TO 35-1-3 - Robins Air Force Base

TO 35-1-3
Figure 1-11. Illustration of a Typical Concentration Corrosion Cell
1.7.7 Stress Corrosion Cracking. Stress corrosion cracking
(SCC) is the intergranular cracking of a metal caused by
the combined effects of constant tensile stress (internal or
applied) and corrosion. Internal or residual stresses are produced
by cold working, forming, and heat treatment operations
during manufacture of a part and remain concealed in the
part unless stress relief operations are used. Other hidden
stresses are induced in parts when press or shrink fits are used
and when slightly mismatched parts are clamped together with
rivets and bolts. All these stresses add to those caused by
applying normal loads to parts in operation. Metals have
threshold stresses below which stress corrosion cracking will
not occur. This threshold stress varies from metal to metal and
depends on the characteristics of the stress that is applied. The
following conditions must be present for SCC to occur. The
component or structure must be under a tensile stress. This
tensile stress may be provided by an externally applied service
load or a residual stress resulting from manufacturing procedures
such as rolling, punching, deep drawing, or welding.
The material must also be exposed to an environment that
causes SCC. Whereas all metals will form stress corrosion
cracks in some environment under the proper conditions, there
is no one environment that will cause SCC in all metals. SCC
is most prevalent and of the most concern in high strength
steels, stainless steels (mostly in the austenitic group), high
strength aluminum alloys (2000 and 7000 series), copperbased
alloys, and titanium alloys.
1.7.8 Hydrogen Embrittlement. Hydrogen embrittlement is
the weakening of materials such as high-strength steel (typically
180 Ksi and above), some high-strength aluminum, titanium,
and some types of stainless steels when they are
exposed to acidic materials such as acid paint removers, acidic
metal pretreatments and cleaners, plating solutions, and some
alkaline materials. This occurs when the materials causes a
cathodic reaction on the metal surface that produces hydrogen.
The hydrogen diffuses into the bulk metal, accumulating at
grain boundaries and weldments weakening the structure. If
the part is under load or contains residual manufacturing
stresses, sudden catastrophic failure occurs when the part can
no longer sustain the internal and/or applied stresses. Hydrogen
embrittlement has been known to occur in parts stressed to
only 15% of nominal tensile strength.
1.7.9 Corrosion Fatigue. Corrosion fatigue is the cracking
of metals caused by the combined effects of cyclic stress and
corrosion and is very similar to stress corrosion cracking. If it
is in a corrosive environment, no metal is immune to some
reduction in resistance to cyclic stressing. Corrosion fatigue
failure occurs in two stages. During the first stage, the combined
action of corrosion and cyclic stress damages the metal
by pitting and forming cracks in the pitted area. The second
stage is the continuation of crack propagation, in which the
rate of cracking is controlled by. In simplified terms, corrosion
fatigue is mechanical fatigue aggravated by a corrosive environment.
In corrosion fatigue, the corrosive environment
causes a lowering or reduction of the fatigue limit (the ability
of a metal to resist fatigue cracking) of a metal as it undergoes
cycles of stress. In the absence of a corrosive environment,
this same metal would be able to withstand significantly more
cycles of stress before cracking. Corrosion fatigue seems to be
most prevalent in environments that cause pitting corrosion.
1.7.10 Filiform Corrosion. Filiform corrosion is a special
form of oxygen concentration cell corrosion (or crevice corrosion)
that occurs on metal surfaces having an organic coating
system. It is recognizable by its characteristic wormlike trace
of corrosion products beneath the paint film (see Figure 1-12).
Filiform occurs when the relative humidity of the air is
between 78% and 90% and when the surface is slightly acidic.
It starts at breaks in the coating system (such as scratches and
cracks around fasteners and seams) and proceeds underneath
the coating because of the diffusion of water vapor and oxygen
from the air through the coating. Filiform corrosion can attack
steel, magnesium, and aluminum surfaces and may lead to
more serious corrosion in some locations. Filiform corrosion
can be prevented by: storing equipment in an environment
with a relative humidity below 70%; using coating systems
with a low rate of diffusion for oxygen and water vapors;
maintaining coatings in good conditions; and washing equipment
to remove acidic contaminants from the surface (such as
those created by air pollutants). Filiform corrosion most often
occurs in humid environments. Once the humidity drops
below 65%, Filiform corrosion stops. When the humidity rises
above 95%, blisters form rather than filaments. Filiform corrosion
forms mostly on steel, aluminum, magnesium, and zinc.
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