Page 219 - Corrosion Engineering Principles and Practice

194 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 195

• Sheet cavitation is also known as fixed, attached, cavity or
pocket cavitation. Sheet cavitation is stable in a quasi-steady
sense. The liquid vapor interface becomes wavy and breaks
down in the closure region of the cavity. Downstream flow,
which contains large scale eddies, is dominated by bubble
clusters.
• Supercavitation is when the sheet cavity grows in such a way
it envelopes the whole solid body. Ventilation can be used to
create or to enhance a supercavity. Supercavitation is desir-
able to achieve viscous drag reduction on underwater vehi-
cles operating at high speeds. High speeds in excess of
1500 m/s are reported for supersonic operation of underwa-
ter projectiles.
• Vortex cavitation occurring on the tips of rotating blades is
known as tip vortex cavitation. Cavities form in the cores of
vortices in regions of high shear. This type of cavitation is not
restricted to rotating blades. It can also occur in the separation
zones of bluff bodies.

The cavitation number (s ) is a dimensionless number that
provides an estimate of cavitation tendency in a flowing stream as
described in Eq. (6.9):
2 (⋅ P − P )
s = v (6.9)
rV 2
where P is the liquid vapor pressure.
v
When the cavitation number is null, the pressure is reduced to
the vapor pressure and cavitation will occur. The cavitation number
and the net positive suction head (NPSH) are related according to
Eq. (6.10):

NPSH = (s + )1 V 2 (6.10)
2g

6.5 Mechanically Assisted Corrosion
Mechanical forces (e.g., tensile or compressive forces) will usually
have minimal effects on the general corrosion of metals and
compressive stresses even reduce the susceptibility of metals to crack.
In fact, shot peening is often used to reduce the susceptibility of
metallic materials to fatigue, stress corrosion cracking (SCC), and
other forms of cracking. However, a combination of tensile stresses
and a specific corrosive environment is one of the most important
causes of catastrophic cracking of metal structures.
SCC and other types of environmental cracking are also the most
insidious forms of corrosion because environmental cracks are