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National Advisory Committee for Aeronautics, Technical Notes - Advantage of Oxide Films as Bases for Aluminum Pigmented Surface Coatings for Aluminum Alloys

The advent of metals into aircraft construction has
caused a demand for light—weight, high—strength alloys. It
is highly desirable that such structures, especially when
in the form of sheet material, be resistant to corrosion.
Alloys of aluminum and of magnesium, which conform to the
weight and strength requirements, are susceptible to corro-
sion and are especially sensitive to salt (sea~water) atmos—
pheres. The problem of producing a satisfactory corrosion—
resistant (under severe conditions) alloy having the requi—
'site physical preperties has not yet been completely solved.
The alternative then is to use the alloys now known to have
best physical properties and to obtain improved corrosion
resistance by the application of "protective" coatings or
"films", either chemically or mechanically, Which will tend
to retard corrosion. The investigation of this problem was
carried out at the Bureau of Standards in cooperation with -f
the National'Advisory Committee for Aeronautics. the Bureau
of Aeronautics of the Navy Department, and the Army Air Corps.

The protective coatings utilized on aluminum alloys may
be roughly classified into three groups. One group may be
considered as including all coatings in which the active
protective agent is an oxide film formed directly upon and
from the surface of the material by suitable chemical or
electrochemical processes. A second group may be taken to
include coatings primarily nonmetallic, such as paints, var—
nishes, oils, greases, etc., mechanically applied directly
to a cleansed or slightly roughened surface, and themselves
functioning as the protective agents. Metallic coatings,
superimposed‘on the surface by electrochemical or mechanical
processes. constitute a third group of protective coatings.

A large.number of coatings are available (in all three
groups) which will afford excellent protection to the under—
3 7 d”7"N.A.c.A. etchiicai Note'No. 400

lying metal under ordinary conditions of service exposure,
for example, in a temperate climate and inland. Aircraft,
however, is not infrequently used under saline service con—
ditions, either coming into contact with salt water or salt
spray. and the difficulties in finding a suitable protect—
ive coating become numerous. 'Onide films and coatings of
the second type, although they retard the onset of corrosion
to a greater or'less'extent, have not been found to render '
very satisfactory service over long periods under such cir—
eumstances. » ”'