MOONEY INTERNATIONAL CORPORATION
M20V SERVICE AND MAINTENANCE MANUAL
Page
Date
8
MAR 2017
Rev Date
(1) Intergranular Corrosion - Selective attack
along grain boundaries of metal alloys is referred to as
intergranular corrosion. Aluminum alloys 2024 and
7075 are vulnerable to this type of attack. Aluminum ex-
trusions may contain non- uniform areas, which in turn
may result in galvanic attack along grain boundaries.
This type of corrosion is difficult to detect in its original
stages. When the attack is well advanced, the metal is
usually blistered or delaminated. This is referred to as
“exfoliation”. It is very difficult to completely remove and
stop this type of corrosion, and replacement of the af-
fected part is recommended wherever possible.
C. Concentration Cell Corrosion
Corrosion forming under rivet heads, along faying sur-
faces, at skin to longeron contact surfaces and other
similar areas is called concentration cell corrosion.
Detection requires close inspection. Rivets must be re-
moved and skin laps must be separated to remove con-
centration cell corrosion. Use aluminum wool soaked in
solvent such as methyl- ethyl- ketone to scour corro-
sion deposits from the surface before painting both fay-
ing surfaces with epoxy- polymide primer and re- as-
sembling.
D. Galvanic Corrosion
Dissimilar metals, such as stainless steel and alumi-
num, in contact with each other sometimes develop
galvanic corrosions. To remedy this form of corrosion,
separate the parts, remove corrosion, and paint both
surfaces with epoxy- polymide primer before reassem-
bling.
2. Corrosion Prevention
A. Thoroughly examine unpainted metal surfaces
at inspections and check corrosion when found. Care-
fully examine seams, lap joints, and crevices where
moisture or dirt can collect. Areas exposed to exhaust
gases require frequent inspection and cleaning.
B. Corrosion may attack metal even though the
surface is painted. Inspect painted areas for a blistered
or scaly appearance that warns of corrosion below the
paint layer.
C. Use only liquid (non- alkaline) soap to wash the
exterior airframe. Cover vent scoops when aircraft is
being washed. Rinse aircraft exterior after exposure to
salt air or industrial fallout.
D. Since moisture promotes corrosion, thoroughly
and frequently inspect areas where water is likely to
collect. Use an air hose to drive water from crevices be-
fore wiping exterior surface dry after washing down.
E. Hangar aircraft when not in use.
F. If battery acid is spilled on any part of the aircraft,
immediately wash the area with a solution of sodium bi-
carbonate in water. Rinse with clear water and dry with
clean towels.
-NOTE-
ACF- 50 or equivalent can be fogged into fuse-
lage, empennage or wing skin areas. It is recom-
mended that electrical connections and electrical
components be protected (sealed off) during
fogging procedures.
-NOTE-
FAA Advisory Circular 43- 4 addresses corrosion
problems.
3. Epoxy- polymide Painting.
Epoxy- polymide coating of internal airframe surfaces
will not eliminate the necessity for periodic inspections.
The most likely areas for corrosion to begin are in hid-
den crevices such as skin laps, under rivet heads or any
opening where moisture can collect. Periodic inspec-
tion of these areas is most important so that any corro-
sion which may be present can be detected and treated
in its very earliest stages.
When corrosion is detected, it should be treated as fol-
lows:
A. Remove all corrosion product (usually white or
grey- white powder on aluminum) from the corroded
area down to sound metal or up to 10% of original thick-
ness. Consult with Mooney International Corporation.
Customer Service for recommendations.
(1) If the base metal is aluminum, clean off the
corrosion product by scrubbing with aluminum wool or
a non- metallic scouring pad such as the nylon pads
made by the Carborundum Company, P.O. Box 277, Ni-
agara Falls, N.Y. 14302.
(2) If the base metal is steel, remove the corro-
sion product with emery cloth (320 grit or finer) or steel
wool.
(3) If the corrosion is too advanced to remove by
the light mechanical cleaning methods of (1) and (2),
refer to Advisory Circulars 43.13- 1( ) and/or 43- 4 for
alternate methods of cleanup and repair.
(4) Chemical cleaning solutions should be used
with extreme caution on both aluminum and steel parts.
Such solutions are not recommended for use in areas
where they cannot be easily and completely neutral-
ized.
B. Wash all areas to be epoxy coated with a clean-
ing solvent such as methyl ethyl ketone (MEK), Turco
T- 657, lacquer thinner, #3094 wash thinner (Sherwin-
Williams [was Pratt & Lambert], P.O. Box 2153, Wichi-
ta, KS. 67201) or similar solvents.
(1) To get a good clean surface for epoxy priming
use a clean cloth or piece of cheesecloth and apply one
of the solvents noted above. The surface should be
wiped dry with a second piece of clean cloth. Do not al-
low the solvent to dry on the cleaned area as it will rede-
posit dissolved soil and grime upon evaporation.
20-00-02
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