Page 212 - Corrosion Engineering Principles and Practice

186 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 187

FIGURE 6.38 Cavitation corrosion inside a deaerator unit. (Courtesy of
Defence R&D Canada-Atlantic)

which itself is a function of dimensionless Reynolds and Schmidt
numbers in Eq. (6.4):
Sh = a Re b Sc g (6.4)

where a, b, and g are experimental constants.
One of the most accepted mass transfer correlations gives values
of 0.0165, 0.86, and 0.33 for respectively, a, b, and g for fully devel-
oped turbulent flow in smooth pipes [27;28]. The Sherwood number
represents the ratio of total mass transport to diffusion mass trans-
port (D). Sh can therefore be directly related to corrosion rates ([29])
and it can be expressed in terms of the mass transfer coefficient k
m
with Eq. (6.5):

m
Sh = k L (6.5)
D
where L is a characteristic length (m) describing the system. In the
case of a tube L is the internal diameter of that tube.
The acceleration of corrosion may sometimes be accompanied by
erosion of the underlying metal while in other cases erosion of the
base metal is not a factor [23]. The relative roles of corrosion and
erosion following damage to the protective film have been expressed
in the relationship based on measurable bulk flow [30] expressed as
Eq. (6.6):
Erosion rate + Corrosion rate ∝V n (6.6)

The exponent n in Eq. (6.6) depends on the relative contributions of
corrosion and erosion to the total metal loss as illustrated in Table 6.2.