—Never attempt to take readings on operating
machinery.
We have to understand that even with the best tools,
it is the person behind the tool who makes things work.
ABFs can take measurements accurately and new
parts to be installed can be on hand. If the one who
finally assembles the pump does not know how to
torque a casing or pipe flange, he or she can destroy
all the hard work and money that have been put into
the job. For maintenance and repair on all equipment,
use the appropriate technical manuals.
CORROSION CONTROL
LEARNING OBJECTIVES: Describe the types
of corrosion the ABF will confront. Identify
their signs and explain the corrective action.
The most effective repair protection is prevention.
A thorough maintenance program continuously carried
out prevents most equipment failure. With higher
strength and closer tolerances being demanded of met-
als, equipment would rapidly become inoperable with-
out regular anticorrosion maintenance.
Corrosion endangers the equipment by reducing the
strength and changing the mechanical characteristics of
the metals used in its construction. Materials are de-
signed to carry certain loads and withstand given
stresses as well as to provide an extra margin of strength
for safety. Corrosion can weaken the structure, thereby
reducing or eliminating this safety factor.
Corrosion may take place over the entire surface of
a metal from chemical reaction with the surrounding
environment. It may be electrochemical in nature be-
tween two metallic materials or two points on the sur-
face of the same alloy, differing in chemical activity.
The presence of moisture is essential in both types of
attacks. The most familiar example of corrosion is rust
found on iron or steel.
All metals are affected to some extent by the atmos-
phere. Water and water vapor containing salt combine
with oxygen in the atmosphere and produce the main
source of corrosion. There are many forms of corrosion;
the form of corrosion depends upon the metal involved,
atmospheric conditions, and the corrosion-producing
agents present. For this discussion, we may consider
corrosion as three general types-surface, galvanic, and
intergranular corrosion.
SURFACE CORROSION
The effect of the atmosphere produces a corrosion
that appears on the surface of a metal as a general
roughening, etching, or pitting. Iron rust is the most
common example of surface corrosion.
Although aluminum, magnesium, and other nonfer-
rous metals do not rust, these metals are subject to
surface corrosion. Surface corrosion on unpainted alu-
minum alloy is evident as white or gray powdery depos-
its on the metal surface. The condition is first indicated
by the powdery residue deposited on the area of contact;
later pitting and searing appear on the aluminum sur-
face, and finally complete deterioration of the alumi-
num. Corrosion on painted aluminum-alloy surfaces
cannot be recognized by either the roughened surface
or by the powdery deposit. Instead, the paint or plating
appears to lift off the surface, indicated by a blistered
appearance and/or discoloration that results from the
pressure of the underlying accumulation of the corro-
sion products.
Surface corrosion on magnesium alloys can be rec-
ognized by powdered or roughened surfaces. Magne-
sium corrosion products are white and quite large
compared to the size of the base metal being corroded.
The deposits have a tendency to raise slightly, and the
corrosion spreads rapidly. When white, puffy areas are
discovered on magnesium, prompt attention is required
to prevent the corrosion from penetrating entirely
through the structure. This can occur in a very short
time.
It has been generally established that surface cor-
rosion is caused by moisture in the air. Since this type
of corrosion is visible, it can be detected in its early
stages by close visual inspection. Surface corrosion
can be prevented or retarded by protecting the metal
surface with a plating or paint and by keeping the
plating or paint in good condition.
GALVANIC CORROSION
Galvanic (or electrolytic) corrosion occurs when
two different metals are connected and exposed to an
electrolyte such as water, especially salt water. When
aluminum pieces are attached with steel bolts or screws,
galvanic corrosion may occur between the aluminum
and steel in the presence of moisture. An electrical
potential is setup, current flows between the two metals,
and an effect similar to that which occurs in batteries is
produced. Galvanic corrosion can usually be recog-
nized by the presence of a buildup of corrosion products
at the joint between two metals. Preventive measures
include painting and plating.
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