FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 2. Types of Corrosion | 6-21. Fatigue Corrosion
9/8/98
AC 43.13-1B
FIGURE 6-11. Galvanic corrosion of magnesium adjacent to steel fastener.
FIGURE 6-12. Stress corrosion cracking.
(1) Salt solutions and sea water cause
stress corrosion cracking of high-strength,
heat-treated steel and aluminum alloys.
(2) Methyl alcohol-hydrochloric acid
solutions will cause stress corrosion cracking
of some titanium alloys.
(3) Magnesium alloys may stress-
corrode in moist air.
(4) Stress Corrosion may be reduced by
• applying protective coatings,
• stress relief heat treatments,
• using corrosion inhibitors, or
• controlling the environment.
6-21. FATIGUE CORROSION. Fatigue
corrosion involves cyclic stress and a corrosive
environment. Metals may withstand cyclic
stress for an infinite number of cycles so long
as the stress is below the endurance limit of the
metal. Once the limit has been exceeded, the
metal will eventually crack and fail from metal
fatigue. However, when the part or structure
undergoing cyclic stress is also exposed to a
corrosive environment, the stress level for fail
ure may be reduced many times. Thus, failure
occurs at stress levels that can be dangerously
low depending on the number of cycles as
signed to the life-limited part.
a. Fatigue corrosion failure occurs in
two stages. During the first stage the combined
action of corrosion and cyclic stress damages
the metal by pitting and crack formations to
such a degree that fracture by cyclic stress will
occur, even if the corrosive environment is
completely removed.
b. The second stage is essentially a fa
tigue stage in which failure proceeds by propa
gation of the crack (often from a corrosion pit
or pits). It is controlled primarily by stress
concentration effects and the physical proper
ties of the metal. Fracture of a metal part due
to fatigue corrosion, generally occurs
Par 6-20
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