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R E C E N T D E V E L O P M E N T S IN C O R R O S IO N - 1 1 1 7 R E S IS T A N T A L U M IN IU M -M A G N E S IU M
A L L O Y S .*
B y P. B R E N N E R ,f Dr. Ing., and W. ROTH.J Dr. Ing.
Sy n o p s i s.
The paper describes an extensive investigation on the influence of chemical composition and thermal treatment on the intercrystalline corrosion of aluminium-magnésium alloys containing 5-9% magnesium.
As a result of the investigation, a new method of heat-treatment, termed a “ stabilizing treatment ” , is suggested in which the alloy is extremely slowly cooled from the homogenizing temperature at a rate of about 50° C./hr. Corrosion tests in the laboratory and in the North Sea, as well as long-term service trials, indicate that alloys treated by this method are immune to intercrystalline-corrosion and stress-corrosion failure even at the high temperatures encountered under tropical conditions.
While cold reduction generally increases the tendency to inter- crystalline corrosion, small additions of copper, manganese, chromium, and zinc have very different effects. Some of these additions, e.g. copper, reduce the stress-corrosion-resistance in the homogenized state and improve it in the aged condition. The high stress-corrosion-resistance of alloys containing small additions of chromium instead of the usual manganese is of special interest.
The close correlation between the thermal treatment, the micro
structure, and the susceptibility to intercrystalline corrosion and, especially, stress-corrosion of these alloys is clearly indicated.
I .— In t b o d u c t i o n.
O f th e aluminium alloys, those of the aluminium-magnesium type are of special interest, since in both the supersaturated-solid-solution and the aged states they show no marked age-hardening, although the solubility of magnesium is very large and decreases rapidly with decreasing tem perature.1’ 2 In alloys of very high magnesium con
te n t (10% ), more marked changes have, of course, been observed, especially in the proof-stress and elongation values in the tem perature range 100°-300° C.3 These changes are preceded by grain-boundary precipitation and are accompanied later by precipitation w ithin the grains. Despite this, the processes of solubility and decomposition of the solid solution in alum inium -m agnesium alloys are connected with essentially much smaller changes of the mechanical and forming
* Manuscript received 14 March 1947.
t Metallurgy Department, Royal Aircraft Establishment, South Farnborough, Hants.
Î High Duty Alloys, Ltd., Redditch, Worcester.