Pitting

Surface Pitting

Pitting corrosion or pitting is a localized form of corrosion by which cavities or "holes" are produced in metal. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Pitting can cause failure due to perforation while the total corrosion, as measured by weight loss, might be rather minimal.  The rate of penetration may be 10 to 100 times that by general corrosion. In some cases, pits may be masked due to general corrosion.

WIKIPEDIA ENTRY FOR PITTING

Pitting Picture      Pitting 2 Picture