Page 180 - Corrosion Engineering Principles and Practice

154 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 155

6.3 Localized Corrosion

6.3.1 Pitting Corrosion
Probably the most common type of localized corrosion is pitting, in
which small volumes of metal are removed by corrosion from certain
areas on the surface to produce craters or pits that may culminate in
complete perforation of a pipe or vessel wall (Fig. 6.8). Pitting
corrosion may occur on a metal surface in a stagnant or slow-moving
liquid. It may also be the first step in crevice corrosion, poultice
corrosion, and many of the corrosion cells described in Chap. 7.
Pitting is considered to be more dangerous than uniform corrosion
damage because it is more difficult to detect, predict, and design
against. A small, narrow pit with minimal overall metal loss can lead
to the failure of an entire engineering system. Only a small amount of
metal is corroded, but perforations can lead to costly repair of
expensive equipment.
One spectacular catastrophe resulting from a single pit has been
described in the television series called Seconds from Disaster. The
sewer explosion that killed 215 people in Guadalajara, Mexico, in

FIGURE 6.8 Water freely flowing out of a silver-plated water jug with pit
through corrosion.