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The Corrosion of Magnesium and of the Magnesium Aluminum Alloys Containing Manganese

The extensive use of magnesium and its alloys in aircraft has been seriously handicapped by
the uncertainties surrounding their resistance to corrosion. This problem has been given
intense study by the American Magnesium Corporation and at the request of the Subcommittee
on Materials for Aircraft of the National Advisory Commitee for Aeronautics, Mr. J. A. Boyer,
of the American Magnesium Corporation, has prepared the following report on the corrosion
of magnesium.

Although considerable progress has been made in the development of strong magnesium
alloys, the corrosion of these alloys has received very little study. For that reason the corro-
dibility of the metal itself has been the subject of considerable misunderstanding. Magnesium
of inferior quality has often been placed on the market; this metal usually contained chloride and
corroded badly when subject to even ordinary atmospheric conditions. Alloys have been pre-
pared more or less indiscriminately from the standpoint of physical properties alone, and their
rapid corrosion has been attributed to the tendency of magnesium itself to corrode rather than
to the nature and proportion of the alloying ingredients.

Contrary to popular belief, pure magnesium is quite resistant to corrosion. However,
the physical properties of the pure metal make it unfit for most structural purposes, and the
alloys having desirable physical properties are usually not resistant to corrosion. The present
nvestigation deals with the development of a resistant magnesium alloy by a variation in com—
iposition.

The natural alloying element for magnesium is aluminum. The latter, like magnesium,
has a very low density and the addition required does not cause any appreciable increase in the
density of the alloy over that of the pure metal. Aluminum is soluble in solid magnesium up
to at least 10 per cent at the eutectic temperature, and the alloys may be subject to heat treat-
ment and artificial aging. An exhaustive survey of the alloy field has not shown any alloying
ingredient as satisfactory irom the physical standpoint as aluminum, and for that reason the
study of corrosion has been chiefly confined to the magnesium-aluminum series. The corro-
sion resistance of these alloys has been increased approximately 50 times without any sacrifice
in physical properties.