Avionics News June 2020 - 72

series
B Y

T H O M A S

I N M A N

Corrosion
D

uring the last several months in this Theory
& Practice column, we have introduced you
to several subject areas covered in ASTM
International's Aircraft Electronics Technician certification
exam. For those intending to earn an AET certificate, keep
in mind that the stories here are mere introductions into
each subject area. In addition to the electronic and electrical
subjects, the AET exam covers other areas such as tools,
safety and corrosion. This month's column will be the last
one directly related directly to the AET exam.
ASTM considers it important for technicians to be
familiar with corrosion, and for good reason. Over
the years, we have dealt with many corrosion-related
problems. An argument can be made that the majority
of avionics problems in the aircraft, and a significant
portion of problems inside radios, are caused by corrosion.
In Advisory Circular 43-206, the Federal Aviation
Administration lists 16 corrosion issues along with their
expected failure modes. The table is recreated, with
additions in italics, in Table 1. Understanding corrosion will
make anyone a better avionics technician.
For an introduction into corrosion and its control, I urge
readers to consult FAA AC 8083-30, Chapter 6, Aircraft
Cleaning and Corrosion Control. Some airframe and
powerplant Part 147 schools are using this AC as their
primary textbook. For continued study, have a look at AC
43-4B, Corrosion Control for Aircraft. Finally, for specifics
aimed at our facet of the industry, read AC 43-206,
Inspection, Prevention, Control, and Repair of Corrosion on
Avionics Equipment. All of these documents are available
via the FAA website. With at least three ACs addressing
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2020

corrosion, it's safe to say this is an important subject.
Corrosion is a deterioration of material due to a reaction
with the environment. Corrosion may be chemical or
electrochemical in nature. We can categorize corrosion
into different types, but we should keep in mind multiple
kinds of corrosion can happen simultaneously. Furthermore,
the corrosive process never takes a break. Once corrosion
begins, it will continue every hour of the day and every
day of the year. Corrosion can create problems as minor
as a little extra noise in a radio receiver, or as major as
a complete failure of a system. It does not take much
corrosion to create a serious problem.
An example of a potentially dangerous situation is
corrosion in the connector of a navigation indicator. The
voltages driving the electromechanical meter assembly
inside a course directing indicator or a horizontal situation
indicator are small, typically about + 150 mV. The current
flowing through the mechanism is even smaller, + 150 µA.
With such a small current flow, it takes little corrosion to
render the navigation indicator useless. This is one reason
why aircraft have two navigation systems. This is also a
good reason to upgrade to electronic instrumentation.
Types of corrosion can be divided into two broad
categories: direct chemical attack and electrochemical
attack. In either type, metal is transformed from its original
state into a compound such as oxide, hydroxide or sulfate.
If you look at lead acid battery theory, you'll find we take
advantage of corrosion in order to produce voltage. As
current flows, the internal plates of the battery turn from
lead to lead sulfate. In other words, the plates corrode.
Fortunately, within a lead acid battery, the corrosion



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