Cessna 172 Maintenance Manual page 19

1. General
A. Corrosion is a natural phenomenon which destroys metal by chemical or electrochemical action and
converts it to a metallic compound such as an oxide, hydroxide, or sulfate. All metals used in airplane
construction are subject to corrosion. If exposed, attack may take place over an entire metal surface.
It may penetrate a surface at random forming deep pits or may follow grain boundaries. Corrosion may
be accentuated by stresses from external loads or from lack of homogeneity in the metallic structure
or from improper heat treatment. It is promoted by contact between dissimilar metals or with materials
which absorb moisture such as wool, rubber, felt, dirt, and so forth.
For additional information on corrosion control for aircraft, refer to the FAA Advisory Circular
NOTE:
No. 43-4.
(1) Refer to Figure 1 for a simplified illustration of the conditions which must exist for electrochemical
corrosion to occur.
(a) There must be a metal that corrodes and acts as the anode.
(b) There must be a less corrodible metal that acts as the cathode
(c) There must be a continuous liquid path between the two metals which acts as the
electrolyte, usually condensation and salt or other contamination.
There must be a conductor to carry the flow of electrons from the cathode to the anode.
(d)
This conductor is usually in the form of a metal-to-metal contact (rivets, bolts, welds, etc.)
(2) The elimination of any one of the four conditions described above will stop the corrosion reaction
process as shown in Figure 1.
(3) One of the best ways to eliminate one of the four described conditions is to apply an organic film
(such as paint, grease, plastic, etc.) to the surface of the metal affected. This will prevent the
electrolyte from connecting the cathode to the anode, and since current cannot flow, it prevents
corrosive reaction.
(4) At normal atmospheric temperatures, metals do not corrode appreciably without moisture, but
the moisture in the air is usually enough to start corrosive action.
(5) When components and systems constructed of many different types of metals must perform
under various climatic conditions, corrosion becomes a complex problem. The presence of salts
on metal surfaces (from sea coast operation) greatly increases the electrical conductivity of any
moisture present and accelerates corrosion.
(6) Other environmental conditions which contribute to corrosion are:
(a) Moisture collecting on dirt particles.
(b) Moisture collecting in crevices between lap joints, around rivets, bolt, and screws.
2. Types of Corrosion
A. Direct Surface Attack.
(1) The most common type of general surface corrosion results from direct reaction of a metal
surface with oxygen in the atmosphere. Unless properly protected, steel will rust and aluminum
and magnesium will form oxides. The attack may be accelerated by salt spray or salt bearing
air, by industrial gasses, or by engine exhaust gasses.
B. Pitting.
(1) While pitting can occur in any metal, it is particularly characteristic of passive materials such
as alloys of aluminum, nickel, and chromium. It is first noticeable as a white or gray powdery
deposit similar to dust, which blotches the surface. When the deposits are cleaned away, tiny
pits can be seen in the surface.
C. Dissimilar Metal Corrosion.
(1) When two dissimilar metals are in contact and are connected by an electrolyte (continuous liquid
or gas path), accelerated corrosion of one of the metals occurs. The most easily oxidized surface
becomes the anode and corrodes. The less active member of the couple becomes the cathode
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
CORROSION AND CORROSION CONTROL - GENERAL
© Cessna Aircraft Company
51-11-00
Page 1
Jun 1/2005