions. Outside the areas of pitting there are practically no signs of surface erosion. As a rule, the depth of the pitting is equal to or bigger than its diameter. N. B.: In border-line cases it is not possible to distinguish between scarring and pitting." The most frequent cause of pitting damage to mechanical seal parts are halogens (mostly chlorine or chloride) in the process medium, which were not taken into account in the (metal) material selection, because these constituents were either unknown or unspecified.

Mechanical seal housing damaged by pitting