Corrosive Agents Mercury

 Hazardous cargo are often carried in

 Corrosive agents that are capable of
aircraft, and damage from a shifting
causing a corrosive reaction. Most
load can result in damaged containers
corrosive agents fall into of two
and hazardous spills
categories, acids, and alkalis. However,
 Mercury attacks aluminum by a
care must be taken not to overlook
chemical reaction known as
other less obvious corrosive agents such
Amalgamation. In this process, the
as the atmosphere which contains
mercury attacks along the grain
moisture, salts, or corrosive industrial
boundaries within the alloy, and in a
agents.
very short time completely destroys it.
Different types of Corrosive Agents
 Extreme care must be exercised when
Acids and Alkalis
removing spilled mercury, as it is
 Almost all acids and alkalis form
"slippery" and flows through tiny cracks
effective electrolytes as they react with
to get to the lowest part of the
metals to form metallic salts, but some
structure where it causes extensive
electrolytes are more active than
damage.
others. For example, the sulfuric acid
 its vapors are poisonous to humans and
found in aircraft batteries is especially
precautions must be taken to avoid
active in corroding aluminum. However,
exposure to it
a weak solution of chromic or
 If mercury is spilled, remove every
phosphoric acid is often used as a
particle with a vacuum cleaner having a
surface treatment to prepare metal for
mercury trap in the suction line, or with
painting
a rubber suction bulb or medicine
 Ferrous metals are subject to damage
dropper. Never attempt to remove
from both acids and alkalis, but
mercury by blowing it with compressed
aluminum is more vulnerable to strong
air. This only scatters it and spreads the
alkaline solutions than it is to acids.
damage.
Salt
Water
 It is very important to remember that
 Pure water reacts with metals to form
many compounds other than sodium
corrosion or oxidation, but water
chloride fall into the category of salts. In
holding a concentration of salts or other
general, salts are the result of a metallic
contaminants causes much more rapid
element combining with a nonmetal.
corrosion Pure water reacts with metals
The resulting compound is almost
to form corrosion or oxidation, but
always a good electrolyte, and can
water holding a concentration of salts
promote corrosive attack
or other contaminants causes much
 Magnesium is particularly vulnerable to
more rapid corrosion
corrosive attack from an electrolyte
 Seaplanes are in a continual battle with
formed by salt solutions.
the elements, and every precaution
 must be taken to stay ahead of water from freezing. Should entrained
corrosion formation. water freeze, the resulting slush could plug
 Seaplanes operating in salt water are the fuel screens and cause fuel starvation.
especially vulnerable to attack, and Corrosion Detection
when one is taken out of salt water it  Exotic inspection equipment is often
should be hosed down with large needed for certain parts of an aircraft.
volumes of fresh water to get every However, corrosion can often be detected
trace of salt off the structure. Seaplane by careful visual inspection of the airplane
ramps are often located in areas where structure.
there is a concentration of industrial Detection Methods
wastes, making the water even more  To aid in the detection of corrosion you
corrosive should use every inspection aid that is
 Marine atmosphere and air above available to you.
industrial areas hold large  For example, the complex structure of
concentrations of salts. The chemicals modern aircraft makes the use of
precipitate out of the air and collect on magnifying glasses, mirrors, borescopes,
the surface of an aircraft where they fiber optics, and other optical inspection
attract moisture from the air. tools imperative for a good visual inspection
Organic Growth Pulse-Echo Method
 For years, water which condensed in fuel  A pulse of ultrasonic energy is directed into
tanks produced relatively minor corrosion the structure. This energy travels through
problems This water contains microscopic the material to its opposite side and then
animal and plant life called microbes. These bounces back. When the return pulse is
organic bodies live in the water and feed on received, it is displayed on a CRT screen as a
the hydrocarbon fuel spike, which establishes a time base
 the dark insides of the fuel tank promote representing the material's thickness. Any
their growth, and in very short periods of change in a material's thickness, such as
time these tiny creatures multiply and form that caused by corrosion, causes the return
a scum inside the tank this provides a place to occupy a shorter space and thus indicate
for concentration cell corrosion to form. If the extent of damage.
the scum forms along the edge of the Radiological Inspection
sealant in an integral fuel tank, the sealant  x-ray is used to determine if there is any
can pull away from the structure, causing a corrosion on the inside of a structure.
leak and an expensive resealing operation. However, x-ray inspection requires
 The most successful solution to the problem extensive training and experience for
has been to use an additive in fuel which proper interpretation of the results the use
kills these organic growths and prevents the of x-ray involves some danger, because
formation of the corrosion-forming scum exposure to the radiation energy used in
 In addition to preventing corrosion by its this process can cause burns, damage to the
biocidal action, this same fuel additive also blood, and possibly death.
serves as an antifreeze to prevent entrained
Dry Penetrant Inspection  During an inspection, the areas around
 Any fault that is open to the surface may be a battery must be carefully checked,
found through a dye penetrant inspection especially under the battery, and all
traces of corrosion must be removed
 However, the main limitation of dye
and the area refinished. Furthermore,
penetrant inspection is the fact that it can you should verify that the battery has a
fail to find cracks that are so full of sump vent jar containing an absorbent
corrosion products that the dye cannot pad moist with a neutralizing agent.
penetrate. Also, penetrant cannot get in a Lead-acid batteries require bicarbonate
crack that is filled with oil or grease. of soda, and boric acid is used for a
nickel-cadmium battery. These sump
vent jars should be checked to see that
Corrosion Prone Areas
the pads are moist and that there is no
Engine Exhaust leakage.
