Chemical Treatments
Anodizing
Anodizing is the most common surface treatment of nonclad aluminum alloy
surfaces. It is typically done in specialized facilities in accordance with Mil-C-
5541E or AMS-C-5541. The aluminum alloy sheet or casting is the positive pole in
an electrolytic bath in which chromic acid or other oxidizing agent produces an
aluminum oxide film on the metal surface.
Aluminum oxide is naturally protective, and anodizing merely increases the
thickness and density of the natural oxide film. When this coating is damaged in\
service, it can only be partially restored by chemical surface treatments.
Therefore, when an anodized surface is cleaned including corrosion removal, the
technician should avoid unnecessary destruction of the oxide film.
The anodized coating provides excellent resistance to corrosion. The coating is
soft and easily scratched, making it necessary to use extreme caution when
handling it prior to coating it with primer.
Aluminum wool, nylon webbing impregnated with aluminum oxide abrasive, fine
grade nonwoven abrasive pads or fiber bristle brushes are the approved tools for
cleaning anodized surfaces. The use of steel wool, steel wire brushes, or harsh
abrasive materials on any aluminum surfaces is prohibited. Producing a buffed or
wire brush finish by any means is also prohibited Otherwise, anodized surfaces
are treated in much the same manner as other aluminum finishes.
In addition to its corrosion resistant qualities, the anodic coating is also an
excellent bond for paint. In most cases, parts are primed and painted as soon as
possible after anodizing. The anodic coating is a poor conductor of electricity;
therefore, if parts require bonding, the coating is removed where the bonding
wire is to be attached. Alclad surfaces that are to be left unpainted require no
anodic treatment; however, if the Alclad surface is to be painted, it is usually
anodized to provide a bond for the paint.
Alodizing
Alodizing is a simple chemical treatment for all aluminum alloys to increase
their corrosion resistance and to improve their paint bonding qualities.
Because of its simplicity, it is rapidly replacing anodizing in aircraft work.
The process consists of precleaning with an acidic or alkaline metal cleaner
that is applied by either dipping or spraying. The parts are then rinsed with
fresh water under pressure for 10 to 15 seconds. After thorough rinsing,
Alodine® is applied by dipping, spraying, or brushing. A thin, hard coating
results which ranges in color from light, bluish green with a slight iridescence
on copper free alloys to an olive green on copper bearing alloys.
The Alodine is first rinsed with clear cold or warm water for a period of 15 to
30 seconds. An\ additional 10 to 15 second rinse is then given in a
Deoxylyte® bath. This bath is to counteract alkaline material and to make the
alodined aluminum surface slightly acid on drying.
Aircraft Cleaning
Cleaning an aircraft and keeping it clean are extremely important. From an
aircraft maintenance technician’s viewpoint, it should be considered a regular
part of aircraft maintenance. Keeping the aircraft clean can mean more
accurate inspection results, and may even allow a flight crewmember to spot
an impending component failure.
There are many different kinds of cleaning agents approved for use in
cleaning aircraft. It is impractical to cover each of the various types of cleaning
agents since their use varies under different conditions, such as the type of
material to be removed, the aircraft finish, and whether the cleaning is
internal or external.
In general, the types of cleaning agents used on aircraft are solvents, emulsion
cleaners, soaps, and synthetic detergents. Their use must be in accordance
with the applicable maintenance manual. The types of cleaning agents named
above are also classed as light or heavy duty cleaners.
The soap and synthetic detergent type cleaners are used for light duty
cleaning, while the solvent and emulsion type cleaners are
used for heavy duty cleaning. The light duty cleaners, which are nontoxic and
nonflammable, should be used whenever possible. As mentioned previously,
cleaners that can be effectively rinsed and neutralized must be used, or an
alkaline cleaner may cause corrosion within the lap joints of riveted or spot-
welded sheet metal components.
Exterior Cleaning
There are three methods of cleaning the aircraft exterior:
(1) wet wash, (2) dry wash, and (3) polishing.
polishing and mechanical polishing. The type and extent of soiling and the
final desired appearance determine the cleaning method to be used.
Wet wash removes oil, grease, or carbon deposits and most soils, with the
exception of corrosion and oxide films. The cleaning compounds used are
usually applied by spray or mop, after which high pressure running water is
used as a rinse. Either alkaline or emulsion cleaners can be used in the wet
wash method.
Dry wash is used to remove airport film, dust, and small accumulations of dirt
and soil when the use of liquids is neither desirable nor practical. This method
is not suitable for removing heavy deposits of carbon, grease, or oil, especially
in the engine exhaust areas. Dry wash materials are applied with spray, mops,
or cloths, and removed by dry mopping or wiping with clean, dry cloths.
Polishing restores the luster to painted and unpainted surfaces of the aircraft,
and is usually performed after the surfaces have been cleaned. Polishing is
also used to remove oxidation and corrosion. Polishing materials are available
in various forms and degrees of abrasiveness. It is important that the aircraft
manufacturer’s instructions be used in specific applications
Rinse with fresh water and dry with a chamois, synthetic wipes designed for
use on plastic windshields, or absorbent cotton. In view of the soft surface, do
not rub plastic with a dry cloth since this is not only likely to cause scratches,
but it also builds up an electrostatic charge that attracts dust particles to the
surface.
The charge, as well as the dust, may be removed by patting or gently blotting
with a clean, damp chamois. Do not use scouring powder or other material
that can mar the plastic surface. Remove oil and grease by rubbing gently with
a cloth wet with soap and water. Do not use acetone, benzene, carbon
tetrachloride, lacquer thinners, window cleaning sprays, gasoline, fire
extinguisher or deicer fluid on plastics because they soften the plastic and
will cause crazing. Finish cleaning the plastic by coating with a plastic polish
intended for aircraft windows and windshields.
These polishes can minimize small surface scratches and will also help keep
static charges from building up on the surface of the windows. Surface oil,
hydraulic fluid, grease, or fuel can be removed from aircraft tires by washing
with a mild soap solution. After cleaning, lubricate all grease fittings,
hinges, and so forth, where removal, contamination, or dilution of the grease
is suspected during
washing of the aircraft.
