NAMLT Test

This test method consists of immersing test specimens in concentrated nitric acid at 30°C (86°F) for 24 h and determining the mass loss per unit area as a measure of susceptibility to intergranular corrosion 

This test method provides a quantitative measure of the susceptibility to intergranular corrosion of Al-Mg and AlMg-Mn alloys. The nitric acid dissolves a second phase, an aluminum-magnesium intermetallic compound (βAl-Mg), in preference to the solid solution of magnesium in the aluminum matrix. When this compound is precipitated in a relatively continuous network along grain boundaries, the effect of the preferential attack is to corrode around the grains, causing them to fall away from the specimens. Such dropping out of the grains causes relatively large mass losses of the order of 25 to 75 mg/cm2 (160 to 480 mg/in.2 ), whereas, samples of intergranular-resistant materials lose only about 1 to 15 mg/ cm2 (10 to 100 mg/in.2 ).