PITTING

Pitting is a type of localized attack commonly found in water-bearing equipment, and is caused by the formation of highly active, local anodic sites. These may result from unequal ionic concentrations or oxygen differentials. The sites may be found at high temperature zones, at points of metallurgical defects or at cuts, scratches or crevices on the metal surface.

The primary consideration is that pitting is the most common cause of metal failure. One perforation may damage a critical heat exchanger sufficiently to disrupt an entire plant process.

The depth of the pit is in direct proportion to the ratio of the large cathodic area to the small active anodic site. The seriousness of a pitting problem is often expressed in terms of a parameter called the "pitting factor"; the ratio of the pit depth to the average metal penetration.

Average metal penetration is often measured by recording the weight loss of corrosion test specimens over a period of time. Corrosion rates can then be expressed as mils per year (mpy). The higher the ratio of pit depth (in mils) to corrosion rate (mpy), the more serious the localized corrosion and the greater the danger of metal failure.