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National Advisory Committee for Aeronautics, Technical Notes - ''ALCLAD'' A New Corrosion Resistant Aluminum Product

The corrosion of aluminum alloys has been the cause of
much concern to those‘interested.in the successfulfapplication’
of metal in airplane and airship construction."Within the .
past few years it has been made the subject of much discussion,
both constructive and destructive. Examples of corrosion oc—
curring under unusual and severe conditions have been cited as
reasons why alUminum alloys should not be used even under rel—
atively protected conditions.

As a result of this situation the whole subject has re—
ceived careful consideration in many quarters and much exper—
imental work is now in progress. Although much information
has been obtained concerning the nature of the corrosion attack
and protective coatings, yet very little has hitherto been ac-
complished towards making major improvements in the corrosion
resistance of the strong alloys themselves. One conclusion is
outstanding, namely, that uhile in comparison with steel alumi-
num is highly corrosion resistant, or "rust proof, " yet for
satisfactory service, thin sections of the strong aluminum al—
loys of to—day require some protection, especially when sub—
jected to severe conditions such as salt water or mist. It is
worthy of note, however, that under normal atmospheric condi—
tions unprotected strong aluminum alloys are giving very satis~
factory service, the radiator shells used in a popular automo—
bile being one excellent and familiar example.

In aircraft, however, where so much depends upon the abso—
lute reliability of every unit, it is essential that the struc—
tural parts have a maximum of resistance to deterioration under
service conditions. Aircraft, especially seaplanes, are often
subjected to atmospheres Which have a very corrosive effect on
most metals. Hence, it is now the practice to protect all
metal parts by paint or varnish. If this is carefully done the
aluminum alloys suffer little deterioration; but there is al—
waYs the possibility of a careless job where parts are hurried~
1y put together without first having a shop coat, and so the
joints are not properly protected. Such conditions may easily
escape subsequent inspection, and corrosion and failure may
result. Blisters, checking, and abrasion*may further cause the
failure of the protective coating. Under severe conditions as
in the case of the pontoons of seaplanes, the life of the best
protective paints is short and the structure must be freQuently
reconditioned.