01 March 2005
NAVAIR 01-1A-509-1
TM 1-1500-344-23-1
TO 1-1-689-1
Electrolyte
Low Oxygen Concentration
(Corrosion Site)
High Oxygen Concentration
A. Oxygen Differential Cell
Foreign Material
Passive Film
Creates Low Oxygen
Protects Exposed
Region Which Prevents
Surface
the Re-Formation
of Passive Film
Active Metal
Pitting Corrosion
B. Active Passive Cell
creates an oxygen differential cell. The passive film is
then broken in the area of the salt deposit and the more
active metal beneath the passive film will be exposed
to corrosive attack. This small anodic area will then
corrode rapidly due to the much larger area of the
surrounding cathode (passive film). The result is rapid
Paint Coating on a Magnesium Panel
View B.
or destroy the base metal. Filiform corrosion occurs
3-9.7. FILIFORM CORROSION. Filiform corrosion is
when the relative humidity of the air is between 65 and
a form of crevice corrosion which occurs on metal
90%, and the air temperature is between 70 and 100F.
surfaces having a thin (~4 mils) organic protective
It starts at breaks in the coating system, such as
coating. It is recognized by its characteristic wormlike
scratches and cracks around fasteners and seams, and
trace of corrosion products beneath the coating (see
proceeds underneath the coating, due to the diffusion
of water vapor and oxygen from the air through the
only affects surface appearance, but does not weaken
Organic
Break in
Cathode
Coating
Coating
Base Metal
Anode
Figure 3-18. Schematic of the Development of Filiform Corrosion on an Aluminum Alloy