01 March 2005
steel, magnesium, and aluminum surfaces and may
lead to more serious corrosion in some locations.
Filiform corrosion can be prevented by storing
equipment and aircraft in an environment with a relative
humidity below 65%, by using coating systems having
a low rate of diffusion for oxygen and water vapors, by
maintaining coatings in good condition (prompt touchup
around fasteners), and applying corrosion preventive
compounds (CPCs) when the coating is damaged.
3-9.8. EROSION CORROSION. Erosion corrosion is
the increase in the rate of attack on a metal due to the
action of a corrosive fluid against the metal surface.
Generally the movement is rapid, and wear or abrasion
occurs with the corrosion. Erosion corrosion is
characterized by grooves, gullies, waves, rounded holes
and/or valleys in the metal surface. Metals that are soft
(copper, lead) or metals that depend upon the
development of a protective surface film (aluminum,
stainless steel) are susceptible to erosion corrosion
damage. Equipment exposed to moving fluids (e.g.
heat exchanger tubing, pumps, propellers, impellers)
are also susceptible.
3-9.9. STRESS CORROSION. Also called stress
corrosion cracking (SCC). Stress corrosion
cracking of a metal caused by the combined effects of
constant tensile stress (internal or applied) and corrosion.
Internal or residual stresses may be produced by welding,
cold working, forming, and heat treatment operations
during the manufacture of a part. Stresses remain
concealed in the part unless stress relief operations
occur at stress levels far below the rated strength of a
are used. Other hidden stresses are induced in parts
metal, starting from what appears to be a very minor
when press or shrink fits are used and when slightly
corrosion pit. This type of failure can be catastrophic
mismatched parts are clamped together with rivets and
and occur without warning. Parts can completely sever
bolts. All these stresses add to those caused by applying
in a split second or they can crack slowly. The rate of
normal loads to parts in operation. Stress corrosion is
cracking and the stress limit is very unpredictable in
normally localized and appears in the form of cracks.
operating service. For example, 7075-T6 aluminum
During SCC, the metal is unattacked over most of its
alloy can fail by stress corrosion cracking when subjected
surface, while fine cracks progress through the interior
to a stress which is only 10% of its rated strength.
of the part. Cracking is generally perpendicular to the
applied stress. Metals have threshold stresses below
3-9.9.2. Causes. Specific environments have been
which stress corrosion cracking will not occur. This
identified which cause stress corrosion cracking of
threshold stress varies from metal to metal, is different
certain alloys. Salt solutions, sea water, and moist salt
for different tempers of the same metal, and is different
laden air may cause stress corrosion cracking of heat
for each of the three directions in which stress can be
treatable aluminum alloys, stainless steels, and some
applied. In aircraft, high strength steel parts (e. g.
titanium alloys. Magnesium alloys may stress corrode
landing gear) and high strength aluminum parts are
in moist air. Stress corrosion can be prevented by
particularly susceptible to stress corrosion.
placing an insulating barrier between the metal and the
corrosive environment, or by applying protective coatings
3-9.9.1. Associated Hazards. Stress corrosion cracking
and/or water displacing corrosion preventive
is an extremely dangerous type of failure because it can
compounds. Stress relief operations during fabrication