Cessna 172 Maintenance Manual page 23

(4) Severe Corrosion.
(a) General appearance may be similar to moderate corrosion with severe blistering exfoliation
and scaling or flaking. Pitting depths may be as deep as 15 percent of the material
thickness. This type of damage is normally repaired by complete part replacement, but
patches or other types of repair may be available. Contact Cessna Propeller Aircraft
Product Support, P.O. Box 7706, Wichita, KS 67277 USA, for assistance.
6. Corrosion Removal
A. The following methods are provided as an aid in determining the correct method for corrosion removal.
(1) Standard Methods
(a) Several standard methods are available for corrosion removal. The method normally used
to remove corrosion are chemical treatments, hand sanding with aluminum oxide or metal
wool that is of similar material to the surface being treated, and mechanical sanding or
buffing with abrasive mats or grinding mats. The method used depends on the metal and
the degree of corrosion. Select appropriate materials from the abrasives chart as illustrated
in Figure 2.
(2) Aluminum and Aluminum Alloys.
(a) Most formed aluminum parts and skins of this airplane consist of various gauges of sheet
2024-T3 and 2024-T42 Alclad. Alclad is formed by laminating a thin layer of relatively
pure aluminum, one to five mils thick, over the higher strength base alloy surface. Since
pure aluminum has relatively greater corrosion resistance than the stronger alloy, it is
imperative the clad surface be maintained intact to the maximum extent possible and to
avoid unnecessary mechanical removal of the protective coating. In addition, aluminum
parts receive a chemical conversion coating and are then epoxy-primed.
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(3) Steel.
(a) Unlike some other metal oxides, the red oxide of steel (rust) will not protect the underlying
base metal.
moisture from the air and acting as a catalyst in causing additional corrosion to take
place. Light red rust on bolt heads, hold-down nuts, and other nonstructural hardware
is generally not dangerous. However, it is indicative of a general lack of maintenance
and possible attack in more critical areas, such as highly stressed steel landing gear
components and flight control surface actuating components. When paint failures occur
or mechanical damage exposes highly stressed steel surfaces to the atmosphere, even
small amounts of rusting are potentially dangerous and must be removed. The most
practical means of controlling corrosion of steel is the complete removal of the corrosion
products by mechanical means. Except on highly stressed steel surfaces, the use of
abrasive papers, small power buffers and buffing compounds, and wire brushes are
acceptable for clean up procedures. However, residual rust usually remains in the bottom
of small pits and crevices.
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CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
Clean area to be reworked. Strip paint as required.
To determine the extent of corrosion damage refer to Corrosion Damage Limits.
Remove light corrosion by light hand sanding.
Mechanically remove moderate or severe corrosion by hand scraping with a carbide-
tipped scraper or fine-fluted rotary file.
Remove residual corrosion by hand sanding. Select appropriate abrasive from Figure
2.
Blend into surrounding surface any depressions resulting from rework and surface
finish with 400 grit abrasive paper.
Clean reworked area.
Determine depth of faired depressions to ensure that rework limits have not been
exceeded.
Chemically conversion-coat rework area.
Restore original finish (epoxy prime).
The presence of rust actually promotes additional attack by attracting
Clean area to be reworked.
Strip paint as required.
Remove all degrees of corrosion from steel parts using a stainless steel hand brush
or hand operated power tool. Alternatively, use dry abrasive blasting process.
© Cessna Aircraft Company
51-11-00
Page 5
Jun 1/2005