2-74. CORROSION CONTROL. Corrosion is the deterioration of metal by chemical or electrochemical attack.
Water which is allowed to remain on the aircraft and industrial pollution are the major causes of corrosion in
aircraft. The two general types of corrosion are: 1. a direct chemical attack (ex. spilled battery acid); and, 2.
electrochemical attack which requires a medium (usually water). The latter is the most common and is
responsible for most forms of aircraft corrosion.
Since corrosion is a constant threat, the only effective method to control it is a routine of regular inspection,
cleaning, and surface refinishing.
2-75. FORM OF CORROSION. The following are the most common forms of corrosion:
a. Surface Corrosion appears as a general roughening or pitting on the surface usually accompanied by a
powdery deposit of corrosion products. It may spread under the surface and not be recognized until the paint or
plating is lifted off the surface in small blisters.
b. Dissimilar Metal Corrosion may occur when two dissimilar metals are contacting each other. This type
may be serious because it usually takes place out of sight. The only way to find it before structural failure is by
disassembly and inspection. Insulating is necessary between two contacting dissimilar surfaces (2-3 coats of zinc
chromate on each surface; plus a .003 thick piece of vinyl tape if one of the surfaces is magnesium).
c. Intergranular Corrosion is difficult to detect in its early stages. When severe, it causes the surface of the
metal to "exfoliate" (flake or lift).
d. Stress Corrosion is the result of sustained tensile stresses and corrosive environment. It usually occurs in
assemblies such as aluminum alloy bellcranks with pressed in bushings; landing gear shock struts with pipe
thread grease fittings, clevis pin joints and shrink fit parts.
e. Fretting Corrosion takes place when two parts rub together, constantly exposing fresh active metal to the
corrosive effects of the atmosphere.
f. Filiform Corrosion is the appearance of numerous meandering threadlike filaments of corrosion on the
surface of various types of metal.
2-76. CONDITIONS AFFECTING CORROSION. Some conditions which affect the occurrence of corrosion
are:
a. The environmental conditions affect the corrosion characteristics. A hot, humid climate increases
corrosion. One of the worst conditions would be allowing the aircraft to be constantly exposed to the corrosive
elements found near the ocean.
b. Different metals and their sizes affect resistance to corrosion.
c. The foreign materials which most frequently contribute to corrosion are:
1. Soil and atmospheric dust.
2. Oil, grease and exhaust residues.
3. Salt water and salt moisture condensation.
4. Spilled battery acids and caustic cleaning solution.
5. Welding, brazing and soldering flux residue.
A clean aircraft will resist corrosion better than a dirty one. Cleaning frequency depends on several factors
(such as geographical location, type of operation, etc.). Soil should be removed as soon as possible, especially
when it is on a high temperature area.
After cleaning, ensure that no cleaning solution remains in any holes, crevices or joints, as it may lead to
increased corrosion. Also, all exposed areas (landing gear, flap tracks, control surface, hinge parts, etc.) should
be lubricated after cleaning.
PIPER AIRCRAFT
PA-28-161 CADET
AIRPLANE MAINTENANCE MANUAL
Issued: FEBRUARY 28, 1989
HANDLING AND SERV I C I N G
1C11
