TB 43-0213
CHAPTER 2
CORROSION PREVENTION AND CONTROL
Section I. RUSTING THE ELECTROCHEMICAL REACTION
2-1.
What is Rust?
presence of a moisture film, which serves as a conduc-
tor, iron atoms in the purer areas of the metal create a
positive electric pole while iron atoms in adjacent areas
a. The flaky metal deposit known as rust is, in fact,
with more impurities become the opposite pole, a nega-
metal returning to a more natural state. Metals such as
tive area. The result is similar to a simple electric bat-
iron or steel are not natural elements found in the
tery with moisture serving as the electrolyte, and the
earth's crust. Rather, they are man-made by-products
stronger-weaker iron atoms becoming the positive and
resulting from the melting of ore in high temperature
negative terminals of the circuit.
furnaces. This melting separates the iron particles from
the ore. Once separated, the iron particles flow directly
f. In the rusting process, iron atoms in the weaker,
from the furnace into molds that shape them. The
negative areas will split apart. These atoms give up or
cooling and hardening of the iron begins immediately.
give off some of their electrons, which are subatomic
What we call "steel" is produced in the same manner.
particles found in all atoms. These lost electrons
Steel is iron mixed with other molten alloys during the
migrate through the metal toward the positive area. As a
furnace refining process.
result of this loss, electron deficient iron atoms in the
negative area gravitate toward the metal surface and
b. The process described above requires the use of a
dissolve in the film of moisture. This half of the
large amount of energy in order to separate the iron
"Oxidation-Reduction Reaction", or rusting process, is
particles from their ore and shape the molten metal into
referred to as the "oxidation half". This half weakens
the desired form. As a result, these metals are in a high
the metal in all negative areas, but it does not cause
energy state after cooling (like a tightly wound spring
rust. In order for actual rust to form, the second half of
forever straining to unwind), and would naturally like to
the "oxidation-reduction reaction" must complete the
return to a low energy state. This straining to return to a
cycle.
low energy state is the driving force behind corrosion.
Rusting, therefore, is nature's way of returning metals
to their natural state.
g. The "reduction half" of the rusting process re-
turns to the lost electrons migrating through the metal
toward the positive area. These free electrons are
c. Rust is iron hydroxide. It is iron transformed by a
picked up by oxygen atoms in the water film on the sur-
natural process. This process is caused by the attraction
face of the metal. On this surface, the products of the
of negatively charged electrons in the atoms of iron
two reactions then combine to from a flaky, crusty third
toward oxygen in water. Corrosion prevention
technicians today refer to this natural process as the
substance called iron hydroxide or rust. Stated another
"Oxidation-Reduction Reaction" (see Figure 2-1).
way, the dissolved electron deficient iron atoms that
gravitated to the wet surface during the oxidation half of
the process have now combined with electron charged
d. The "Oxidation-Reduction Reaction" begins with
oxygen atoms produced by the reduction half of the
water on a metal surface as a result of exposure of the
process. The result of all this is a transformation of iron
metal to rain, snow, dew, or humidity. This moisture
atoms into iron hydroxide or rust.
forms a film on the metal. Beneath this film is the metal
itself which, as noted above, is a formed, man-made
construction of earth minerals that have been stressed
h. The rusting process described above actually
into a shape that better suits mankind's needs. That
begins the moment moisture comes into contact with
which was once an aggregate of iron ore in the earth's
bare metal. This process is actually occurring at the
crust has now been molted from its ore and molded into
atomic level; the results are not instantly apparent.
a fender, stake pocket, or cargo bed.
While salt and dirt particles added to the moisture will
accelerate the process, it still takes weeks, months, or
e. The smelting process that makes iron is imperfect
years (depending upon the thickness of the metal) for
and the resultant metal itself always contains impurities,
the process to transform the iron or steel to an
some areas of the metal more so than others. In the
unserviceable state.
2-1
