Corrosion endangers the gas turbine and its support
equipment by reducing the strength and changing the
structural characteristics of the materials used in their
construction. All such materials are designed to carry
certain loads and withstand given stresses and
temperatures, as well as to provide an extra margin of
strength for safety. Corrosion can weaken the structure,
thereby reducing or eliminating this safety factor.
Replacement or repair operations are costly, time
consuming, and restrict the usage of the equipment.
Corrosion in electronic and electrical components can
cause serious malfunctions. These malfunctions reduce
the effectiveness and reliability of the engineering plant
and can often completely destroy these components.
A thorough comprehension of the dangers of
corrosion and the ability to recognize and cope with the
various types of corrosion should be included in the
objectives of any maintenance training program. As a
work center supervisor, you may find that corrosion
prevention and control frequently turn out to be an
all-hands evolution. To some extent you can avoid this
situation through frequent inspections, effective use of
available manpower, and proper training of your
The problem of gas turbine engines and support
equipment protection is threefold: (1) prevention of
corrosion of the metal parts; (2) control of deterioration
of nonmetallic materials; and (3) elimination of physical
damage during replacement, repair, and maintenance.
Of the three basic problems, corrosion of metals is the
most difficult to control.
Metal corrosion is the deterioration of a metal.
When the metal is combined with oxygen, it forms
metallic oxides. This combining is a chemical process
that is essentially the reverse of the process of smelting
metal from ore. Very few metals occur in nature in the
pure state. For the most part, they occur as metallic
oxides. The refining process involves the extraction of
relatively pure metal from its ore and the addition of
other elements (both metallic and nonmetallic) to form
After refining, regardless of whether or not they are
alloyed, base metals possess a potential or tendency to
return to their natural state. However, this potential is
not enough in itself to initiate and promote this
There must also exist a corrosive
environment in which the significant element is oxygen.
It is the process of oxidation that causes metals to
It is a well-known fact that the tendency to corrode
varies widely between various metals. For example,
magnesium alloys are very difficult to protect and have
a very low corrosion resistance. Copper alloys have
relatively good corrosion resistance and are very easy
Corrosion may take place over the entire surface of
a metal by having a chemical reaction with the
surrounding environment. Or corrosion may be
electrochemical in nature between two different
metallic materials or two points on the surface of the
same alloy that differ in chemical activity. The presence
of some type of moisture is usually essential for
corrosion to exist.
Prevention and control of corrosion begins with an
understanding of the causes and nature of this
phenomenon. As stated earlier, corrosion is caused by
an electrochemical or a direct chemical reaction of a
metal with other elements. In the direct chemical attack,
the reaction is similar to that which occurs when acid is
applied to bare metal. Corrosion in its most familiar
form is a reaction between metal and water and is
electrochemical in nature.
In an electrochemical attack, metals of different
electrical potential are involved and they need not be in
direct contact. When one metal contains positively
charged ions and the other metal contains negatively
charged ions and an electrical conductor is bridged
between them, current will flow as in the discharge of a
dry-cell battery. In this type of reaction, the conductor
bridge may be any foreign material such as water, dirt,
grease, or any debris that is capable of acting as an
electrolyte. The presence of salt in any of the foregoing
media tends to accelerate the current flow and hence
speed the rate of corrosive attack.
Once the electrolyte has completed the circuit (fig.
2-32), the electron flow is established within the metal
in the direction of the negatively charged area (cathode).
The positively charged area (anode) is eventually
destroyed. All preventive measures taken with respect
to corrosion prevention and control are designed
primarily to avoid the establishment of an electrical
circuit. Or secondly, to remove electron flow as soon as
possible after its establishment before serious damage