FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 2. Types of Corrosion | 6-18. Exfoliation Corrosion
9/27/01
AC 43.13-1B CHG 1
6-16. FILIFORM CORROSION. Filiform
corrosion is a special form of oxygen concen
tration cell which occurs on metal surfaces
having an organic coating system. It is recog
nized by its characteristic worm-like trace of
corrosion products beneath the paint film.
(See figure 6-8.) Polyurethane finishes are es
pecially susceptible to filiform corrosion. Fili
form occurs when the relative humidity of the
air is between 78 and 90 percent and the sur
face is slightly acidic. This corrosion usually
attacks steel and aluminum surfaces. The
traces never cross on steel, but they will cross
under one another on aluminum which makes
the damage deeper and more severe for alumi
num. If the corrosion is not removed, the area
treated, and a protective finish applied, the cor
rosion can lead to inter-granular corrosion, es
pecially around fasteners and at seams. Fili
form corrosion can be removed using glass
bead blasting material with portable abrasive
blasting equipment or sanding. Filiform corro
sion can be prevented by storing aircraft in an
environment with a relative humidity below
70 percent, using coating systems having a low
rate of diffusion for oxygen and water vapors,
and by washing the aircraft to remove acidic
contaminants from the surface.
6-17. INTERGRANULAR CORROSION.
Inter-granular corrosion is an attack on the
grain boundaries of a metal. A highly magni
fied cross section of any commercial alloy
shows the granular structure of the metal. It
consists of quantities of individual grains, and
each of these tiny grains has a clearly
FIGURE 6-8. Filiform corrosion.
defined boundary which chemically differs
from the metal within the grain. The grain
boundary and the grain center can react with
each other as anode and cathode when in con
tact with an electrolyte. (See figure 6-9.)
Rapid selective corrosion of the grain bounda
ries can occur. High-strength aluminum alloys
such as 2014 and 7075 are more susceptible to
inter-granular corrosion if they have been im
properly heat-treated and then exposed to a
corrosive environment.
6-18. EXFOLIATION CORROSION. Ex-
foliation corrosion is an advanced form of in
ter-granular corrosion and shows itself by lift
ing up the surface grains of a metal by the
force of expanding corrosion products occur
ring at the grain boundaries just below the sur
face. (See figure 6-10.) It is visible evidence
of inter-granular corrosion and is most often
seen on extruded sections where grain thick
ness are usually less than in rolled forms.
FIGURE 6-7. Active-passive cell.
Par 6-16
Page 6-7