Page 183 - Corrosion Engineering Principles and Practice

158 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 159

The complex interactions between these factors may cause major
differences on how pitting corrosion will initiate and develop in real
situations. Copper, for example, a relatively simple material in terms
of its metallurgy, can suffer three well-documented types of pitting
corrosion depending on specific conditions in the water it carries:
Type I pitting is associated with hard or moderately hard waters
with a pH between 7 and 7.8, and it is most likely to occur in cold
water. The pitting is deep and narrow, and results in pipe failure
(Fig. 6.12).
Type II pitting occurs only in certain soft waters, with a pH below
7.2 and occurs rarely in temperatures below 60°C. The pitting that
occurs is narrower than Type I, but still may result in pipe failure
(Fig. 6.13).
Type III pitting occurs in cold soft waters having a pH above 8.0.
It is a more generalized form of pitting, which tends to be wide
and shallow and results in blue water, byproduct releases, or pipe
blockage (Fig. 6.14).

The practical importance of pitting corrosion depends on the
thickness of the metal and on the penetration rate. In general, the rate
of penetration decreases if the number of pits increases. This is be-
cause adjacent pits have to share the available adjacent cathodic area,
which controls the corrosion current that can flow. A pit may go
through four separate stages: (1) initiation, (2) propagation, (3) termi-
nation, and (4) reinitiation.

Cu(OH) (SO ) Blue
x
4 y
and
CuCO Cu(OH) Green
2
3

Cuprous oxide (Cu O) layer Copper
2
Cuprous chloride (CuCl) pipe wall

FIGURE 6.12 Type I copper pitting corrosion. (Courtesy of Russ Green, TMI)