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National Advisory Committee for Aeronautics, Technical Notes - Corrosion Embrittlement of Duralumin - II - Accelerated Corrosion Tests and the Behavior of High Strength Aluminum Alloys of Different Compositions

As shown in the previofis‘report,-light aluminum alloys of
the duralumin type, that is, wrought alloys whose properties
can be improved decidedly by‘heat treatment and which, for this
reason, are widelv'employed in-aircratt construction, at times
have shown evidence of a lack of permanence and reliability
while in use. This laok'of.permanence has been shown to be
the direct result of an intercrystalline corrosive attack which,
although starting-at the surface of the material, gives very
little evidence in the changed appearance of the corroded part
of the extent of the attack and the seriousness of the result—
ing'iupairment of the properties of the material.. This form of
deterioration of duralumin is most serious in the case of sheet
material.' The tendency tOWard this change, according to the
observations which have been made, has been exhibited by only a
relatively small amount of the total quantity of sheet duralumin

which has been put into use. However, such a change, whereby a
sheet may be converted from a strong ductile material into a
very weak brittle.one with no accompanying-marked change in the
external appearance, is of such an insiduous nature that, until
reliable information.is available, all duralumin must be regard—
ed, more or less, with suspicibn.» To obtain such information
so that precautionary measures to'guard against possible deterio—
ration of this type may be taken, a comprehensive investigation
was necessary.

The results of this investigation form the basis
of this series of reports (Reference lIL I The work has been car—
ried' out. at the Bureau of Standards in cooperation with the
National Advisory Committee for Aeronautics, Bureau of Aeronau—
tics of the Navy Department, lland Army. Air Corps. The leading
manufacturers have also participated in the investigation by
furnishing practically all of the materials needed. The inves—
tigation, which was started in the latter 'part of 1985, is still
in progress and final and complete answers have not _een reached
on all points concerning the permanence of duralumin in service.
The information which has been obtained, however, is of very
considerable value to both manufacturers and users of aircraft
and its publication at th time would seem to be warranted al—
though possibly some of the statements made in this progress
report may be modified slightly in the light of future results.