Effect of Addition of Bi on Stress-Corrosion Cracking of Al-Mg Alloy

In order to improve the resistance to stress-corrosion of Al-Mg alloys containing more than 5% magnesium, effects of additional elements, especially Bi, were investigated. Stress-corrosion tests were carried out on A1-6%Mg and Al-8%Mg alloys containing additional elements, subjected to solution-treatment at 400°C and aging at 130°C for 7 days, in 3.5%NaCl solution at room temperature under conditions of constant load and deflection. The mechanism that the addition of 0.2 to 0.4%Bi reduced the susceptibility of stress-corrosion of the Al-Mg alloys was studied by means of electrical resistivity measurement, EPMA, X-ray diffraction and polarization measurement. The results obtained are as follows:(1) Resistance to stress-corrosion cracking Al-6%Mg and Al-8%Mg alloys is greatly improved by the addition of 0.2 to 0.4%Bi.(2) Tensile properties, grain size and aging characteristics of Al-Mg alloys containing 0.2 to 0.4%Bi are the same as those of the base alloys.(3) The Bi content exists as an insoluble compound Bi2Mg3 and makes the potential of the base alloy less noble. The favorable effect of Bi addition may be due to the sacrificial protective action of the electronegative Bi2Mg3 compound.