Avionics News June 2020 - 74

Figure 2: Surface corrosion

THEORY & PRACTICE
Continued from page 73

outgassing from plastic assemblies and high-humidity
environments.
Electrochemical attack requires a medium, which most
of the time is water. The water will have the capability of
conducting a small amount of electricity so current will
flow. Like in a battery, the metal will oxidize as the current
flows. You may be surprised to learn that metals will have
different electrical potentials within different areas of the
same piece. One reason for this is we use metal alloys. An
alloy is a metal created from the combination of two or
more other metals. Each different type of metal has its own
natural electrical potential. As a result, an alloy has a builtin mechanism to create voltage.

Figure 1: Electrochemical attack

One area of the metal will carry a negative charge
in relation to the other area. The negative area will be
considered cathodic (think of the cathode of a diode); the
less negative area will be considered anodic (think of the
anode of a diode). Add water to the surface, which acts as
an electrolyte, and current will begin to flow. As the current
flows, the metal will oxidize. Figure 1 gives an illustration
of electrochemical attack along with a common form of
protection, which is paint. Keep in mind, these cathodic
and anodic areas will exist regardless of whether there is a
coat of paint or not. As a result, electrons will move around
within the metal; however, a covering will keep oxygen
74

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2020

away. Without oxygen, metal cannot oxidize.
One proof of your familiarity of corrosion is the ability to
recognize common forms of corrosion. Let's review some here.
Surface corrosion looks like an overall roughening,
etching or pitting of the metal surface. Often, the technician
will also notice powder, which is produced by the corrosive
process. Surface corrosion, shown in Figures 2 and 3, can
be caused by either direct chemical or electrical chemical
attack. This type of corrosion may be hidden underneath
paint, which will loosen. A variation of this is filiform
corrosion, which is the result of improper preparation of the
surface to be painted. Often, filiform corrosion will show up
quickly as the paint dries.
Dissimilar metal corrosion is self-explanatory, and
another name for this is galvanic corrosion. The corrosion
results from two different types of metals being in contact
with each other. An obvious example is a steel fastener
being used with aluminum materials. One metal becomes
anodic, and the other cathodic. Less obvious is when
someone uses steel wool or a wire brush to clean aluminum.
Microscopic bits of the steel will be left behind, starting the
dissimilar corrosion process. Figure 4 shows an example of
dissimilar metal corrosion.
Intergranular corrosion occurs along the grain
boundaries of alloys. Since aluminum alloy is used in

Figure 3: Filiform corrosion



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