01 March 2005
NAVAIR 01-1A-509-1
TM 1-1500-344-23-1
TO 1-1-689-1
of parts will help, because it lowers the internal stress
level of the part. Shot peening a metal increases
resistance to stress corrosion cracking by creating
compressive stresses on the surface, which must be
overcome by an applied tensile stress before the
surface sees any tension load. Changing the alloy (for
example, replacing an aluminum 7075-T6 part with
one made from 7075-T73 or T76 alloy) can greatly
3-9.10. CORROSION FATIGUE. Corrosion fatigue
the combined effects of cyclic stress and corrosion. No
metal is immune to reduction in its resistance to cyclic
stress if the metal is in a corrosive environment. Corrosion
damages the metal by pitting, followed by crack formation
in the pitted area due to cyclic stress. The crack is
propagated predominantly in the fatigue mode, in which
occurs. Moisture does not appear to increase the
the rate of cracking is controlled by the stress
corrosion; in fact, it tends to slow down the reaction.
concentration in the main cross section, the physical
Fretting corrosion is normally encountered in heavily
properties of the metal, and the presence of corrosion
loaded static joints which are subject to vibration, such
products on the crack face. Fracture of a metal part due
as landing gear component attachment areas having
to corrosion fatigue occurs when the remaining cross-
lug holes with slight press fits, slip fit bushings with
sectional area is unable to carry the applied loads. Like
very close tolerance bolts passing through the bushings,
stress corrosion, corrosion fatigue is normally localized
wing root access panels or wing-to-body fairings, and
and appears in the form of cracks. The metal is generally
engine blade roots. Practical means of reducing fretting
unattacked over most of its surface, while the crack
corrosion include reducing the amount of relative motion
progresses through the part. Cracking is generally
at the surface, adding a lubricant at the interface to
perpendicular to the applied stress. Protection of all
reduce friction and seal out oxygen, increasing the
parts subject to cyclic stress is particularly important,
surface hardness of the part, and increasing the overall
even in environments that are only mildly corrosive.
hardness of one or both contacting metals.
Preventive measures include reducing the stress on the
part, using corrosion inhibitors, and applying a metallic
3-9.12. H O T C O R R O S I O N . A l s o c a l l e d h i g h
coating (e.g. chromium, cadmium, or ion vapor deposition
temperature oxidation. Corrosion in the absence of
(IVD) aluminum) to the part.
water can occur at high temperatures, such as those
found in turbine engine combustors, turbine sections,
3-9.11. FRETTING CORROSION. Fretting corrosion
and afterburners. When hot enough, metals can react
occurs at contact areas between materials under load
directly with the surrounding gases, producing oxide
subject to repeated vibration. The relative motion needed
to produce fretting is extremely small (sometimes as
such as chlorides and sulfates, can accelerate the hot
little as 10-8 cm). The corrosion products increase the
corrosion reaction by reducing the melting point of the
wear of the surface, and the wear exposes more bare
metallic oxide and promoting its vaporization. High
metal surface to be corroded. The overall effect is
temperature ceramic coatings can reduce this type of
greater than the single effects of corrosion and wear
corrosion but are usually applied only by the
added together. Fretting has the general appearance of
manufacturer due to the highly specialized equipment
galling, in which chunks of metal are torn from the
required for application.
surface with corrosion at the torn areas or ragged pits
place on any metal, aluminum, stainless steel, and
metal has perfect environmental integrity and is totally
titanium alloys are most susceptible. These metals
resistant to corrosion. As a result, all metals will corrode
depend on an oxide surface film to inhibit further
sooner or later. The characteristics of corrosion on
corrosion. With rapid movement under pressure at the
interface, the oxides are removed and rapid oxidation