Filiform Corrosion
Filiform corrosion is a special form of oxygen concentration
cell that occurs on metal surfaces having an organic coating system. It is
recognized by its characteristic worm[1]like trace of
corrosion products beneath the paint film.
Polyurethane finishes are especially susceptible to filiform
corrosion. Filiform occurs when the relative humidity of the air is between
78–90 percent, and the surface is slightly acidic.
This corrosion
usually attacks steel and aluminum surfaces. The traces never cross on steel,
but they cross under one another on aluminum, making the damage deeper and more
severe for aluminum.
If the corrosion is not removed, the area treated, and a
protective finish applied, the corrosion can lead to intergranular corrosion,
especially around fasteners and at seams.
Filiform corrosion can be removed using glass bead blasting
material with portable abrasive blasting equipment or sanding.
Filiform corrosion 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, such as those created
by pollutants in the air.