TO 35-1-3 - Robins Air Force Base

TO 35-1-3
1.8 METALS AFFECTED BY CORROSION.
The characteristics of corrosion on SE metals are summarized
in Table 1-2. The following is a discussion of corrosion characteristics
of commonly used metals.
1.8.1 Magnesium. Magnesium alloys are the lightest structural
metals used for SE construction. These alloys are highly
susceptible to corrosion, which appears as white, powdery
mounds or spots when the metal surface is exposed to the
environment without a protective finish. The normal oxidecarbonate
film formed on magnesium alloys does not provide
sufficient corrosion protection even in the mildest environment.
The rate of corrosion of a magnesium alloy increases
when the alloy is immersed in water or periodically subjected
to moisture. Corrosion may also be accelerated by dissimilar
metal couples and when conductive contaminants are dissolved
in water. Corrosion of magnesium alloys can be greatly
diminished by the use of the proper protective finish. Some
magnesium parts in use were originally protected by anodizing
processes, such as HAE and DOW 17 coatings. The HAE
process can be identified by the brown to mottled gray appearance
of the unpainted surface. DOW 17 coatings have a green
to grayish-green color. Electrolytic coatings are thicker than
those applied by immersion or brushing. Electrolytic finishes
cannot be restored in the field. Take care to minimize removal
of these coatings. At the field level, corrosion removal and
surface pretreatment on magnesium parts should be referred to
the Aircraft Structural Maintenance Technicians for repair of
the conversion coatings.
1.8.2 Steel. Ferrous (iron) alloys are used to manufacture
many components and assemblies in SE and other equipment
such as frames and bodies of trailers, and lesser structural
parts such as brackets, racks, and panels. If unprotected, ferrous
alloy surfaces, (with the exception of stainless steels also
known as corrosion resistant steel (CRES)), corrode easily in
the presence of moisture. Ferrous alloy surfaces are normally
painted or plated to prevent corrosion. Corrosion of steel is
easily recognized because the corrosion product is red or black
color. When ferrous alloys corrode, a dark corrosion product
usually forms first. When moisture is present, this coating is
converted to the common red or black corrosion product.
1.8.3 Aluminum. Aluminum and aluminum alloys are
widely used for SE construction. Aluminum is highly anodic,
as evidenced by its position in the galvanic series. However,
the formation of a tightly adhering oxide film offers increased
resistance under mild corrosive conditions. Some aluminum
parts are protected with an electrochemically applied anodized
coating. Aluminum oxide film on aluminum is a naturally
occurring protective film; anodizing increases the thickness of
the oxide film. When this coating is damaged, it can only be
partially restored by chemical surface treatment. Avoid unnecessary
destruction of the anodized surface layer during the
processing of anodized aluminum.
1.8.3.1 The corrosion product of aluminum is a white to gray
powdery material (aluminum oxide or hydroxide), which can
be removed by mechanical polishing or brushing with an abrasive.
Aluminum is anodic to most other metals, and, when in
contact with them, galvanic corrosion of the aluminum will
occur. Aluminum alloys are subject to pitting, intergranular
corrosion, and stress corrosion cracking. In some cases, the
corrosion products of a metal in contact with aluminum are
corrosive to aluminum. Therefore, it is necessary to clean and
protect aluminum and its alloys against corrosion.
1.8.3.2 Because pure aluminum is more corrosion-resistant
than most alloys, aluminum sheet stock is often covered with a
thin layer of nearly pure aluminum called cladding. Cladding
is often removed by harsh treatment with abrasives and tooling,
exposing a more corrodible surface. In such areas, paints
and CPCs are especially important. However, in environments
where SE is exposed to salt-laden atmosphere or significant
industrial pollutants, all bare aluminum surfaces require protection.
1.8.4 Copper and Copper Alloys. Copper and copper
alloys are quite corrosion-resistant, with corrosion usually limited
to staining and tarnish. Generally, such changes in surface
conditions are not dangerous and usually do not affect the part.
Copper corrosion is evidenced by the accumulation of blue or
blue-green corrosion products. Protective paint coatings are
seldom required because of the inherent resistance of the
metal. However, paint finishes may be applied for appearance
purposes. The green patina is merely a thin coating consisting
mainly of basic copper sulfate and perhaps some hydrated
copper carbonate. The patina in the thin, firmly adhering state
actually offers increased corrosion protection to the base
metal, but the patina in a rough or frosted state should be
removed. When coupled with most metals used in SE construction,
copper is the less active metal and greatly accelerates
the corrosion of other metals. This is especially true in
copper/aluminum couples. Examples are usually found in
electrical components and in areas where copper bonding
strips or wires are fastened to an aluminum chassis or structural
components.
1.8.5 Stainless Steel. Stainless steel or corrosion-resistant
steel (CRES), as they are more properly described, are alloys
of iron with chromium. Many other elements such as nickel,
sulfur, molybdenum, vanadium, cobalt, columbium, titanium,
and aluminum are added in various amounts and combinations
to develop special characteristics. Stainless steels are much
more resistant to common rusting, chemical actions, and hightemperature
oxidation than ordinary steels because of the formation
of an invisible oxide film or passive layer on the surface
of these alloys. Corrosion and heat-resistance are the
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