FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 8. Aluminum and Aluminum Alloys | 6-134. Repair of Aluminum Alloy Sheet Metal
9/8/98
SECTION 8. ALUMINUM AND ALUMINUM ALLOYS
AC 43.13-1B
6-132. GENERAL. Aluminum and alumi
num alloys are the most widely used material
for aircraft construction. Aluminum appears
high in the electro-chemical series of elements
and corrodes very easily. However, the forma
tion of a tightly-adhering oxide film offers in
creased resistance under most corrosive condi
tions. Most metals in contact with aluminum
form couples that undergo galvanic corrosion
attack. The alloys of aluminum are subject to
pitting, intergranular corrosion and inter-
granular stress corrosion cracking. In some
cases the corrosion products of metal in con
tact with aluminum are corrosive to aluminum.
Therefore, aluminum and its alloys must be
cleaned and protected.
6-133. SPECIAL TREATMENT OF
ANODIZED SURFACES. Anodizing is the
most common surface treatment of aluminum
alloy surfaces. The aluminum sheet or casting
is made the positive pole in an electrolyte bath
in which chromic acid or other oxidizing
agents produce a supplemental protective ox
ide film on the aluminum surface. The ano
dized surface coating offers the alloy a great
deal of protection as long as it is not damaged.
Once the film is damaged, it can only be par
tially restored by chemical surface treatment.
Therefore exercise care to avoid breaking of
the protective film, particularly at the edges of
the sheet.
6-134. REPAIR OF ALUMINUM ALLOY
SHEET METAL. After extensive corrosion
removal the following procedures should be
followed:
a. If water can be trapped in blended ar
eas, chemical conversion coat in accordance
with MIL-C-81706 and fill the blended area
with structural adhesive or sealant to the same
level and contour as the original skin. When
areas are small enough that structural strength
has not been significantly decreased, no other
work is required prior to applying the protec
tive finish.
b. When corrosion removal exceeds the
limits of the structural repair manual, contact a
DER or the aircraft manufacturer for repair in
structions.
c. Where exterior doublers are installed,
it is necessary to seal and insulate them ade
quately to prevent further corrosion.
d. Doublers should be made from alclad,
when available, and the sheet should be ano
dized (preferred) or a chemical conversion coat
applied after all cutting, drilling, and counter
sinking has been accomplished.
e. All rivet holes should be drilled, coun
tersunk, surface treated, and primed prior to
installation of the doubler.
f. Apply a suitable sealing compound in
the area to be covered by the doubler. Apply
sufficient thickness of sealing compound to fill
all voids in the area being repaired.
g. Install rivets wet with sealant. Suffi
cient sealant should be squeezed out into holes
so that all fasteners, as well as all edges of the
repair plate, will be sealed against moisture.
h. Remove all excess sealant after fasten
ers are installed. Apply a fillet sealant bead
around the edge of the repair. After the sealant
has cured apply the protective paint finish to
the reworked area.
Par 6-132
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