01 March 2005
3-10.2. ANODIZED ALUMINUM. Some aluminum
following paragraphs discuss the corrosion
parts are protected with an electrochemically applied
characteristics of commonly used aircraft metals.
oxide coating called anodize. Aluminum oxide film on
aluminum is a naturally occurring protective film, and
3-10.1. ALUMINUM. Aluminum and aluminum alloys
anodizing merely increases the thickness of the oxide
are the most widely used materials for aircraft
film. When this coating is damaged in service, it can be
construction. In addition to its uses in aircraft structure,
only partially restored by chemical surface treatment.
aluminum and aluminum alloys are widely used in
Unnecessary destruction (e.g. nicks and scratches) of
equipment housings, chassis, mounting racks,
the anodized surface must be avoided.
supports, frames and electrical connector shells.
Aluminum is highly anodic, as evidenced by its position
3-10.3. MAGNESIUM. Magnesium alloys are the
in the galvanic series table. It is anodic to most other
lightest structural metals used for aircraft and missile
metals, and, when in contact with them, galvanic
airframes. Magnesium alloys are used extensively
corrosion of the aluminum will occur. Aluminum alloys
throughout avionic systems as antennas, structures,
are subject to pitting, intergranular corrosion, and
chassis, supports, and frames (radar). Magnesium is
stress corrosion cracking. In some cases, the corrosion
also used extensively for transmission and gearbox
products of a metal in contact with aluminum are
housings. These alloys are highly susceptible to
corrosive to aluminum. However, the formation of a
corrosion when the metal surface is exposed to the
tightly adhering oxide film offers increased resistance
environment without a protective finish. The corrosion
under mild corrosive conditions. The corrosion product
products are white powdery snow-like mounds (see
of aluminum is a white to gray powdery material
(aluminum oxide or hydroxide) which can be removed
slightly and the corrosion spreads rapidly. When the
by mechanical polishing or brushing with abrasives
white puffy areas are discovered, it requires prompt
treatment or the corrosion will penetrate entirely through
protect aluminum and its alloys against corrosion.
the structure. The natural oxide-carbonate film formed
Since pure aluminum is more corrosion resistant than
on magnesium alloys does not provide sufficient
most alloys, aluminum sheet stock is often covered
corrosion protection even in the mildest environment.
with a thin layer of nearly pure aluminum called cladding
The rate of corrosion of a magnesium alloy increases
or alclad. However, in a marine environment, all
when the alloy is immersed in water or periodically
aluminum surfaces require protection. Cladding is
subjected to moisture. Corrosion may also be
easily removed by harsh treatment with abrasives and
accelerated by dissimilar metal couples and when
tooling, exposing the more corrodible alloy surface.
Chemical conversion coating, paints, and corrosion
Corrosion of magnesium alloys can be greatly
preventive compounds are the main methods of
diminished by the use of the proper protective finish,
such as magnesium conversion coating and paint.
Some magnesium parts in current aircraft have been
originally protected by anodizing processes. Coatings