TO 1-1-700 - Robins Air Force Base
TO 1-1-700 - Robins Air Force Base
TO 1-1-700 - Robins Air Force Base
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<strong>TO</strong> 1-1-<strong>700</strong><br />
3.8.7 CRES/Stainless Steel. Basically, stainless steels or<br />
corrosion resistant steels (CRES) as they are more properly<br />
described, are alloys of iron with chromium and nickel. Many<br />
other elements, such as sulfur, molybdenum, vanadium,<br />
cobalt, columbium, titanium, and aluminum are added in various<br />
amounts and combinations to develop special characteristics.<br />
Stainless (CRES) steels are much more resistant to<br />
common rusting, chemical action, and high temperature oxidation<br />
than ordinary steels, due to the formation of an invisible<br />
oxide film or passive layer on the surface of these alloys.<br />
Corrosion and heat resistance are the major factors in selecting<br />
stainless (CRES) steels for a specific application. However, it<br />
should be well understood that stainless (CRES) steels are not<br />
the cure-all for all corrosion problems due to service conditions<br />
which can destroy the oxide film on their surfaces. Stainless<br />
(CRES) steels are highly susceptible to crevice/<br />
concentration cell corrosion in moist, salt laden environments<br />
and can cause galvanic corrosion of almost any other metal<br />
with which they are in contact if proper techniques of sealing<br />
and protective coating are ignored. Stainless (CRES) steels<br />
may be magnetic or non-magnetic. The magnetic steels are<br />
identified by numbers in the American Iron and Steel Institute<br />
(AISI) 400-series, such as 410, 430, etc. These steels are not<br />
as corrosion resistant as the non-magnetic steels which are<br />
identified by numbers in the AISI 300-series, such as 304,<br />
316, etc. The AISI 300-series steels have nickel contents ranging<br />
from 6% to 22%, while the 400-series steels have nickel<br />
contents of only 2%.<br />
3.8.8 Nickel and Chromium. Nickel and chromium are<br />
used as protective platings. Chromium plating is also used to<br />
provide a smooth, wear-resistant surface and to reclaim worn<br />
parts. Where corrosion resistance in a marine environment is<br />
required, a nickel under-coat is used. The degree of protection<br />
is dependent upon plating thickness. Both of these metals form<br />
continuous oxide coatings that can be polished to a high luster<br />
and still protect not only themselves but also any underlying<br />
metal. Chromium platings contain micro-cracks, and corrosion/rust<br />
originates on the base metal below these separations<br />
and spalls the plating from the surface. Figure 3-22 shows the<br />
results of a failed chromium plate.<br />
3.8.9 Silver, Platinum, and Gold. These metals do not corrode<br />
in the ordinary sense, although silver tarnishes in the<br />
presence of sulfur. The tarnish is a brown-to-black film. Gold<br />
tarnish is not really corrosion but is a very thin layer of soils or<br />
contaminants that shows up as a darkening of the reflecting<br />
surfaces. Silver and gold are used extensively in C-E-M equipment<br />
because of their high degree of conductivity and solderability.<br />
All these metals are highly cathodic to almost all other<br />
metals and can cause severe galvanic corrosion of almost any<br />
metal with which they are in contact in the presence of moisture<br />
if joint areas are not sealed or otherwise insulated.<br />
3.8.10 Graphite (Carbon) Fiber/Epoxy Composites and<br />
Fiberglass Materials. Graphite or carbon fiber/epoxy composites<br />
are materials consisting of reinforcing fibers in a<br />
matrix of an organic epoxy resin. They are an important class<br />
of materials because of their high strength-to-weight ratios and<br />
high stiffness. Since carbon is the least active metal in the<br />
galvanic series, it will accelerate the corrosion of any metal to<br />
which it is coupled; so insulation, usually with sealants,<br />
between graphite/epoxy composites and other metals is an<br />
absolute necessity to prevent dissimilar metal attack on the<br />
attached part. Graphite/epoxy composites are not frequently<br />
encountered in C-E-M and associated equipment, but fiberglass<br />
materials are used extensively as the foundation of<br />
printed circuit/wiring boards and skin panels of vans and shelters.<br />
Fiberglass materials consist of a mat of cut glass fibers or<br />
a woven mesh of long glass fibers in a matrix of either an<br />
epoxy or a polyester resin. Skin panels of some vans and shelters<br />
may be fabricated from Kevlar® fiber mesh in an epoxy<br />
resin matrix. The fiberglass and Kevlar® materials present no<br />
galvanic couple problems, but the joint areas between these<br />
types of panels and their metal support frames or structures<br />
should be faying surface sealed to prevent fluid intrusion that<br />
can lead to corrosion of the metal components.<br />
3.9 CORROSIVE ENVIRONMENTS.<br />
Corrosion of C-E-M and associated equipment is caused by<br />
both natural and man-made environments. Natural conditions,<br />
which affect the corrosion process, are moisture, temperature,<br />
salt atmospheres, ozone, sand, dust, solar radiation, insects and<br />
birds, and microorganisms. Man-made conditions, which also<br />
affect the corrosion process, are industrial pollution, manufacturing<br />
operations, storage conditions, and shipment. By understanding<br />
these conditions, maintenance personnel will be<br />
better able to prevent damage by corrosion.<br />
3.9.1 Moisture. Moisture is present in air as a gas (water<br />
vapor) or as finely divided droplets of liquid (mist or fog) and<br />
often contains contaminants such as chlorides, sulfates, and<br />
nitrates, which increase its corrosive effects. Moisture enters<br />
all areas of equipment that air can enter. All enclosed areas<br />
that are not sealed allow air to enter and leave as the pressure<br />
between inside and outside changes. These pressure changes<br />
occur when the atmospheric pressure changes and when the air<br />
temperature inside or outside of an enclosed area changes.<br />
Moisture condenses out of air when the air becomes too cool<br />
to hold all of the moisture in it. The dew found on equipment<br />
exterior surfaces, and many times on their interior surfaces,<br />
after a cool night is the result of condensation.<br />
3.9.2 Condensed Moisture. Condensed moisture will usually<br />
evaporate as surrounding air warms; but its dissolved contaminants,<br />
including salts, will be left behind as residues or<br />
deposits on the surfaces. This can result in the build-up of<br />
soils and salt contamination. Condensed moisture and its con-<br />
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