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