FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 9. Magnesium and Magnesium Alloys | 6-150. Repair of Magnesium Sheet Metal After Extensive Corrosion Removal
9/8/98
SECTION 9. MAGNESIUM AND MAGNESIUM ALLOYS AC 43.13-1B
6-148. GENERAL. Magnesium and mag
nesium alloys are the most chemically active
of the metals used in aircraft construction and
are the most difficult to protect. However, cor
rosion on magnesium surfaces is probably the
easiest to detect in its early stages. Since mag
nesium corrosion products occupy several
times the volume of the original magnesium
metal destroyed, initial signs show a lifting of
the paint films and white spots on the magne
sium surface. These rapidly develop into
snow-like mounds or even white whiskers.
The prompt and complete correction of the
coating failure is imperative if serious struc
tural damage is to be avoided.
6-149. TREATMENT OF WROUGHT
MAGNESIUM SHEETS AND FORGINGS.
Corrosive attack on magnesium skins will usu
ally occur around the edges of skin panels, un
derneath hold-down washers, or in areas
physically damaged by shearing, drilling, abra
sion, or impact. Entrapment of moisture under
and behind skin crevices is frequently a con
tributing factor. If the skin section can be eas
ily removed, this should be accomplished to
ensure complete inhibition and treatment.
a. Complete mechanical removal of cor
rosion products should be practiced when
practical. Mechanical cleaning should nor
mally be limited to the use of stiff bristle
brushes and similar nonmetallic cleaning tools.
b. Any entrapment of steel particles
from steel wire brushes, steel tools, or con
tamination of treated surfaces, or dirty abra
sives, can cause more trouble than the initial
corrosive attack. The following procedural
summary is recommended for treatment of cor
roded magnesium areas when accomplished
under most field conditions.
c. When aluminum insulating washers
are used and they no longer fasten tightly to
magnesium panels, corrosion is likely to occur
under the washers if corrective measures are
not taken.
(1) When machine screw fasteners are
used, aluminum insulating washers must be
removed from all locations to surface treat the
magnesium panel.
(2) Where permanent fasteners other
than machine screws are used, the insulating
washer and fastener must be removed.
(3) When located so water can be
trapped in the counter-bored area where the
washer was located, use sealants to fill the
counterbore. If necessary, fill several areas
adjacent to each other. It may be advantageous
to cover the entire row of fasteners with a strip
of sealant.
6-150. REPAIR OF MAGNESIUM
SHEET METAL AFTER EXTENSIVE
CORROSION REMOVAL. The same gen
eral instructions apply when making repairs in
magnesium as in aluminum alloy skin, except
that two coats of epoxy primer may be re
quired on both the doubler and skin being
patched instead of one. Where it is difficult to
form magnesium alloys in the contour, alumi
num alloy may be utilized. When this is done,
it is necessary to ensure effective dissimilar
metal insulation. Vinyl tape will ensure posi
tive separation of dissimilar metals, but edges
will still have to be sealed to prevent entrance
of moisture between mating surfaces at all
points where repairs are made. It is recom
mended that only non-corrosive type sealant be
used, since it serves a dual purpose of material
separation and sealing.
Par 6-148
Page 6-29