Page 75 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 53

drop erosion on helicopter blades can generate tensile stresses just below the surface
and lead to cracking (52).
The following measures are useful in reducing or preventing erosion–corrosion:
(i) a proper geometric design to get laminar flow with minimum turbulence, as in
large diameter pipelines to avoid abrupt changes or streamline bends; (ii) the com-
position of the metal or alloy with inherent resistance to corrosion and the ease with
which a protective film is formed, which is resistant to erosion–corrosion, plays an
important role. Addition of 2% aluminum to brass, 1.2% iron to cupronickel results
in a marked increase in resistance to impingement attack leading to corrosion. Con-
densation tubes made of brass-aluminum resist impingement because of the presence
of iron in the protective film, which arises from corrosion products in water. It is also
known that the addition of molybdenum to 18-8 stainless to form steel type 316 results
in high resistance to erosion–corrosion; (iii) toughness can have an influence on the
performance of materials subject to erosion–corrosion. Toughness is a good crite-
rion for resistance to the mechanical erosion or abrasion, but this is not applicable to
erosion–corrosion. Stellite (Co, Cr, W, Fe, C) alloy that has better toughness than 18-8
stainless steel showed better resistance to cavitation erosion on a water brake (2); (iv)
There are several process procedures to harden alloys. Hardening can be done by the
formation of a solid solution. Cold work can harden an alloy. For example, stainless
steel resists cavitation erosion; (v) deaeration and addition of inhibitors are useful in
reducing the aggressiveness of the environment. These methods are not economical.
The suspended solid particles may be removed by filtration, and the temperature may
be lowered; (vi) hard and tough coatings made of rubber and plastics may be used
(9); (vii) cathodic protection may help reduce the electrochemical attack.

1.7.18 Cavitation
This form of corrosion is caused by repeated nucleation, growth, and violent col-
lapse of vapor bubbles in a liquid against a metal surface. Cavitation erosion occurs
when a solid and fluid are in relative motion, and bubbles formed in the fluid become
unstable and implode against the surface of the solid. Cavitation erosion is similar
to surface wear fatigue and the appearance of sample metal is similar to a pitted
metal (9). Figure 1.17 shows the damage caused by cavitation erosion on a cylinder
of a diesel motor. The cavitation is similar to pitting, except that surfaces in the pits
are usually much rougher. On immediate observation, the surface affected region is
free of deposits and accumulated corrosion products. Cavitation corrosion involves
the destruction of the oxide layer formed by corrosion. When the mechanical effect
damages the metal, it is known as cavitation erosion. This type of cavitation erosion
damage is found in components such as ship propellers, runners of hydraulic turbines,
centrifugal pumps, pump impellers, and on surfaces in contact with high-velocity
liquids subject to variable pressure.

1.7.19 Cavitation Erosion
In certain conditions, a thin layer of liquid, nearly static at the metal–liquid interface,
can prevent impingement of the surface by the turbulent flow of the liquid. However,