Environmental Engineering Reference
In-Depth Information
thickness at the vulnerable areas. Impingement attack can be avoided by smooth-
ing the bends in the piping systems. Increasing the diameter of the pipe will
ensure a laminar flow and less turbulence. Provision of baffles or barriers will
restrict the impact of fluid. Design should also provide for the easy replacement
of the affected parts. These are only a few examples. A well-regarded modifica-
tion of the design has in many instances proven to be the most economical solu-
tion to erosion corrosion problems.
Selection of Materials
As discussed in the previous section, inherently resistant materials may be used
to replace the frequently affected parts. However, galvanic corrosion must also
be considered if such replacement forms part of an integrated system involving
components made of more than one metal or alloy.
Alteration of Environment
Lowering of solution corrosivity through control of pH, inhibitor addition, and
lowering of temperature can have beneficial effects.
Cathodic Protection
Provision of sacrificial anodes as a component or additional attachment may be
effective in some cases.
3.8.4 Cavitation Damage
When the conditions of velocity are such that repetitive low- and high-pressure
areas are developed, bubbles form and collapse at the metal-liquid interface.
This phenomenon is called
cavitation
and the damage to the metal caused by it
is known as
cavitation erosion
or
cavitation damage
. A pressure drop develops
when a high-velocity liquid flows across a curved surface. A pressure drop below
the vapor pressure of the liquid causes local boiling and bubble formation. The
bubbles collapse as the bulk of liquid falls on them. The collapse of vapor bubbles
produces shock waves with high pressures sufficient to dislodge metal particles
or destroy the protective film. If the liquid is corrosive, rapid attack takes place
on the exposed metal until the protective film is restored. The sequence is shown
schematically in Fig. 3.30. The repetition of the process leads to a cluster of pit
formation resembling the appearance of a honeycomb. Eventually, perforation
may take place.
The trailing faces of ship propellers and pump impellers are particularly vul-
nerable to cavitation damage and many failure cases have been reported. Cavita-
tion damage is also encountered on rotors of pumps, water turbine blades, and
on the water-cooled side of diesel engine cylinders.
Cavitation damage can be minimized by the proper choice of materials. Tita-
nium alloys, austenitic and precitation-hardening stainless steels, nickel-chro-