 Reciprocating and turbine engines
generate power by converting chemical Battery Compartment and Vents
energy from a hydrocarbon fuel into  All battery box vent openings should be
heat energy. Because of the inefficiency clear and the intake and exhaust lines
of the engine, much of this heat along free and open. If battery electrolyte is
with energy-rich gases pass out of the spilled during servicing, it must be
engine through the exhaust. The gases cleaned up immediately and the area
contain all of the constituents for a neutralized. To verify an area is
potent electrolyte, and because of their completely neutralized you can blot the
elevated temperature, corrosion forms water on the surface with a piece of
extremely rapidly. litmus paper. If the area is acidic, the
paper turns pink, and if it is alkaline, it
 Exhaust areas must be carefully
turns blue. The entire area should be
inspected and all exhaust residue
neutral and it should not change the
removed before corrosion has a chance
color of the paper.
to start. Cracks and seams in the
exhaust track are prime areas for Lavatories and Food serve areas
corrosion. Fairings on the nacelles,  Food service areas can be troublesome
hinges, and inspection door fasteners if there is a possibility of food debris
all contain crevices which invite the getting into cracks under or behind the
formation of corrosion. galley where it cannot be removed.
While this material in itself may not be
Battery Compartment and Vents corrosive, it can hold water which can
 Airplanes having lead-acid batteries cause the structure to corrode.
must have their boxes protected by a  The lavatory area is an especially
material that resists corrosion from the important area to check for corrosion.
sulfuric acid fumes, and airplanes with Human wastes are usually acidic, and
nickel-cadmium batteries must have rapidly promote corrosion if allowed to
their battery areas protected with an remain on the skin of the airplane or to
alkaline-resistant finish. These finishes get into cracks and seams in the
can have a bitumastic (tar) base, a rub- structure. Furthermore, some
ber base, or can be polyurethane disinfectants used in lavatory areas can
finishes cause further damage to the aircraft.
Because of this, you should check the
 disinfectant carried in the airplane to be External skin areas
sure that it is not of a type that is harm-  One of the first places corrosions
ful to aluminum. appears on the surface of an aircraft is
along seams and lap joints. It is here
 In addition to lavatories, the area that concentration cell corrosion
around and behind the discharge of frequently appears and, in clad skins, it
relief tubes must be inspected carefully is here that the sheared edges of Alclad
for indications of corrosion. If corrosion material are exposed without the
is present, it should be removed and protection of the pure aluminum. There
the area painted with an acid-proof is also a danger of water or cleaning
paint. solvents becoming entrapped in the lap
joints and providing an effective
Wheel wells and landing Gears electrolyte.
 Because of the many complicated  Relatively little corrosion trouble is
shapes, assemblies, and fittings found in experienced with magnesium skins if
the wheel well and landing gear areas, the original surface finish and insulation
complete coverage of the area with are adequately maintained. However,
paint film is difficult to attain and a trimming, drilling, or riveting destroys
partially applied preservative tends to some of the original surface treatment,
mask corrosion rather than prevent it. which can never be completely restored
Furthermore, due to heat generated by by touchup procedures. Any inspection
braking action, preservatives cannot be for corrosion on magnesium skins
used on some main landing gear should include all magnesium skin
wheels. During inspection of this area, surfaces, with special attention to
pay particular attention to the following edges, areas around fasteners, and
trouble spots: cracked, chipped, or missing paint.
1. Magnesium wheels, especially
around bolt heads, lugs, and wheel- Fuel tanks
web areas, particularly for the  There is probably no single place in an
presence   of entrapped water or its aircraft that is less accessible for
effects. inspection and repair than the inside of
an integral fuel tank. Unfortunately,
2. Exposed rigid tubing, especially at fuel tanks are highly susceptible to
B-nuts and corrosion formation, especially in jet
ferrules, under clamps and tubing aircraft. As discussed in the previous
identification section, organic growth is the primary
tapes. cause of corrosion in fuel tanks that
hold turbine fuel. If ignored, this
3. Exposed position indicator switches organic growth can grow into the
and other water-holding scum which attaches to
electrical equipment. the aluminum alloy skin.
