Page 272 - Corrosion Engineering Principles and Practice

246 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 247

FIGURE 7.38 Cavitation downstream side of the service water system
downstream of a valve. (Courtesy of Russ Green, TMI)

have been found with sprinkler drops completely plugged with the
biofilm or biosludge debris generated by the MIC process. Sprinkler
system feed mains have also been found with up to 60 percent
obstruction from biological growth.
The second example relates to service water systems that supply
river water flow through emergency cooling coils used by the nuclear
industry. Water typically passes through a set of coils before being
returned to the river. Such systems are normally kept in standby mode
except for a quarterly test during which water is passed through a
minimum recirculation valve back to the river. In one case that was fully
investigated, cavitation was experienced immediately downstream of
the valve disc, cutting many deep grooves in the valve body and the
outlet piping. Since the leak was discovered as a drip and not a spray it
was postulated that microbiological action caused the final wall break-
through (Fig. 7.38).

7.5 Corrosion Avoidance
The forms of corrosion discussed in Chap. 6 are generally triggered
by one or many of the factors described earlier in the present chapter.
Some of these factors are conveniently described as corrosion cells
and corrosion avoidance starts by considering how to reduce the
effects of these factors and cells.