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This test was developed to indicate the resistance of aluminum-magnesium alloys 5086 and 5456 in the H116 and H117 tempers to exfoliation and intergranular corrosion. The test consists of measuring the weight loss per unit area of a 2 by 0.25 in. by gage specimen immersed in concentrated nitric acid at 30 C (86 F) for 24 h. The acid dissolves a second phase, an aluminum-magnesium intermetallic compound, in preference to solid solution of magnesium in aluminum. When this compound is precipitated in a continuous network along grain boundaries, the effect of the preferential attack is to eat around the grains, causing them to fall away from the specimen. Weight losses for materials proven susceptible to exfoliation or intergranular corrosion amount to the hundreds of milligrams per square inch, while samples of 5086 and 5456 in the H116 and H117 tempers lose only from 10 up to 100 mg/in.2.
Results of tests performed on commercially produced materials are presented, as well as several examples using other tempers and alloys which have shown varied resistance to exfoliation or intergranular corrosion in service.
corrosion, aluminum magnesium alloys, exfoliation corrosion, intergranular corrosion, corrosion tests, corrosion resistance, nitric acid, salt spray tests, immersion tests (corrosion), etching
Research supervisor, Reynolds Metals Co., Richmond, Va.