4. Crevices between stiffeners, ribs,
and lower skin   surfaces which are  In addition to organic growth, sealants
typical water and debris traps. used to convert the structure of a wing
into a fuel tank are impervious to fuel,
but only resistant to water. Therefore, it
is possible for water to seep through
the sealer and cause oxygen
 concentration cell corrosion. Corrosion to protect the skin lap joints in these
under the sealant is extremely difficult areas.
to detect and must usually be found
with x-ray or ultrasonic inspection from Bilge areas
the outside of the wing.  The bottom of the fuselage below the
floor is an area where water and all
Piano Hinges forms of liquid and solid debris can
 Piano hinges on control surfaces and accumulate and cause corrosion. These
access doors are ideal locations for areas are ideal for the formation of
dissimilar metal corrosion to develop. corrosion because  of the almost
The reason for this is because the hinge constant exposure to  an electrolyte.
body is usually made of aluminum alloy, Furthermore, because of their inacces-
while the pin is made of hard carbon sibility, corrosion often goes undetected
steel. Furthermore, since it is almost until it has caused major damage.
impossible to keep hinge crevices clean,
dirt and dust accumulate and hold  Airplanes having areas prone to
moisture between the pin and the accumulate water are typically provided
hinge body. Dissimilar metal corrosion with drain holes. However, dirt and
then develops. other debris also collect here, and drain
holes often become clogged. Therefore,
 If corrosion within a piano hinge goes on every inspection, you should
undetected the pin may rust or freeze carefully inspect any area where water
in the hinge, or break off and become might accumulate and make sure all
impossible to remove. To help prevent drain holes are clear. 
this, piano hinges should be kept as
clean and dry as possible and should be  Air-powered vacuum cleaners can be
lubricated with a spray which displaces used to remove dirt or water collected
water and leaves an extremely thin film in these areas, and a water-displacing
of lubricant. These water-displacing liquid spray that forms a thin film on
lubricants are manufactured to meet the surface of the metal can be used to
MIL-C-16713 specifications. prevent further contact with moisture.

Control Surface Recesses

 Any place on an airplane that is difficult
to inspect is an area where corrosion
has an opportunity to grow. For
example, some airplanes have areas in
the wing or empennage where the
movable surfaces recess back into the
fixed structure. Hinges are buried back
in these cavities and are difficult to
lubricate. Furthermore, special
attention must be paid when inspecting
these areas to remove every trace of
corrosion and provide drains for any
water that might collect. A thin film of a
water-displacing lubricant can be used
Landing Gear boxes
  Few areas in a modern fixed-gear the strands by twisting them
aircraft are as highly stressed, yet as against the lay, allowing you to see
difficult to inspect, as the landing between the strands. Cable
gear box structure. The landing gear showing any indication of corrosion
attaches into the fuselage by a should be replaced. To prevent this
strong, heavy-gauge aluminum alloy type of corrosion you should spray
box structure which is under the the cable with a water-displacing
floor and is accessible for inspection type of lubricant. For cables in a
only through a relatively small seaplane or those exposed to
access hole. Although this area is agricultural chemicals, coat them
well-protected, water can collect if with a waxy grease such as Par-Al-
the drain holes become plugged. Ketone.
Therefore, during inspections all
drains should be opened, and the Welded Areas
entire enclosed area generously  Aluminum torch welding requires
sprayed with a water-displacing the use of a flux to exclude oxygen
lubricant film. from the weld. This flux may
contain lithium chloride, potassium
Engine Mount Structure chloride, potassium bisulphide, or
 When a reciprocating aircraft potassium fluoride. All of these
engine is started, the heavy current compounds are extremely corrosive
from the starter must return to the to aluminum and, therefore, all
battery through the engine mount. traces of flux must be removed
This current flows through joints in after welding is completed. Welding
the mount and creates the flux is soluble in water and can be
potential difference required for removed with hot water and a
corrosion to form in these areas. To nonmetallic bristle brush.
protect welded steel tube mounts
from internal corrosion the tubing Electronic Equipment
should be periodically filled with  The use of copper, lead, tin, and
hot linseed oil or other type of other metals in electronic wiring
tubing oil. The oil is then drained and printed circuit boards makes
and the drain hole plugged with a them a target for corrosion.
drive screw or a self-tapping metal Therefore, circuit boards are
screw. typically protected by sealing the
wiring and circuit boards with a
Control Cables transparent film which excludes all
 The cables used in an aircraft oxygen and moisture from the
control system are made of either surface. Detection and repair of
carbon steel or corrosion-resistant corrosion in these areas is a highly
steel. If carbon steel cable is left specialized field and is not usually
unprotected and water is allowed the job of an A&P technician.
to get between the cable strands,
the cable will corrode. The
corrosion that forms on the inside
of the cable is difficult to detect. If
corrosion is suspected, release the
tension from the cable and open